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diff --git a/39142-h/39142-h.htm b/39142-h/39142-h.htm new file mode 100644 index 0000000..a02be01 --- /dev/null +++ b/39142-h/39142-h.htm @@ -0,0 +1,15449 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" + "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> +<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> + <head> + <meta http-equiv="Content-Type" content="text/html;charset=iso-8859-1" /> + <meta http-equiv="Content-Style-Type" content="text/css" /> + <title> + The Project Gutenberg eBook of Astronomy: The Science of the Heavenly Bodies, by David Todd. + </title> + <style type="text/css"> + +body {margin-left: 10%; margin-right: 10%;} + +h1,h2,h3 {text-align: center; clear: both;} + +p {margin-top: .75em; text-align: justify; + text-indent: 1.5em; margin-bottom: .75em;} + +hr {width: 33%; margin-top: 2em; margin-bottom: 2em; + margin-left: auto; margin-right: auto; clear: both;} + +hr.tb {width: 15%;} +hr.chap {width: 65%} + +table {margin-left: auto; margin-right: auto; + margin-top: 1em; margin-bottom: 1em; + text-align: center; border-collapse: collapse;} +td {padding: 2px 7px;} + +.text_lf {text-align: left;} +.text_rt {text-align: right;} +.center {text-align: center; text-indent: 0;} +.justify {text-align: justify} +.caption2 {font-weight: bolder; text-align: center; font-size: 2em; margin: 1.5em 0; text-indent: 0;} +.caption3 {font-weight: bolder; text-align: center; font-size: 1.5em; margin: 1em 0; text-indent: 0;} + +.pagenum {position: absolute; left: 92%; font-size: 0.8em; + color: #c0c0c0; text-align: right;} + +.brdbt {border-bottom: solid #000 1px;} +.brdbt2 {border-bottom: solid #000 2px;} +.brdbtl {border-bottom: solid #d0d0d0 1px;} +.brdlf {border-left: solid #000 1px;} +.brdtp2 {border-top: solid #000 2px;} +.mrt1 {margin-top: 1em;} +.mrt2 {margin-top: 2em;} +.mrb1 {margin-bottom: 1em;} +.mrb2 {margin-bottom: 2em;} + +.smcap {font-variant: small-caps;} +.smcap2 {font-variant: small-caps; font-size: 0.8em;} + +.fig_caption {font-size: 0.8em;} + +/* Images */ +.fig_center {margin: auto; text-align: center;} + +.fig_left {float: left; clear: left; margin-left: 0; margin-bottom: 1em; + margin-top: 1em; margin-right: 1em; padding: 0; text-align: center;} + +.fig_right {float: right; clear: right; margin-left: 1em; + margin-bottom: 1em; margin-top: 1em; margin-right: 0; + padding: 0; text-align: center;} + +/* Poetry */ +.poem {margin-left:35%;} +.poem.i4 {padding-left: 3em; text-indent: -3em; text-align: left;} + +/* Transcriber's notes */ +.trans_notes {background-color: #E6E6FA; color: #000; font-size:smaller; padding:0.5em; + margin-bottom:5em; font-family:sans-serif, serif; } + + </style> + </head> +<body> + + +<pre> + +The Project Gutenberg EBook of Astronomy, by David Todd + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org/license + + +Title: Astronomy + The Science of the Heavenly Bodies + +Author: David Todd + +Release Date: March 15, 2012 [EBook #39142] + +Language: English + +Character set encoding: ISO-8859-1 + +*** START OF THIS PROJECT GUTENBERG EBOOK ASTRONOMY *** + + + + +Produced by K Nordquist, Tom Cosmas, Brenda Lewis and the +Online Distributed Proofreading Team at http://www.pgdp.net +(This file was produced from images generously made +available by The Internet Archive/American Libraries.) + + + + + + +</pre> + + + +<div class="fig_center" style="width: 288px"> +<img src="images/cover.jpg" width="288" height="452" alt="" title="" /> +</div> + +<div class="fig_center mrb2" style="width: 633px;"> +<a name="Frontispiece" id="Frontispiece"></a> +<img src="images/pl_f001.png" width="633" height="467" alt="" title="" /> +<div class="fig_caption">Photo, Mt. Wilson Solar Observatory +<br /> +<i>An Active Prominence of the Sun, 140,000 Miles High, +photographed July 9, 1917.</i></div> +</div> + +<p><span class="pagenum"><a name="Page_p001" id="Page_p001">[Pg 1]</a></span></p> + +<div class="center mrt2"> +<h1 style="font-size: 3em">ASTRONOMY</h1> + +<p class="caption2 smcap">The Science of the Heavenly Bodies</p> + +<p class="caption3">BY</p> + +<h2>DAVID TODD</h2> + +<p class="caption3 smcap">Director Emeritus, Amherst College Observatory</p> + +<div class="fig_center" style="width: 84px;"> +<img src="images/logo.png" width="84" height="121" alt="" /> +</div> + +<p class="center mrb2" style="margin: 3em 0;">NEW YORK AND LONDON<br /> +HARPER & BROTHERS<br /> +PUBLISHERS MCMXXII</p> +</div> + +<p><span class="pagenum"><a name="Page_p002" id="Page_p002">[2]</a></span></p> + + +<div class="center" style="margin: 5em 0;"> +Copyright 1922<br /> +<span class="smcap">By P. F. Collier & Son Company</span><br /> +<br /> +MANUFACTURED IN U. S. A.<br /> +<br /> +<br /> +</div> + +<p><span class="pagenum"><a name="Page_p003" id="Page_p003">[3]</a></span></p> + + +<h2><a name="PREFACE" id="PREFACE"></a>PREFACE</h2> + + +<p>Sir William Rowan Hamilton, the eminent +mathematician of Dublin, has, of all writers +ancient and modern, most fittingly characterized +the ideal science of astronomy as man's golden chain +connecting the heavens to the earth, by which we +"learn the language and interpret the oracles of the +universe."</p> + +<p>The oldest of the sciences, astronomy is also the +broadest in its relations to human knowledge and +the interests of mankind. Many are the cognate +sciences upon which the noble structure of astronomy +has been erected: foremost of all, geometry and the +higher mathematics, which tell us of motions, magnitudes +and distances; physics and chemistry, of +the origin, nature, and destinies of planets, sun, and +star; meteorology, of the circulation of their atmospheres; +geology, of the structure of the moon's +surface; mineralogy, of the constitution of meteorites; +while, if we attack, even elementally, the +fascinating, though perhaps forever unsolvable, +problem of life in other worlds, the astronomer must +invoke all the resources that his fellow biologists +and their many-sided science can afford him.</p> + +<p>The progress of astronomy from age to age has +been far from uniform—rather by leaps and bounds: +from the earliest epoch when man's planet earth +was the center about which the stupendous cosmos +wheeled, for whom it was created, and for whose +edification it was maintained—down to the modern +<span class="pagenum"><a name="Page_p004" id="Page_p004">[4]</a></span> +age whose discoveries have ascertained that even +our stellar universe, the vast region of the solar +domain, is but one of the thousands of island universes +that tenant the inconceivable immensities of +space.</p> + +<p>Such results have been attainable only through +the successful construction and operation of monster +telescopes that bring to the eye and visualize on +photographic plates the faintest of celestial objects +which were the despair of astronomers only a few +years ago.</p> + +<p>But the end is not yet; astronomy to-day is but +passing from infancy to youth. And with new and +greater telescopes, with new photographic processes +of higher sensitivity, with the help of modern invention +in overcoming the obstacle of the air—that +constant foe of the astronomer—who will presume +to set down any limit to the leaps and bounds of +astronomy in the future?</p> + +<p>So rapid, indeed, has been the progress of astronomy +in very recent years that the present is +especially favorable for setting forth its salient +features; and this book is an attempt to present +the wide range of astronomy in readable fashion, +as if a story with a definite plot, from its origin +with the shepherds of ancient Chaldea down to +present-day ascertainment of the actual scale of the +universe, and definite measures of the huge volume +of supersolar giants among the stars.</p> + +<p style="margin-left:90%" class="smcap mrb1">David Todd</p> + +<p style="text-indent:0"><span class="smcap">Amherst College Observatory</span><br /> +November, 1921<br /> +</p> + +<p><span class="pagenum"><a name="Page_p005" id="Page_p005">[5]</a></span></p> + + +<h2><a name="CONTENTS" id="CONTENTS"></a>CONTENTS</h2> + + +<table width="100%" summary="ToC"> +<tr> + <td class="text_rt"><small>CHAPTER</small></td> + <td></td> + <td class="text_rt"><small>PAGE</small></td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_I">I</a></td> + <td class="text_lf smcap">Astronomy a Living Science</td> + <td class="text_rt">9</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_II">II</a></td> + <td class="text_lf smcap">The First Astronomers</td> + <td class="text_rt">19</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_III">III</a></td> + <td class="text_lf smcap">Pyramid, Tomb, and Temple</td> + <td class="text_rt">23</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_IV">IV</a></td> + <td class="text_lf smcap">Origin of Greek Astronomy</td> + <td class="text_rt">27</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_V">V</a></td> + <td class="text_lf smcap">Measuring the Earth—Eratosthenes</td> + <td class="text_rt">30</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_VI">VI</a></td> + <td class="text_lf smcap">Ptolemy and His Great Book</td> + <td class="text_rt">33</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_VII">VII</a></td> + <td class="text_lf smcap">Astronomy of the Middle Ages</td> + <td class="text_rt">37</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_VIII">VIII</a></td> + <td class="text_lf smcap">Copernicus and the New Era</td> + <td class="text_rt">42</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_IX">IX</a></td> + <td class="text_lf smcap">Tycho, the Great Observer</td> + <td class="text_rt">45</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_X">X</a></td> + <td class="text_lf smcap">Kepler, the Great Calculator</td> + <td class="text_rt">49</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XI">XI</a></td> + <td class="text_lf smcap">Galileo, the Great Experimenter</td> + <td class="text_rt">53</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XII">XII</a></td> + <td class="text_lf smcap">After the Great Masters</td> + <td class="text_rt">57</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XIII">XIII</a></td> + <td class="text_lf smcap">Newton and Motion</td> + <td class="text_rt">62</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XIV">XIV</a></td> + <td class="text_lf smcap">Newton and Gravitation</td> + <td class="text_rt">66</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XV">XV</a></td> + <td class="text_lf smcap">After Newton</td> + <td class="text_rt">73</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XVI">XVI</a></td> + <td class="text_lf smcap">Halley and His Comet</td> + <td class="text_rt">83</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XVII">XVII</a></td> + <td class="text_lf smcap">Bradley and Aberration</td> + <td class="text_rt">90</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XVIII">XVIII</a></td> + <td class="text_lf smcap">The Telescope</td> + <td class="text_rt">93</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XIX">XIX</a></td> + <td class="text_lf smcap">Reflectors—Mirror Telescopes</td> + <td class="text_rt">102</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XX">XX</a></td> + <td class="text_lf smcap">The Story of the Spectroscope</td> + <td class="text_rt">111</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XXI">XXI</a></td> + <td class="text_lf smcap">The Story of Astronomical Photography</td> + <td class="text_rt">125</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XXII">XXII</a></td> + <td class="text_lf smcap">Mountain Observatories</td> + <td class="text_rt">139</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XXIII">XXIII</a></td> + <td class="text_lf smcap">The Program of a Great Observatory</td> + <td class="text_rt">152</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XXIV">XXIV</a></td> + <td class="text_lf smcap">Our Solar System</td> + <td class="text_rt">162</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XXV">XXV</a></td> + <td class="text_lf smcap">The Sun and Observing It</td> + <td class="text_rt">165</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XXVI">XXVI</a></td> + <td class="text_lf smcap">Sun Spots and Prominences</td> + <td class="text_rt">174</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XXVII">XXVII</a></td> + <td class="text_lf smcap">The Inner Planets</td> + <td class="text_rt">189</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XXVIII">XXVIII</a></td> + <td class="text_lf smcap">The Moon and Her Surface</td> + <td class="text_rt">193</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XXIX">XXIX</a></td> + <td class="text_lf smcap">Eclipses of the Moon</td> + <td class="text_rt">206</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XXX">XXX</a></td> + <td class="text_lf smcap">Total Eclipses of the Sun</td> + <td class="text_rt">209</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XXXI">XXXI</a></td> + <td class="text_lf smcap">The Solar Corona + <span class="pagenum"><a name="Page_p006" id="Page_p006">[6]</a></span></td> + <td class="text_rt">219</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XXXII">XXXII</a></td> + <td class="text_lf smcap">The Ruddy Planet</td> + <td class="text_rt">227</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XXXIII">XXXIII</a></td> + <td class="text_lf smcap">The Canals of Mars</td> + <td class="text_rt">235</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XXXIV">XXXIV</a></td> + <td class="text_lf smcap">Life in Other Worlds</td> + <td class="text_rt">242</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XXXV">XXXV</a></td> + <td class="text_lf smcap">The Little Planets</td> + <td class="text_rt">254</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XXXVI">XXXVI</a></td> + <td class="text_lf smcap">The Giant Planet</td> + <td class="text_rt">260</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XXXVII">XXXVII</a></td> + <td class="text_lf smcap">The Ringed Planet</td> + <td class="text_rt">264</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XXXVIII">XXXVIII</a></td> + <td class="text_lf smcap">The Farthest Planets</td> + <td class="text_rt">267</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XXXIX">XXXIX</a></td> + <td class="text_lf smcap">The Trans-Neptunian Planet</td> + <td class="text_rt">270</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XL">XL</a></td> + <td class="text_lf smcap">Comets—the Hairy Stars</td> + <td class="text_rt">273</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XLI">XLI</a></td> + <td class="text_lf smcap">Where Do Comets Come From?</td> + <td class="text_rt">279</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XLII">XLII</a></td> + <td class="text_lf smcap">Meteors and Shooting Stars</td> + <td class="text_rt">283</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XLIII">XLIII</a></td> + <td class="text_lf smcap">Meteorites</td> + <td class="text_rt">290</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XLIV">XLIV</a></td> + <td class="text_lf smcap">The Universe of Stars</td> + <td class="text_rt">294</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XLV">XLV</a></td> + <td class="text_lf smcap">Star Charts and Catalogues</td> + <td class="text_rt">300</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XLVI">XLVI</a></td> + <td class="text_lf smcap">The Sun's Motion Toward Lyra</td> + <td class="text_rt">304</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XLVII">XLVII</a></td> + <td class="text_lf smcap">Stars and Their Spectral Type</td> + <td class="text_rt">307</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XLVIII">XLVIII</a></td> + <td class="text_lf smcap">Star Distances</td> + <td class="text_rt">311</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_XLIX">XLIX</a></td> + <td class="text_lf smcap">The Nearest Stars</td> + <td class="text_rt">319</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_L">L</a></td> + <td class="text_lf smcap">Actual Dimensions of the Stars</td> + <td class="text_rt">321</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_LI">LI</a></td> + <td class="text_lf smcap">The Variable Stars</td> + <td class="text_rt">324</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_LII">LII</a></td> + <td class="text_lf smcap">The Novæ, or New Stars</td> + <td class="text_rt">331</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_LIII">LIII</a></td> + <td class="text_lf smcap">The Double Stars</td> + <td class="text_rt">334</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_LIV">LIV</a></td> + <td class="text_lf smcap">The Star Clusters</td> + <td class="text_rt">336</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_LV">LV</a></td> + <td class="text_lf smcap">Moving Clusters</td> + <td class="text_rt">341</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_LVI">LVI</a></td> + <td class="text_lf smcap">The Two Star Streams</td> + <td class="text_rt">345</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_LVII">LVII</a></td> + <td class="text_lf smcap">The Galaxy or Milky Way</td> + <td class="text_rt">350</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_LVIII">LVIII</a></td> + <td class="text_lf smcap">Star Clouds and Nebulæ</td> + <td class="text_rt">357</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_LIX">LIX</a></td> + <td class="text_lf smcap">The Spiral Nebulæ</td> + <td class="text_rt">361</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_LX">LX</a></td> + <td class="text_lf smcap">Cosmogony</td> + <td class="text_rt">366</td> +</tr> +<tr> + <td class="text_rt"><a href="#CHAPTER_LXI">LXI</a></td> + <td class="text_lf smcap">Cosmogony in Transition</td> + <td class="text_rt">380</td> +</tr> +</table> + +<p><span class="pagenum"><a name="Page_p007" id="Page_p007">[7]</a></span></p> + + +<h2><a name="LIST_OF_ILLUSTRATIONS" id="LIST_OF_ILLUSTRATIONS"></a>LIST OF ILLUSTRATIONS</h2> + + +<table width="100%" summary="LoI"> +<tr> + <td class="text_lf smcap"><a href="#Frontispiece">Active Prominence of the Sun, 140,000 Miles High</a></td> + <td class="text_rt"><i>Frontispiece</i><br /></td> +</tr> +<tr> + <td> </td> + <td class="text_rt"><small>FACING PAGE</small></td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p064p1">Nicholas Copernicus</a></td> + <td class="text_rt">64</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p064p1">Galileo Galilei</a></td> + <td class="text_rt">64</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p064p2">Johann Kepler</a></td> + <td class="text_rt">65</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p064p2">Sir Isaac Newton</a></td> + <td class="text_rt">65</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p096p1">The Hundred-Inch Reflecting Telescope at Mount Wilson</a></td> + <td class="text_rt">96</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p096p1">The Forty-Inch Refracting Telescope, Yerkes Observatory</a></td> + <td class="text_rt">96</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p096p2">150-Foot Tower, Mount Wilson, a Diagram of Tower and Pit</a></td> + <td class="text_rt">97</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p096p2">150-Foot Tower—Exterior View</a></td> + <td class="text_rt">97</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p096p2">View Looking Down into the Pit Beneath 150-Foot Tower</a></td> + <td class="text_rt">97</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p128p1">Mount Wilson Solar Observatory—the 100-Foot Dome</a></td> + <td class="text_rt">128</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p128p1">Mount Chimborazo, the Best Site in the World for an Observatory</a></td> + <td class="text_rt">128</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p128p2">Lick Observatory, Mount Hamilton, California</a></td> + <td class="text_rt">129</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p128p2">Photographing with the 40-inch Refractor</a></td> + <td class="text_rt">129</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p160p1">Great Sunspot Group of August 8, 1917</a></td> + <td class="text_rt">160</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p160p2">Calcium Flocculi on the Sun</a></td> + <td class="text_rt">161</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p160p2">Eclipse of the Moon, with the Lunar Surface Visible</a></td> + <td class="text_rt">161</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p192p1">Moon's Surface in the Region of Copernicus</a></td> + <td class="text_rt">192</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p192p2">South Central Portion of the Moon, at Last Quarter</a> + <span class="pagenum"><a name="Page_p008" id="Page_p008">[8]</a></span></td> + <td class="text_rt">193</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p224p1">Corona of the Sun During an Eclipse</a></td> + <td class="text_rt">224</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p224p2">Venus, in the Crescent Phase</a></td> + <td class="text_rt">225</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p224p2">Mars, Showing Bright Polar Cap</a></td> + <td class="text_rt">225</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p256p1">Jupiter, the Giant Planet</a></td> + <td class="text_rt">256</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p256p1">Neptune and Its Satellites</a></td> + <td class="text_rt">256</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p256p2">Saturn, with Edge of Rings only in View</a></td> + <td class="text_rt">257</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p256p2">Saturn, with Rings Displayed to Fullest Extent</a></td> + <td class="text_rt">257</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p288p1">Two Views of Halley's Comet</a></td> + <td class="text_rt">288</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p288p1">Swift's Comet, which Showed Remarkable Transformations</a></td> + <td class="text_rt">288</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p288p2">Meteor Trail in Field with Fine Nebulæ</a></td> + <td class="text_rt">289</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p320p1">Ring Nebula in Lyra</a></td> + <td class="text_rt">320</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p320p2">Dumb-bell Nebula</a></td> + <td class="text_rt">321</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p352p1">Star Clouds and Black Holes in Sagittarius</a></td> + <td class="text_rt">352</td> +</tr> +<tr> + <td class="text_lf smcap"><a href="#Page_p352p2">Great Nebula in Andromeda</a></td> + <td class="text_rt">353</td> +</tr> +</table> + +<p><span class="pagenum"><a name="Page_p009" id="Page_p009">[9]</a></span></p> + +<h2><a name="CHAPTER_I" id="CHAPTER_I"></a>CHAPTER I<br /> +<br /> +ASTRONOMY A LIVING SCIENCE</h2> + +<p>Like life itself we do not know when astronomy +began; we cannot conceive a time when it was +not. Man of the early stone age must have begun +to observe sun, moon, and stars, because all the +bodies of the cosmos were there, then as now. With +his intellectual birth astronomy was born.</p> + +<p>Onward through the childhood of the race he +began to think on the things he observed, to make +crude records of times and seasons; the Chaldeans +and Chinese began each their own system of +astronomy, the causes of things and the reasons +underlying phenomena began to attract attention, +and astronomy was cultivated not for its own sake, +but because of its practical utility in supplying the +data necessary to accurate astrological prediction. +Belief in astrology was universal.</p> + +<p>The earth set in the midst of the wonders of the +sky was the reason for it all. Clearly the earth +was created for humanity; so, too, the heavens were +created for the edification of the race. All was subservient +to man; naturally all was geocentric, or +earth-centered. From the savage who could count +only to five, the digits of one hand, civilized man +very slowly began to evolve; he noted the progress +of the seasons; the old records of eclipses showed +Thales, an early Greek, how to predict their +happenings, and true science had its birth when +<span class="pagenum"><a name="Page_p010" id="Page_p010">[10]</a></span> +man acquired the power to make forecasts that +always came true.</p> + +<p>Few ancient philosophers were greater than +Pythagoras, and his conceptions of the order of +the heavens and the shape and motion of the +earth were so near the truth that we sometimes +wonder how they could have been rejected for +twenty centuries. We must remember, however, +that man had not yet learned the art of measuring +things, and the world could not be brought into +subjection to him until he had. To measure he must +have tools—instruments; to have instruments he +must learn the art of working in metals, and all +this took time; it was a slow and in large part +imperceptible process; it is not yet finished.</p> + +<p>The earliest really sturdy manifestation of +astronomical life came with the birth of Greek +science, culminating with Aristarchus, Hipparchus +and Ptolemy. The last of these great philosophers, +realizing that only the art of writing prevents man's +knowledge from perishing with him, set down all +the astronomical knowledge of that day in one of +the three greatest books on astronomy ever written, +the Almagest, a name for it derived through the +Arabic, and really meaning "the greatest."</p> + +<p>The system of earth and heaven seemed as if +finished, and the authority of Ptolemy and his Almagest +were as Holy Writ for the unfortunate centuries +that followed him. With fatal persistence +the fundamental error of his system delayed the +evolutionary life of the science through all that +period.</p> + +<p>But man had begun to measure. Geometry had +been born and Eratosthenes had indeed measured +the size of the earth. Tools in bronze and iron were +<span class="pagenum"><a name="Page_p011" id="Page_p011">[11]</a></span> +fashioned closely after the models of tools of stone; +astrolabes and armillary spheres were first built +on geometric spheres and circles; and science was +then laid away for the slumber of the Dark +Ages.</p> + +<p>Nevertheless, through all this dreary period the +life of the youthful astronomical giant was maintained. +Time went on, the heavens revolved; sun, +moon, and stars kept their appointed places, and +Arab and Moor and the savage monarchs of the East +were there to observe and record, even if the world-mind +was lying fallow, and no genius had been born +to inspire anew that direction of human intellect on +which the later growth of science and civilization +depends. With the growth of the collective mind +of mankind, from generation to generation, we note +that ordered sequence of events which characterizes +the development of astronomy from earliest peoples +down to the age of Newton, Herschel, and the +present. It is the unfolding of a story as if with a +definite plot from the beginning.</p> + +<p>Leaving to philosophical writers the great fundamental +reason underlying the intellectual lethargy +of the Dark Ages, we only note that astronomy and +its development suffered with every other department +of human activity that concerned the intellectual +progress of the race. To knowledge of every +sort the medieval spirit was hostile. But with the +founding and growth of universities, a new era +began. The time was ripe for Copernicus and a +new system of the heavens. The discovery of the +New World and the revival of learning through +the universities added that stimulus and inspiration +which marked the transition from the Middle Ages +to our modern era, and the life of astronomy, long +<span class="pagenum"><a name="Page_p012" id="Page_p012">[12]</a></span> +dormant, was quickened to an extraordinary development.</p> + +<p>It fell to the lot of Copernicus to write the second +great book on astronomy, "De Revolutionibus Orbium +Cœlestium." But the new heliocentric or sun-centered +system of Copernicus, while it was the true +system bidding fair to replace the false, could not +be firmly established except on the basis of accurate +observation.</p> + +<p>How fortunate was the occurrence of the new +star of 1572, that turned the keen intellect of Tycho +Brahe toward the heavens! Without the observational +labors of Tycho's lifetime, what would the +mathematical genius of Kepler have availed in discovery +of his laws of motion of the planets?</p> + +<p>Historians dwell on the destruction and violent +conflicts of certain centuries of the Middle Ages, +quite overlooking the constructive work in progress +through the entire era. Much of this was of a nature +absolutely essential to the new life that was to +manifest itself in astronomy. The Arabs had made +important improvements in mathematical processes, +European artisans had made great advances in the +manufacture of glass and in the tools for working +in metals.</p> + +<p>Then came Galileo with his telescope revealing +anew the universe to mankind. It was the north of +Italy where the Renaissance was most potent, recalling +the vigorous life of ancient Greece. Copernicus +had studied here; it was the home of Galileo. +Columbus was a Genoese, and the compass which +guided him to the Western World was a product +of deft Italian artisans whose skill with that of +their successors was now available to construct the +instruments necessary for further progress in the +<span class="pagenum"><a name="Page_p013" id="Page_p013">[13]</a></span> +accurate science of astronomical observation. Even +before Copernicus, Johann Müller, better known as +Regiomontanus, had imbibed the learning of the +Greeks while studying in Italy, and founded an observatory +and issued nautical almanacs from Nuremberg, +the basis of those by which Columbus was +guided over untraversed seas.</p> + +<p>About this time, too, the art of printing was +invented, and the interrelation of all the movements +then in progress led up to a general awakening of +the mind of man, and eventually an outburst in +science and learning, which has continued to the +present day. Naturally it put new life into astronomy, +and led directly up from Galileo and his experimental +philosophy to Newton and the <i>Principia</i>, +the third in the trinity of great astronomical books +of all time.</p> + +<p>To get to the bottom of things, one must study +intimately the history of the intellectual development +of Europe through the fifteenth and sixteenth +centuries. Many of the western countries were ruled +by sovereigns of extraordinary vigor and force of +character, and their activities tended strongly toward +that firm basis on which the foundations of +modern civilization were securely laid.</p> + +<p>Contemporaneously with this era, and following +on through the seventeenth century, came the +measurements of the earth by French geodesists, +the construction of greater and greater telescopes +and the wonderful discoveries with them by Huygens, +Cassini, and many others.</p> + +<p>Most important of all was the application of +telescopes to the instruments with which angles are +measured. Then for the first time man had begun +to find out that by accurate measures of the heavenly +<span class="pagenum"><a name="Page_p014" id="Page_p014">[14]</a></span> +bodies, their places among the stars, their sizes and +distances, he could attain to complete knowledge of +them and so conquer the universe.</p> + +<p>But he soon realized the insufficiency of the +mathematical tools with which he worked—how unsuited +they were to the solution of the problem of +three bodies (sun, earth, and moon) under the Newtonian +law of gravitation, let alone the problem of +n-bodies, mutually attracting each the other; and +every one perturbing the motion of every other one. +So the invention of new mathematical tools was +prosecuted by Newton and his rival Leibnitz, who, +by the way, showed himself as great a man as +mathematician: "taking mathematics," wrote Leibnitz, +"from the beginning of the world to the times +when Newton lived, what he had done was much +the better half." Newton was the greatest of astronomers +who, since the revival of learning, had observed +the motions of the heavenly bodies and +sought to find out why they moved.</p> + +<p>Copernicus, Tycho Brahe, Galileo, Kepler, Newton, +all are bound together as in a plot. Not one of +them can be dissociated from the greatest of all +discoveries. But Newton, the greatest of them all, +revealed his greatness even more by saying: "If I +have seen further than other men, it is because I +have been standing on the shoulders of giants." +Elsewhere he says: "All this was in the two plague +years of 1665 and 1666 [he was then but twenty-four], +for in those days I was in the prime of my +age for invention, and minded mathematics and +philosophy more than at any time since." All school +children know these as the years of the plague and +the fire; but very few, in school or out, connect +these years with two other far-reaching events in +<span class="pagenum"><a name="Page_p015" id="Page_p015">[15]</a></span> +the world's history, the invention of the infinitesimal +calculus and the discovery of the law of gravitation.</p> + +<p>We have passed over the name of Descartes, almost +contemporary with Galileo, the founder of +modern dynamics, but his initiation of one of the +greatest improvements of mathematical method +cannot be overlooked. This era was the beginning +of the Golden Age of Mathematics that embraced +the lives of the versatile Euler, equally at home in +dynamics and optics and the lunar theory; of La +Grange, author of the elegant "Mécanique Analytique"; +and La Place, of the unparalleled "Mécanique +Céleste." With them and a fully elaborated +calculus Newton's universal law had been extended +to all the motions of the cosmos. Even the tides and +precession of the equinoxes and Bradley's nutation +were accounted for and explained. Mathematical or +gravitational astronomy had attained its pinnacle—it +seemed to be a finished science: all who were to +come after must be but followers.</p> + +<p>The culmination of one great period, however, +proved to be but the inception of another epoch in +the development of the living science.</p> + +<p>The greatest observer of all time, with a telescope +built by his own hands, had discovered a great +planet far beyond the then confines of the solar system. +Mathematicians would take care of Uranus, +and Herschel was left free to build bigger telescopes +still, and study the construction of the stellar universe. +Down to his day astronomy had dealt almost +wholly with the positions and motions of the celestial +bodies—astronomy was a science of <i>where</i>. +To inquire <i>what</i> the heavenly bodies <i>are</i>, seemed +to Herschel worthy of his keenest attention also. +While "a knowledge of the construction of the +<span class="pagenum"><a name="Page_p016" id="Page_p016">[16]</a></span> +heavens has always been the ultimate object of my +observations," as he said, and his ingenious method +of star-gauging was the first practicable attempt to +investigate the construction of the sidereal universe, +he nevertheless devoted much time to the description +of nebulæ and their nature, as well as their +distribution in space. He was the founder of double-star +astronomy, and his researches on the light of +the stars by the simple method of sequences were +the inception of the vast fields of stellar photometry +and variable stars. The physics of the sun, also, was +by no means neglected; and his lifework earned +for him the title of father of descriptive astronomy.</p> + +<p>While progress and discovery in the earlier fields +of astronomy were going on, the initial discoveries +in the vast group of small planets were made at the +beginning of the nineteenth century. The great +Bessel added new life to the science by revolutionizing +the methods and instruments of accurate +observation, his work culminating in the measure +of the distance of 61 Cygni, first of all the stars +whose distance from the sun became known.</p> + +<p>Wonderful as was this achievement, however, a +greater marvel still was announced just before the +middle of the century—a new planet far beyond +Uranus, whose discovery was made as a direct result +of mathematical researches by Adams and Le +Verrier, and affording an extraordinary verification +of the great Newtonian law. These were the days of +great discoveries, and about this time the giant +of all the astronomical tools of the century was +erected by Lord Rosse, the "Leviathan" reflector +with a speculum six feet in diameter, which remained +for more than half a century the greatest +telescope in the world, and whose epochal discovery +<span class="pagenum"><a name="Page_p017" id="Page_p017">[17]</a></span> +of spiral nebulæ has greater significance than we +yet know or perhaps even surmise.</p> + +<p>The living science was now at the height of a +vigorous development, when a revolutionary discovery +was announced by Kirchhoff which had been +hanging fire nearly half a century—the half century, +too, which had witnessed the invention of +photography, the steam engine, the railroad, and the +telegraph: three simple laws by which the dark +absorption lines of a spectrum are interpreted, and +the physical and chemical constitution of sun and +stars ascertained, no matter what their distance +from us.</p> + +<p>Huggins in England and Secchi in Italy were +quick to apply the discovery to the stars, and Draper +and Pickering by masterly organization have photographed +and classified the spectra of many hundred +thousand stars of both hemispheres, a research of +the highest importance which has proved of unique +service in studies of stellar movements and the +structure of the universe by Eddington and Shapley, +Campbell and Kapteyn, with many others who are +still engaged in pushing our knowledge far beyond +the former confines of the universe.</p> + +<p>Few are the branches of astronomy that have not +been modified by photography and the spectroscope. +It has become a measuring tool of the first order of +accuracy; measuring the speed of stars and nebulæ +toward and from us; measuring the rotational speed +of sun and planets, corona and Saturnian ring; +measuring the distances of whole classes of stars +from the solar system; measuring afresh even the +distance of the sun—the yardstick of our immediate +universe; measuring the drift of the sun with his +entire family of planets twelve miles every second +<span class="pagenum"><a name="Page_p018" id="Page_p018">[18]</a></span> +in the direction of Alpha Lyræ; and discovering +and measuring the speed of binary suns too close +together for our telescopes, and so making real +the astronomy of the invisible.</p> + +<p>Impatient of the handicap of a turbulent atmosphere, +the living science has sought out mountain +tops and there erected telescopes vastly greater than +the "Leviathan" of a past century. There the sun +in every detail of disk and spectrum is photographed +by day, and stars with their spectra and the nebulæ +by night. Great streams of stars are discovered +and the speed and direction of their drift ascertained. +The marvels of the spiral nebulæ are unfolded, +their multitudinous forms portrayed and +deciphered.</p> + +<p>And their distances? And the distances of the still +more wonderful clusters? Far, inconceivably far +beyond the Milky Way. And are they "island universes"? +And can man, the measurer, measure the +distance of the "mainland" beyond?</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p019" id="Page_p019">[19]</a></span></p> + +<h2><a name="CHAPTER_II" id="CHAPTER_II"></a>CHAPTER II<br /> +<br /> +THE FIRST ASTRONOMERS</h2> + +<p>Who were the first astronomers? And who +wrote the first treatise on astronomy, oldest +of the sciences?</p> + +<p>Questions not easy to answer in our day. With +the progress of archæological research, or inquiry +into the civilization and monuments of early +peoples, it becomes certain that man has lived on +this planet earth for tens of thousands of years in +the past as an intelligent, observing, intellectual +being; and it is impossible to assign any time so +remote that he did not observe and philosophize +upon the firmament above.</p> + +<p>We can hardly imagine a people so primitive that +they would fail to regard the sun as "Lord of the +Day," and therefore all important in the scheme of +things terrestrial. Says Anne Bradstreet of the +sun in her "Contemplations":</p> + +<div class="poem"> +<div class="i4">What glory's like to thee?</div> +<div class="i4">Soul of this world, this universe's eye,</div> +<div class="i4">No wonder some made thee deity.</div> +</div> + +<p>To the Babylonians belongs the credit of the +oldest known work on astronomy. It was written +nearly six thousand years ago, about <small>B. C.</small> 3800, by +their monarch Sargon the First, King of Agade. +Only the merest fragments of this historic treatise +have survived, and they indicate the reverence of +<span class="pagenum"><a name="Page_p020" id="Page_p020">[20]</a></span> +the Babylonians for the sun. Another work by +Sargon is entitled "Omens," which shows the intimate +relationship of astronomy to mysticism and +superstitious worship at this early date, and which +persists even at the present day.</p> + +<p>As remotely as <small>B. C.</small> 3000, the sun-god Shamash +and his wife Aya are carved upon the historic +cylinders of hematite and lapis lazuli, and one of +the oldest designs on these cylinders represents the +sun-god coming out of the Door of Sunrise, while a +porter is opening the Gate of the East. The +Semitic religion had as its basis a reverence for the +bodies of the sky; and Samson, Hebrew for sun, +was probably the sun-god of the Hebrews. The +Phœnician deity, Baal, was a sun-god under differing +designations; and at the epoch of the Shepherd +Kings, about <span class="smcap2">B. C.</span> 1500, during the Hyksos dynasty, +the sun-god was represented by a circle or disk +with extended rays ending in hands, possibly the +precursor of the frequently recurring Egyptian +design of the winged disk or winged solar globe. +Hittites, Persians, and Assyrians, as well as the +Phœnicians, frequently represented the sun-god +in similar fashion in their sacred glyphs or +carvings.</p> + +<p>For a long period in early human history, astronomy +and astrology were pretty much the same. +We can trace the history of astrology back as far +as <span class="smcap2">B. C.</span> 3000 in ancient Babylonia. The motions of +the sun, moon, and the five lucid planets of that +time indicated the activity of the various gods +who influenced human affairs. So the Babylonian +priests devised an elaborate system of interpreting +the phenomena of the heavens; and attaching the +proper significance in human terms to everything +<span class="pagenum"><a name="Page_p021" id="Page_p021">[21]</a></span> +that took place in the sky. In Babylonia and Assyria +it was the king and his people for whom the +prognostications were made out. It was the same +in Egypt. Later, about the fifth century <span class="smcap2">B. C.</span>, +astrology spread through Greece, where astrologers +developed the idea of the influence of planets upon +individual concerns. Astrology persisted through +the Dark Ages, and the great astronomers Copernicus, +Tycho, Kepler, Gassendi, and Huygens were +all astrologers as well. Milton makes many references +to planetary influence, our language has many +words with a direct origin in astrology, and in our +great cities to-day are many astrologers who prepare +individual horoscopes of more than ordinary +interest.</p> + +<p>It is difficult to assign the antiquity of the +Chinese astronomy with any approach to definiteness. +Their earliest records appear to have been +total eclipses of the sun, going back nearly 2,200 +years before the Christian era; and nearly a +thousand years earlier the Hindu astronomy sets +down a conjunction of all the planets, concerning +which, however, there is doubt whether it was +actually observed or merely calculated backward. +Owing to a colossal misfortune, the burning of all +native scientific books by order of the Emperor +Tsin-Chi-Hwang-Ti, in <span class="smcap2">B. C.</span> 221, excepting only +the volumes relating to agriculture, medicine, and +astrology, the Chinese lost a precious mass of astronomical +learning, accumulated through the ages. No +less an authority than Wells Williams credits them +with observing 600 solar eclipses between <span class="smcap2">B. C.</span> 2159 +and <span class="smcap2">A. D.</span> 1223, and there must have been some centuries +of eclipses observed and recorded anterior to +<span class="smcap2">B. C.</span> 2159, as this is the date assigned to the eclipse +<span class="pagenum"><a name="Page_p022" id="Page_p022">[22]</a></span> +which came unheralded by the astronomers royal, +Hi and Ho, who had become intoxicated and forgot +to warn the Court, in accord with their duty. China +was thereby exposed to the anger of the gods, and +Hi and Ho were executed by his Majesty's command. +It is doubtful if there is an earlier record +of any celestial phenomenon.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p023" id="Page_p023">[23]</a></span></p> + +<h2><a name="CHAPTER_III" id="CHAPTER_III"></a>CHAPTER III<br /> +<br /> +PYRAMID, TOMB, AND TEMPLE</h2> + +<p>Inquiry into the beginnings of astronomy in +ancient Egypt reveals most interesting relations +of the origins of the science to the life and work and +worship of the people. Their astronomers were +called the "mystery teachers of heaven"; their +monuments indicate a civilization more or less advanced; +and their temples were built on astronomical +principles and dedicated to purpose of worship. +The Egyptian records carry us back many +thousands of years, and we find that in Egypt, as +in other early civilizations, observation of the +heavenly bodies may be embraced in three pretty +distinct stages. Awe, fear, wonder and worship +were the first. Then came utility: a calendar was +necessary to tell men when "to plow and sow, to +reap and mow," and a calendar necessitated astronomical +observations of some sort. Following this, +the third direction required observations of celestial +positions and phenomena also, because astrology, in +which the potentates of every ancient realm believed, +could only thrive as it was based on astronomy.</p> + +<p>Sun worship was preeminent in early Egypt as +in India, where the primal antithesis between night +and day struck terror in the unformed mind of man. +In one of the Vedas occurs this significant song to +the god of day: "Will the Sun rise again? Will our +old friend the Dawn come back again? Will the +<span class="pagenum"><a name="Page_p024" id="Page_p024">[24]</a></span> +power of Darkness be conquered by the God of +Light?"</p> + +<p>Quite different from India, however, is Egypt in +matters of record: in India, records in papyrus, +but no monuments of very great antiquity; in +Egypt, no papyrus, but monuments of exceeding antiquity +in abundance. Herodotus and Pliny have +told us of the great antiquity of these monuments, +even in their own day, and research by archæologist +and astronomer has made it certain that the pyramids +were built by a race possessing great knowledge +of astronomy. Their temples, too, were constructed +in strict relation to stars. Not only are the temples, +as Edfu and Denderah, of exceeding interest +in themselves, but associated with them are often +huge monoliths of syenite, obelisks of many hundred +tons in weight, which the astronomer recognizes as +having served as observation pillars or gnomons. +Specimens of these have wandered as far from +home as Central Park and the bank of the Thames. +But there is an even more remarkable wealth of +temple inscriptions, zodiacs especially.</p> + +<p>Next to the sun himself was the worship of the +Dawn and Sunrise, the great revelations of nature. +There were numerous hymns to the still more +numerous sun-gods and the powers of sunlight. +Ra was the sun-god in his noontide strength; Osiris, +the dying sun of sunset. Only two gods were associated +with the moon, and for the stars a special +goddess, Sesheta. Sacrifices were made at day-break; +and the stars that heralded the dawn were +the subjects of careful observation by the sacrificial +priests, who must therefore have possessed a good +knowledge of star places and names, doubtless in +belts of stars extending clear around the heavens. +<span class="pagenum"><a name="Page_p025" id="Page_p025">[25]</a></span> +These decans, as they were called, are the exact +counterparts of the moon stations devised by the +Arabians, Indians, and other peoples for a like +purpose.</p> + +<p>The plane or circle of observation, both in Egypt +and India, was always the horizon, whether the sun +was observed or moon or stars. So the sun was +often worshiped by the ancient Egyptians as the +"Lord of the Two Horizons." It is sometimes +difficult to keep in mind the fact, in regard to all +temples of the ancients, whether in Egypt or elsewhere, +that in studying them we must deal with +the risings or settings of the heavenly bodies in +quite different fashion from that of the astronomer +of to-day, who is mainly concerned only with +observing them on the meridian. The axis of the +temple shows by its direction the place of rising or +setting: if the temple faces directly east or west, +its amplitude is 0. Now the sun, moon, and planets +are, as everyone knows, very erratic as to their +amplitudes (i. e., horizon points) of rising and setting; +so it must have been the stars that engrossed +the attention of the earliest builders of temples. +After that, temples were directed to the rising sun, +at the equinox or solstices. Then came the necessity +of finding out about the inclination or obliquity +of the ecliptic, and this is where the gnomon was +employed.</p> + +<p>At Karnak are many temples of the solstitial +order: the wonderful temple of Amen-Ra is so oriented +that its axis stands in amplitude 26 degrees +north of west, which is the exact amplitude of the +sun at Thebes at sunset of the summer solstice. +The axis of a lesser temple adjacent points to 26 +degrees south of east, which is the exact amplitude +<span class="pagenum"><a name="Page_p026" id="Page_p026">[26]</a></span> +of sunrise at the winter solstice. At Gizeh we find +the temples oriented, not solstitially, but by the +equinoxes, that is, they face due east and west. +Peoples who worshiped the sun at the solstice must +have begun their year at the solstice; and Sir Norman +Lockyer shows how the rise of the Nile, which +took place at the summer solstice, dominated not +only the industry but the astronomy and religion of +Egypt.</p> + +<p>Looking into the question of temple orientation +in other countries, as China, for example, Lockyer +finds that the most important temple of that +country, the Temple of the Sun at Peking, is oriented +to the winter solstice; and Stonehenge, as has long +been known, is oriented to sunrise at the summer +solstice.</p> + +<p>In like fashion the rising and setting of many +stars were utilized by the Egyptians, in both temple +and pyramid; and no astronomer who has ever seen +these ancient structures and studied their orientations +can doubt that they were built by astronomers +for use by astronomers of that day. The priests +were the astronomers, and the temples had a deep +religious significance, with a ceremony of exceeding +magnificence wherever observations of heavenly +bodies were undertaken, whether of sun or stars.</p> + +<p>Hindu and Persian astronomy must be passed +over very briefly. Interesting as their systems are +historically, there were few, if any, original contributions +of importance, and the Indian treatises bear +strong evidence of Greek origin.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p027" id="Page_p027">[27]</a></span></p> + +<h2><a name="CHAPTER_IV" id="CHAPTER_IV"></a>CHAPTER IV<br /> +<br /> +ORIGIN OF GREEK ASTRONOMY</h2> + +<p>While the Greeks laid the foundations of modern +scientific astronomy, they were not as a +whole observers: rather philosophers, we should +say. The later representatives of the Greek School, +however, saw the necessity of observation as a basis +of true induction; and they discovered that real +progress was not possible unless their speculative +ideas were sufficiently developed and made definite +by the aid of geometry, so that they became capable +of detailed comparison with observation. This was +the necessary and ultimate test with them, and the +same is true to-day. The early Greek philosophers +were, however, mainly interested, not in observations, +but in guessing the causes of phenomena.</p> + +<p>Thales of Miletus, founder of the Ionian School, +introduced the system of Egyptian astronomy into +Greece, about the end of the seventh century <span class="smcap2">B. C.</span> +He is universally known as the first astronomer who +ever predicted a total eclipse of the sun that +happened when he said it would: the eclipse of <span class="smcap2">B. C.</span> +585. This he did by means of the Chaldean eclipse +cycle of 18 years known as the Saros.</p> + +<p>Aristarchus of Samos was the first and most +eminent of the Alexandrian astronomers, and his +treatise "On the Magnitudes and Distances of the +Sun and Moon" is still extant. This method of +ascertaining how many times farther the sun is +<span class="pagenum"><a name="Page_p028" id="Page_p028">[28]</a></span> +than the moon is very simple, and geometrically +exact. Unfortunately it is impossible, even to-day, +to observe with accuracy the precise time when the +moon "quarters," (an observation essential to his +method), because the moon's terminal, or line between +day and night, is not a straight line as required +by theory, but a jagged one. By his observation, +the sun was only twenty times farther away +than the moon, a distance which we know to be +nearly twenty times too small.</p> + +<p>His views regarding other astronomical questions +were right, although they found little favor among +contemporaries. Not only was the earth spherical, +he said, but it rotated on its axis and also traveled +round the sun. Aristarchus was, indeed, the true +originator of the modern doctrine of motions in the +solar system, and not Copernicus, seventeen centuries +later; but Seleucus appears to have been his +only follower in these very advanced conceptions. +Aristarchus made out the apparent diameters of +sun and moon as practically equal to one another, +and inferred correctly that their real diameters are +in proportion to their distances from the earth. +Also he estimated, from observations during an +eclipse of the moon, that the moon's diameter is +about one-third that of the earth. Aristarchus +appears to have been one of the clearest and most +accurate thinkers among the ancient astronomers; +even his views concerning the distances of the stars +were in accord with the fact that they are immeasurably +distant as compared with the distances of the +sun, moon, and planets.</p> + +<p>Practically contemporary with Aristarchus were +Timocharis and Aristillus, who were excellent observers, +and left records of position of sun and +<span class="pagenum"><a name="Page_p029" id="Page_p029">[29]</a></span> +planets which were exceedingly useful to their successors, +Hipparchus and Ptolemy in particular. +Indeed their observations of star positions were +such that, in a way, they deserve the fame of having +made the first catalogue, rather than Hipparchus, +to whom is universally accorded that honor.</p> + +<p>Spherical astronomy had its origin with the +Alexandrian school, many famous geometers, and in +particular Euclid, pointing the way. Spherics, or +the doctrine of the sphere, was the subject of numerous +treatises, and the foundations were securely +laid for that department of astronomical research +which was absolutely essential to farther advance. +The artisans of that day began to build rude mechanical +adaptations of the geometric conceptions as +concrete constructions in wood and metal, and it +became the epoch of the origin of astrolabes and +armillary spheres.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p030" id="Page_p030">[30]</a></span></p> + +<h2><a name="CHAPTER_V" id="CHAPTER_V"></a>CHAPTER V<br /> +<br /> +MEASURING THE EARTH—ERATOSTHENES</h2> + +<p>All told, the Greek philosophers were probably +the keenest minds that ever inhabited the planet, +and we cannot suppose them so stupid as to reject +the doctrine of a spherical earth. In fact so certain +were they that the earth's true figure is a sphere +that Eratosthenes in the third century <span class="smcap2">B. C.</span> made +the first measure of the dimensions of the terrestrial +sphere by a method geometrically exact.</p> + +<p>At Syene in Upper Egypt the sun at the summer +solstice was known to pass through the zenith at +noon, whereas at Alexandria Eratosthenes estimated +its distance as seven degrees from the zenith +at the same time. This difference being about one-fiftieth +of the entire circumference of a meridian, +Eratosthenes correctly inferred that the distance +between Alexandria and Syene must be one-fiftieth +of the earth's circumference. So he measured the +distance between the two and found it 5,000 stadia. +This figured out the size of the earth with a percentage +of error surprisingly small when we consider +the rough means with which Eratosthenes +measured the sun's zenith distance and the distance +between the two stations.</p> + +<p>Greatest of all the Greek astronomers and one +of the greatest in the history of the science was +Hipparchus who had an observatory at Rhodes in +<span class="pagenum"><a name="Page_p031" id="Page_p031">[31]</a></span> +the middle of the second century <span class="smcap2">B. C.</span> His activities +covered every department of astronomy; he +made extensive series of observations which he +diligently compared with those handed down to +him by the earlier astronomers, especially Aristillus +and Timocharis. This enabled him to ascertain the +motion of the equinoxial points, and his value of +the constant of precession of the equinoxes is exceedingly +accurate for a first determination.</p> + +<p>In 134 <span class="smcap2">B. C.</span> a new star blazed out in the constellation +Scorpio, and this set Hipparchus at work on a +catalogue of the brighter stars of the firmament, a +monumental work of true scientific conception, because +it would enable the astronomers of future +generations to ascertain what changes, if any, were +taking place in the stellar universe. There were +1,080 stars in his catalogue, and he referred their +positions to the ecliptic and the equinoxes. Also he +originated the present system of stellar magnitudes +or orders of brightness, and his catalogue was in +use as a standard for many centuries.</p> + +<p>Hipparchus was a great mathematician as well, +and he devoted himself to the improvement of the +method of applying numerical calculations to geometrical +figures: trigonometry, both plane and +spherical, that is; and by some authorities he is regarded +as the inventor of original methods in trigonometry. +The system of spheres of Eudoxus did not +satisfy him, so he devised a method of representing +the paths of the heavenly bodies by perfectly uniform +motion in circles. There is slight evidence that +Apollonius of Perga may have been the originator +of the system, but it was reserved for Hipparchus +to work it out in final form. This enabled him to +ascertain the varying length of the seasons, and he +<span class="pagenum"><a name="Page_p032" id="Page_p032">[32]</a></span> +fixed the true length of the year as 365¼ days. He +had almost equal success in dealing with the irregularities +of the moon's motion, although the +problem is much more complicated. The distance +and size of the moon, by the method of Aristarchus, +were improved by him, and he worked out, for the +distance of the sun, 1,200 radii of the earth—a +classic for many centuries.</p> + +<p>Hipparchus devoted much attention to eclipses +of both sun and moon, and we owe to him the first +elucidation of the subject of parallax, or the effect +of difference of position of an observer on the earth's +surface as affecting the apparent projection of the +moon against the sun when a solar eclipse takes +place; whereas an eclipse of the moon is unaffected +by parallax and can be seen at the same time by +observers everywhere, no matter what their location +on the earth. Indeed, with all that Hipparchus +achieved, we need not be surprised that astronomy +was regarded as a finished science, and made practically +no progress whatever for centuries after his +time.</p> + +<p>Then came Claudius Ptolemæus, generally known +as Ptolemy, the last great name in Greek astronomy. +He lived in Alexandria about the middle of the +second century <span class="smcap2">A. D.</span> and wrote many minor astronomical +and astrological treatises, also works on +geography and optics, in the last of which the +atmospheric refraction of rays of light from the +heavenly bodies, apparently elevating them toward +the zenith, is first dealt with in true form.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p033" id="Page_p033">[33]</a></span></p> + +<h2><a name="CHAPTER_VI" id="CHAPTER_VI"></a>CHAPTER VI<br /> +<br /> +PTOLEMY AND HIS GREAT BOOK</h2> + +<p>Ptolemy was an observer of the heavens, +though not of the highest order; but he had +all the work of his predecessors, best of all +Hipparchus, to build upon. Ptolemy's greatest +work was the "Megale Syntaxis," generally known +as the Almagest. It forms a nearly complete +compendium of the ancient astronomy, and +although it embodies much error, because built +on a wrong theory, the Almagest nevertheless +is competent to follow the motions of all the +bodies in the sky with a close approach to accuracy, +even at the present day. This marvelous +work written at this critical epoch became as +authoritative as the philosophy of Aristotle, and +for many centuries it was the last word in the +science. The old astrology held full sway, and +the Ptolemaic theory of the universe supplied +everything necessary: further progress, indeed, +was deemed impossible.</p> + +<p>The Almagest comprises in all thirteen books, the +first two of which deal with the simpler observations +of the celestial sphere, its own motion and the apparent +motions of sun, moon, and planets upon it. He +discusses, too, the postulates of his system and exhibits +great skill as an original geometer and mathematician. +In the third book he takes up the length +of the year, and in the fourth book similarly the +<span class="pagenum"><a name="Page_p034" id="Page_p034">[34]</a></span> +moon and the length of the month. Here his mathematical +powers are at their best, and he made a discovery +of an inequality in the moon's motion known +as the evection. Book five describes the construction +and use of the astrolabe, a combination of graduated +circles with which Ptolemy made most of his +observations. In the sixth book he follows mainly +Hipparchus in dealing with eclipses of sun and +moon. In the seventh and eighth books he discusses +the motion of the equinox, and embodies a catalogue +of 1,028 stars, substantially as in Hipparchus. The +five remaining books of the Almagest deal with the +planetary motions, and are the most important of all +of Ptolemy's original contributions to astronomy. +Ptolemy's fundamental doctrines were that the heavens +are spherical in form, all the heavenly motions +being in circles. In his view, the earth too is spherical, +and it is located at the center of the universe, +being only a point, as it were, in comparison. All was +founded on mere appearance combined with the philosophical +notion that the circle being the only +perfect curve, all motions of heavenly bodies must +take place in earth-centered circles. For fourteen +or fifteen centuries this false theory persisted, on +the authority of Ptolemy and the Almagest, rendering +progress toward the development of the true +theory impossible.</p> + +<p>Ptolemy correctly argued that the earth itself is a +sphere that is curved from east to west, and from +north to south as well, clinching his argument, as we +do to-day, by the visibility of objects at sea, the +lower portions of which are at first concealed from +our view by the curved surface of the water which +intervenes. To Ptolemy also the earth is at the +center of the celestial sphere, and it has no motion +<span class="pagenum"><a name="Page_p035" id="Page_p035">[35]</a></span> +of translation from that point; but his argument +fails to prove this. Truth and error, indeed, are so +deftly intermingled that one is led to wonder why +the keen intelligence of this great philosopher permitted +him to reject the simple doctrine of the +earth's rotation on its axis. But if we reflect that +there was then no science of natural philosophy or +physics proper, and that the age was wholly undeveloped +along the lines of practical mechanics, we +shall see why the astronomers of Ptolemy's time and +subsequent centuries were content to accept the doctrines +of the heavens as formulated by him.</p> + +<p>When it came to explaining the movements of the +"wandering stars," or planets, as we term them, the +Ptolemaic theory was very happy in so far as accuracy +was concerned, but very unhappy when it had +to account for the actual mechanics of the cosmos in +space. Sun and moon were the only bodies that went +steadily onward, easterly: whereas all the others, +Mercury, Venus, Mars, Jupiter, Saturn, although +they moved easterly most of the time, nevertheless +would at intervals slow down to stationary points, +where for a time they did not move at all, and then +actually go backward to the west, or retrograde, then +become stationary again, finally resuming their regular +onward motion to the east.</p> + +<p>To help out of this difficulty, the worst possible +mechanical scheme was invented, that known as the +epicycle. Each of the five planets was supposed to +have a fictitious "double," which traveled eastward +with uniformity, attached to the end of a huge but +mechanically impossible bar. The earth-centered +circle in which this traveled round was called the +"deferent." What this bar was made of, what +stresses it would be subjected to, or what its size +<span class="pagenum"><a name="Page_p036" id="Page_p036">[36]</a></span> +would have to be in order to keep from breaking—none +of these questions seems to have agitated the +ancient and medieval astronomers, any more than +the flat-earth astronomy of the Hindu is troubled +by the necessity of something to hold up the +tortoise that holds up the elephant that holds up +the earth.</p> + +<p>But at the end of this bar is jointed or swiveled +another shorter bar, to the revolving end of which is +attached the actual planet itself; and the second bar, +by swinging once round the end of the primary advancing +bar, would account for the backward or retrograde +motion of the planet as seen in the sky. For +every new irregularity that was found, in the motion +of Mars, for instance, a new and additional bar was +requisitioned, until interplanetary space was hopelessly +filled with revolving bars, each producing one +of the epicycles, some large, some small, that were +needed to take up the vagaries of the several planets.</p> + +<p>The Arabic astronomers who kept the science alive +through the Middle Ages added epicycle to epicycle, +until there was every justification for Milton's verses +descriptive of the sphere:</p> + +<div class="poem"> +<div class="i4">With Centric and Eccentric scribbled o'er,</div> +<div class="i4">Cycle and Epicycle, Orb in Orb.</div> +</div> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p037" id="Page_p037">[37]</a></span></p> + +<h2><a name="CHAPTER_VII" id="CHAPTER_VII"></a>CHAPTER VII<br /> +<br /> +ASTRONOMY OF THE MIDDLE AGES</h2> + +<p>With the fall of Alexandria and the victory of +Mohammed throughout the West, and a consequent +decline in learning, supremacy in science +passed to the East and centered round the caliphs +of Bagdad in the seventh and eighth centuries. +They were interested in astronomy only as a practical, +and to them useful, science, in adjusting the +complicated lunar calendar of the Mohammedans, in +ascertaining the true direction of Mecca which +every Mohammedan must know, and in the revival +of astrology, to which the Greeks had not attached +any particular significance.</p> + +<p>Harun al-Rashid ordered the Almagest and +many other Greek works translated, of which the +modern world would otherwise no doubt never have +heard, as the Greek originals are not extant.</p> + +<p>Splendid observatories were built at Damascus +and Bagdad, and fine instruments patterned after +Greek models were continuously used in observing. +The Arab astronomers, although they had no clocks, +were nevertheless so fully impressed with the importance +of time that they added extreme value to +their observations of eclipses, for example, by setting +down the altitudes of sun or stars at the same time. +On very important occasions the records were certified +on oath by a body of barristers and astronomers +conjointly—a precedent which fortunately has +never been followed.</p> + +<p><span class="pagenum"><a name="Page_p038" id="Page_p038">[38]</a></span> +About the middle of the ninth century, the Caliph +Al-Mamun directed his astronomers to revise the +Greek measures of the earth's dimensions, and they +had less reverence for the Almagest than existed in +later centuries: indeed, Tabit ben Korra invented +and applied to the tables of the Almagest a theoretical +fluctuation in the position of the ecliptic which +he called "trepidation," which brought sad confusion +into astronomical tables for many succeeding +centuries.</p> + +<p>Albategnius was another Arab prince whose +record in astronomy in the ninth and tenth centuries +was perhaps the best: the Ptolemaic values of the +precession of the equinoxes and of the obliquity of +the ecliptic were improved by new observations, and +his excellence as mathematician enabled him to make +permanent improvements in the astronomical application +of trigonometry.</p> + +<p>Abul Wefa was the last of the Bagdad astronomers +in the latter half of the tenth century, and his +great treatise on astronomy known as the Almagest +is sometimes confused with Ptolemy's work. +Following him was Ibn Yunos of Cairo, whose +labors culminated in the famous Hakemite Tables, +which became the standard in mathematical and +astronomical computations for several centuries.</p> + +<p>Mohammedan astronomy thrived, too, in Spain +and northern Africa. Arzachel of Toledo published +the Toledan Tables, and his pupils made improvements +in instruments and the methods of calculation. +The Giralda was built by the Moors in Seville +in 1196, the first astronomical observatory on the +continent of Europe; but within the next half century +both Seville and Cordova became Christian +again, and Arab astronomy was at an end.</p> + +<p><span class="pagenum"><a name="Page_p039" id="Page_p039">[39]</a></span> +Through many centuries, however, the science had +been kept alive, even if no great original advances +had been achieved; and Arab activities have modified +our language very materially, adding many such +words as almanac, zenith, and radii, and a wealth of +star names, as Aldebaran, Rigel, Betelgeuse, Vega, +and so on.</p> + +<p>Meanwhile, other schools of astronomy had developed +in the East, one at Meraga near the modern +Persia, where Nassir Eddin, the astronomer of +Hulagu Khan, grandson of the Mongol emperor +Genghis Khan, built and used large and carefully +constructed instruments, translated all the Greek +treatises on astronomy, and published a laborious +work known as the Ilkhanic Tables, based on the +Hakemite Tables of Ibn Yunos.</p> + +<p>More important still was the Tartar school of astronomy +under Ulugh Beg, a grandson of Tamerlane, +who built an observatory at Samarcand in 1420, +published new tables of the planets, and made with +his excellent instruments the observations for a new +catalogue of stars, the first since Hipparchus, the +star places being recorded with great precision.</p> + +<p>The European astronomy of the Middle Ages +amounted to very little besides translation from the +Arabic authors into Latin, with commentaries. Astronomers +under the patronage of Alfonso X of Leon +and Castile published in 1252 the Alfonsine Tables, +which superseded the Toledan tables and were accepted +everywhere throughout Europe. Alfonso +published also the "Libros del Saber," perhaps the +first of all astronomical cyclopedias, in which is said +to occur the earliest diagram representing a planetary +orbit as an ellipse: Mercury's supposed path +round the earth as a center.</p> + +<p><span class="pagenum"><a name="Page_p040" id="Page_p040">[40]</a></span> +Purbach of Vienna about the middle of the 15th +century began his "Epitome of Astronomy" based +on the "Almagest" of Ptolemy, which was finished by +his collaborator Regiomontanus, who was an expert +in mathematics and published a treatise on trigonometry +with the first table of sines calculated for +every minute from 0° to 90°, a most helpful contribution +to theoretical astronomy.</p> + +<p>Regiomontanus had a very picturesque career, +finally taking up his residence in Nuremberg, where +a wealthy citizen named Walther became his patron, +pupil, and collaborator. The artisans of the city +were set at work on astronomical instruments of the +greatest accuracy, and the comet of 1472 was the +first to be observed and studied in true scientific +fashion. Regiomontanus was very progressive and +the invention of the new art of printing gave him an +opportunity to publish Purbach's treatise, which +went through several editions and doubtless had +much to do in promoting dissatisfaction with the +ancient Ptolemaic system, and was thus most significant +in preparing a background for the coming +of the new Copernican order.</p> + +<p>The Nuremberg presses popularized astronomy +in other important ways, issuing almanacs, the first +precursors of our astronomical Ephemerides. Regiomontanus +was practical as well, and invented a +new method of getting a ship's position at sea, with +tables so accurate that they superseded all others +in the great voyages of discovery, and it is probable +that they were employed by Columbus in his discovery +of the American continent. Regiomontanus +had died several years earlier, in 1475 at Rome, where +he had gone by invitation of the Pope to effect a reformation +in the calendar. He was only forty, and +<span class="pagenum"><a name="Page_p041" id="Page_p041">[41]</a></span> +his patron Walther kept on with excellent observations, +the first probably to be corrected for the effect +of atmospheric refraction, although its influence had +been known since Ptolemy. The Nuremberg School +lasted for nearly two centuries.</p> + +<p>Nearly contemporary with Regiomontanus were +Fracastoro and Peter Apian, whose original observations +on comets are worthy of mention because they +first noticed that the tails of these bodies always +point away from the sun. Leonardo da Vinci was +the first to give the true explanation of earth-shine +on the moon, and similarly the moon-illumination +of the earth; and this no doubt had great weight in +disposing of the popular notion of an essential difference +of nature between the earth and celestial +bodies—all of which helped to prepare the way for +Copernicus and the great revolution in astronomical +thought.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p042" id="Page_p042">[42]</a></span></p> + +<h2><a name="CHAPTER_VIII" id="CHAPTER_VIII"></a>CHAPTER VIII<br /> +<br /> +COPERNICUS AND THE NEW ERA</h2> + +<p>Throughout the Middle Ages the progress of +astronomy was held back by a combination of +untoward circumstances. A prolonged reaction from +the heights attained by the Greek philosophers was +to be expected. The uprising of the Mohammedan +world, and the savage conquerors in the East did not +produce conditions favorable to the origin and development +of great ideas.</p> + +<p>At the birth of Copernicus, however, in 1473, the +time was ripening for fundamental changes from +the ancient system, the error of which had helped to +hold back the development of the science for centuries. +The fifteenth century was most fruitful in +a general quickening of intelligence, the invention of +printing had much to do with this, as it spread a +knowledge of the Greek writers, and led to conflict +of authorities. Even Aristotle and Ptolemy were +not entirely in harmony, yet each was held inviolate. +It was the age of the Reformation, too, and near the +end of the century the discovery of America exerted +a powerful stimulus in the advance of thought.</p> + +<p>Copernicus searched the works of the ancient +writers and philosophers, and embodied in this new +order such of their ideas as commended themselves +in the elaboration of his own system.</p> + +<p>Pythagoras alone and his philosophy looked in the +true direction. Many believe that he taught that the +sun, not the earth, is at the center of our solar system; +<span class="pagenum"><a name="Page_p043" id="Page_p043">[43]</a></span> +but his views were mingled with the speculative +philosophy of the Greeks, and none of his writings, +barring a few meager fragments, have come +down to our modern age.</p> + +<p>To many philosophers, through all these long centuries, +the true theory of the celestial motions must +have been obvious, but their views were not formulated, +nor have they been preserved in writing. So +the fact remains that Copernicus alone first proved +the truth of the system which is recognized to-day. +This he did in his great treatise entitled "De Revolutionibus +Orbium Cœlestium," the first printed copy +of which was dramatically delivered to him on his +deathbed, in May, 1543. The seventy years of his +life were largely devoted to the preparation of this +work, which necessitated many observations as +well as intricate calculations based upon them. +Being a canon in the church, he naturally hesitated +about publishing his revolutionary views, his +friend Rheticus first doing this for him in outline +in 1540.</p> + +<p>So simple are the great principles that they may +be embodied in very few words; what appears to +us as the daily revolution of the heavens is not a +real motion, but only an apparent one; that is, the +heavens are at rest, while the earth itself is in +motion, turning round an axis which passes through +its center. And the second proposition is that the +earth is simply one of the six known planets; and +they all revolve round the sun as the true center. +The solar system, therefore, is "heliocentric," or +sun-centered, not "geocentric" or earth-centered, as +taught by the Ptolemaic theory.</p> + +<p>Copernicus demonstrates clearly how his system +explains the retrograde motion of the planets and +<span class="pagenum"><a name="Page_p044" id="Page_p044">[44]</a></span> +their stationary points, no matter whether they are +within the orbit of the earth, as Mercury and Venus, +or outside of it, as Mars, Jupiter, and Saturn. His +system provides also the means of ascertaining with +accuracy the proportions of the solar system, or +the relative distances of the planets from the sun +and from each other. In this respect also his system +possessed a vast advantage over that of +Ptolemy, and the planetary distances which Copernicus +computed are very close approximations to +the measures of the present day.</p> + +<p>Reinhold revised the calculations of Copernicus +and prepared the "Tabulæ Prutenicæ," based on the +"De Revolutionibus," which proved far superior to +the Alfonsine Tables, and were only supplanted +by the Rudolphine Tables of Kepler. On the whole +we may regard the lifework of Copernicus as +fundamentally the most significant in the history +and progress of astronomy.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p045" id="Page_p045">[45]</a></span></p> + +<h2><a name="CHAPTER_IX" id="CHAPTER_IX"></a>CHAPTER IX<br /> +<br /> +TYCHO, THE GREAT OBSERVER</h2> + +<p>Clear as Copernicus had made the demonstration +of the truth of his new system, it nevertheless +failed of immediate and universal acceptance. The +Ptolemaic system was too strongly intrenched, and +the motions of all the bodies in the sky were too +well represented by it. Accurate observations were +greatly needed, and the Landgrave William IV. of +Hesse built the Cassel Observatory, which made a +new catalogue of stars, and introduced the use of +clocks to carry on the time as measured by the uniform +motion of the celestial sphere. Three years +after the death of Copernicus, Tycho Brahe was +born, and when he was 30 the King of Denmark +built for him the famous observatory of Uraniborg, +where the great astronomer passed nearly a quarter +of a century in critically observing the positions of +the stars and planets. Tycho was celebrated as a +designer and constructor of new types of astronomical +instruments, and he printed a large volume +of these designs, which form the basis of many in +use at the present day. Unfortunately for the +genius of Tycho and the significance of his work, +the invention of the telescope had not yet been +made, so that his observations had not the modern +degree of accuracy. Nevertheless, they were destined +to play a most important part in the progress +of astronomy.</p> + +<p><span class="pagenum"><a name="Page_p046" id="Page_p046">[Pg 46]</a></span> +Tycho was sadly in error in his rejection of the +Copernican system, although his reasons, in his day, +seemed unanswerable. If the outer planets were +displaced among the stars by the annual motion of +the earth round the sun, he argued, then the fixed +stars must be similarly displaced—unless indeed +they be at such vast distances that their motions +would be too slight to be visible. Of course we know +now that this is really true, and that no instruments +that Tycho was able to build could possibly have +detected the motions, the effects of which we now +recognize in the case of the nearer fixed stars in +their annual, or parallactic, orbits.</p> + +<p>The remarkably accurate instruments devised by +Tycho Brahe and employed by him in improving +the observations of the positions of the heavenly +bodies were no doubt built after descriptions of +astrolabes such as Hipparchus used, as described +by Ptolemy. In his "Astronomiæ Instauratæ Mechanica" +we find illustrations and descriptions of many +of them.</p> + +<p>One is a polar astrolabe, mounted somewhat as +a modern equatorial telescope is, and the meridian +circle is adjustable so that it can be used in any +place, no matter what its latitude might be. There +is a graduated equatorial ring at right angles to +the polar axis, so that the astrolabe could be used +for making observations outside the meridian as +well as on it. This equatorial circle slides through +grooves, and is furnished with movable sights, and +a plumb line from the zenith or highest point of +the meridian circle makes it possible to give the +necessary adjustment in the vertical. Screws for +adjustment at the bottom are provided, just as in +our modern instruments, and two observers were +<span class="pagenum"><a name="Page_p047" id="Page_p047">[47]</a></span> +necessary, taking their sights simultaneously; unless, +as in one type of the instrument, a clock, or +some sort of measure of time, was employed.</p> + +<p>Another early type of instrument is called by +Tycho the ecliptic astrolabe (<i>Armillæ Zodiacales</i>, +or the Zodiacal Rings). It resembles the equatorial +astrolabe somewhat, but has a second ring inclined +to the equatorial one at an angle equal to the +obliquity of the ecliptic. In observing, the equatorial +ring was revolved round till the ecliptic ring +came into coincidence with the plane of the ecliptic +in the sky. Then the observation of a star's longitude +and latitude, as referred to the ecliptic plane, +could be made, quite as well as that of right ascension +and declination on the equatorial plane. But it +was necessary to work quickly, as the adjustment on +the ecliptic would soon disappear and have to be +renewed.</p> + +<p>Tycho is often called the father of the science of +astronomical observation, because of the improvements +in design and construction of the instruments +he used. His largest instrument was a mural quadrant, +a quarter-circle of copper, turning parallel +to the north-and-south face of a wall, its axis turning +on a bearing fixed in the wall. The radius of +this quadrant was nine feet, and it was graduated +or divided so as to read the very small angle of ten +seconds of arc—an extraordinary degree of precision +for his day.</p> + +<p>Tycho built also a very large alt-azimuth quadrant, +of six feet radius. Its operation was very +much as if his mural quadrant could be swung +round in azimuth. At several of the great observatories +of the present day, as Greenwich and Washington, +there are instruments of a similar type, +<span class="pagenum"><a name="Page_p048" id="Page_p048">[48]</a></span> +but much more accurate, because the mechanical +work in brass and steel is executed by tools that are +essentially perfect, and besides this the power of +the telescope is superadded to give absolute direction, +or pointing on the object under observation.</p> + +<p>Excellent clocks are necessary for precise observation +with such an instrument; but neither +Tycho Brahe, nor Hevelius was provided with such +accessories. Hevelius did not avail himself of the +telescope as an aid to precision of observation, +claiming that pinhole sights gave him more accurate +results. It was a dispute concerning this question +that Halley was sent over from London to +Danzig to arbitrate.</p> + +<p>There could be but one way to decide; the telescope +with its added power magnifies any displacement +of the instrument, and thereby enables the +observer to point his instrument more exactly. So +he can detect smaller errors and differences of +direction than he can without it. And what is of +great importance in more modern astronomy, the +telescope makes it possible to observe accurately the +position of objects so faint that they are wholly +invisible to the naked eye.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p049" id="Page_p049">[49]</a></span></p> + +<h2><a name="CHAPTER_X" id="CHAPTER_X"></a>CHAPTER X<br /> +<br /> +KEPLER, THE GREAT CALCULATOR</h2> + +<p>Most fortunate it was for the later development +of astronomical theory that Tycho Brahe not +only was a practical or observational astronomer of +the highest order, but that he confined himself studiously +for years to observations of the places of the +planets. Of Mars he accumulated an especially long +and accurate series, and among those who assisted +him in his work was a young and brilliant pupil +named Johann Kepler.</p> + +<p>Strongly impressed with the truth of the Copernican +System, Kepler was free to reject the erroneous +compromise system devised by Tycho Brahe, and +soon after Tycho's death Kepler addressed himself +seriously to the great problem that no one had ever +attempted to solve, viz: to find out what the laws of +motion of the planets round the sun really are. Of +course he took the fullest advantage of all that +Ptolemy and Copernicus had done before him, and +he had in addition the splendid observations of +Tycho Brahe as a basis to work upon.</p> + +<p>Copernicus, while he had effected the tremendous +advance of substituting the sun for the earth as the +center of motion, nevertheless clung to the erroneous +notion of Ptolemy that all the bodies of the sky +must perforce move at uniform speeds, and in circular +curves, the circle being the only "perfect +curve." Kepler was not long in finding out that +<span class="pagenum"><a name="Page_p050" id="Page_p050">[50]</a></span> +this could not be so, and he found it out because +Tycho Brahe's observations were much more accurate +than any that Copernicus had employed.</p> + +<p>Naturally he attempted the nearest planet first, +and that was Mars—the planet that Tycho had +assigned to him for research. How fortunate that +the orbit of Mars was the one, of all the planets, to +show practically the greatest divergence from the +ancient conditions of uniform motion in a perfectly +circular orbit! Had the orbit of Mars chanced to +be as nearly circular as is that of Venus, Kepler +might well have been driven to abandon his search +for the true curve of planetary motion.</p> + +<p>However, the facts of the cosmos were on his +side, but the calculations essential in testing his +various hypotheses were of the most tedious nature, +because logarithms were not yet known in his day. +His first discovery was that the orbit of Mars is +certainly not a circle, but oval or elliptic in figure. +And the sun, he soon found, could not be in the +center of the ellipse, so he made a series of trial +calculations with the sun located in one of the +foci of the ellipse instead.</p> + +<p>Then he found he could make his calculated places +of Mars agree quite perfectly with Tycho Brahe's +observed positions, if only he gave up the other +ancient requisite of perfectly uniform motion. On +doing this, it soon appeared that Mars, when in +perihelion, or nearest the sun, always moved +swiftest, while at its greatest distance from the +sun, or aphelion, its orbital velocity was slowest.</p> + +<p>Kepler did not busy himself to inquire why these +revolutionary discoveries of his were as they were; +he simply went on making enough trials on Mars, +and then on the other planets in turn, to satisfy +<span class="pagenum"><a name="Page_p051" id="Page_p051">[51]</a></span> +himself that all the planetary orbits are elliptical, +not circular in form, and are so located in space +that the center of the sun is at one of the two foci +of each orbit. This is known as Kepler's first law +of planetary motion.</p> + +<p>The second one did not come quite so easy; it +concerned the variable speed with which the planet +moves at every point of the orbit. We must remember +how handicapped he was in solving this problem: +only the geometry of Euclid to work with, and +none of the refinements of the higher mathematics +of a later day. But he finally found a very simple +relation which represented the velocity of the planet +everywhere in its orbit. It was this: if we calculate +the area swept, or passed over, by the planet's +radius vector (that is, the line joining its center to +the sun's center) during a week's time near perihelion, +and then calculate the similar area for a +week near aphelion, or indeed for a week when +Mars is in any intermediate part of its orbit, we +shall find that these areas are all equal to each +other. So Kepler formulated his second great law of +planetary motion very simply: the radius vector of +any planet describes, or sweeps over, equal areas in +equal times. And he found this was true for all the +planets.</p> + +<p>But the real genius of the great mathematician +was shown in the discovery of his third law, which +is more complex and even more significant than +the other two—a law connecting the distances of +the planets from the sun with their periods of revolution +about the sun. This cost Kepler many additional +years of close calculation, and the resulting +law, his third law of planetary motion is this: The +cubes of the mean or average distances of the +<span class="pagenum"><a name="Page_p052" id="Page_p052">[52]</a></span> +planets from the sun are proportional to the squares +of their times of revolution around him.</p> + +<p>So Kepler had not only disposed of the sacred +theories of motion of the planets held by the +ancients as inviolable, but he had demonstrated the +truth of a great law which bound all the bodies of +the solar system together. So accurately and completely +did these three laws account for all the motions, +that the science of astronomy seemed as if +finished; and no matter how far in the future a +time might be assigned, Kepler's laws provided +the means of calculating the planet's position for +that epoch as accurately as it would be possible +to observe it. Kepler paused here, and he died +in 1630.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p053" id="Page_p053">[53]</a></span></p> + +<h2><a name="CHAPTER_XI" id="CHAPTER_XI"></a>CHAPTER XI<br /> +<br /> +GALILEO, THE GREAT EXPERIMENTER</h2> + +<p>The fifteenth and sixteenth centuries, containing +the lives and work of Copernicus, Tycho, Galileo, +Kepler, Huygens, Halley, and Newton, were a veritable +Golden Age of astronomy. All these men were +truly great and original investigators.</p> + +<p>None had a career more picturesque and popular +than did Galileo. Born a few years earlier and +dying a few years later than Kepler, the work of +each of these two great astronomers was wholly independent +of the other and in entirely different +fields. Kepler was discovering the laws of planetary +motion, while Galileo was laying the secure foundations +of the new science of dynamics, in particular +the laws of falling bodies, that was necessary before +Kepler's laws could be fully understood. When only +eighteen Galileo's keen power of observation led to +his discovery of the laws of pendulum motion, suggested +by the oscillation to and fro of a lamp in the +cathedral of Pisa.</p> + +<p>The world-famous leaning tower of this place, +where he was born, served as a physical laboratory +from the top of which he dropped various objects, +and thus was led to formulate the laws of falling +bodies. He proved that Aristotle was all wrong in +saying that a heavy body must fall swifter in proportion +to its weight than a lighter one. These and +other discoveries rendered him unpopular with his +associates, who christened him the "Wrangler."</p> + +<p><span class="pagenum"><a name="Page_p054" id="Page_p054">[54]</a></span> +The new system of Copernicus appealed to him; +and when he, first of all men, turned a telescope on +the heavenly bodies, there was Venus with phases +like those of the moon, and Jupiter with satellites +traveling about it—a Copernican system in miniature. +Nothing could have happened that would +have provided a better demonstration of the truth +of the new system and the falsity of the old. His +marvelous discoveries caused the greatest excitement—consternation +even, among the anti-Copernicans. +Galileo published the "Sidereus Nuncius," with +many observations and drawings of the moon, +which he showed to be a body not wholly dissimilar +to the earth: this, too, was obviously of great moment +in corroboration of the Copernican order and +in contradiction to the Ptolemaic, which maintained +sharp lines of demarcation between things terrestrial +and things celestial.</p> + +<p>His telescopes, small as they were, revealed to him +anomalous appearances on both sides of the planet +Saturn which he called <i>ansæ</i>, or handles. But their +subsequent disappearance was unaccountable to +him, and later observers, who kept on guessing +ineffectively till Huygens, nearly a half century +after, showed that the true nature of the appendage +was a ring. Spots on the sun were frequently observed +by Galileo and led to bitter controversies. +He proved, however, that they were objects on the +sun itself, not outside it, and by noticing their repeated +transits across the sun's disk, he showed that +the sun turned round on his axis in a little less than +a month—another analogy to the like motion of the +earth on the Copernican plan.</p> + +<p>Galileo's appointment in 1610 as "First Philosopher +and Mathematician" to the Grand Duke of +<span class="pagenum"><a name="Page_p055" id="Page_p055">[55]</a></span> +Tuscany gave him abundant time for the pursuit +of original investigations and the preparation of +books and pamphlets. His first visit to Rome the +year following was the occasion of a reception with +great honor by many cardinals and others of high +rank. His lack of sympathy with others whose +views differed from his, and his naturally controversial +spirit, had begun to lead him headlong into +controversies with the Jesuits and the church, which +culminated in his censure by the authorities of the +church and persecution by the Inquisition.</p> + +<p>In 1618 three comets appeared, and Galileo was +again in controversial hot water with the Jesuits. +But it led to the publication five years later of "Il +Saggiatore" (The Assayer), of no great scientific +value, but only a brilliant bit of controversial literature +dedicated to the newly elevated Pope, Urban +VIII. Later he wrote through several years a great +treatise, more or less controversial in character, +entitled a "Dialogue on the Two Chief Systems of +the World" between three speakers, and extending +through four successive days. Simplicio argues for +the Aristotelians, Salviati for the Copernicans, +while Sagredo does his best to be neutral. It will +always be a very readable book, and we are fortunate +to have a recent translation by Professor +Crew of Evanston.</p> + +<p>Here we find the first suggestion of the +modern method of getting stellar parallaxes, the +relative parallax, that is, of two stars in the same +field—a method not put into service till Bessel's +time, two centuries later. But the most important +chapters of the "Dialogue" deal with Galileo's investigations +of the laws of motion of bodies in general, +which he applied to the problem of the earth's +<span class="pagenum"><a name="Page_p056" id="Page_p056">[56]</a></span> +motion. In this he really anticipated Newton in the +first of his three laws of motion, and in a subsequent +work, dealing with the theory of projectiles, +he reaches substantially the results of Newton's +second law of motion, although he gave no general +statement of the principle. Nevertheless, in the +epoch where his life was lived and his work done, +his telescopic discoveries, combined with his dynamic +researches in untrodden fields, resulted in the +complete and final overthrow of the ancient system +of error, and the secure establishment of the Copernican +system beyond further question and discussion. +Only then could the science of astronomy +proceed unhampered to the fullest development by +the master minds of succeeding centuries.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p057" id="Page_p057">[57]</a></span></p> + +<h2><a name="CHAPTER_XII" id="CHAPTER_XII"></a>CHAPTER XII<br /> +<br /> +AFTER THE GREAT MASTERS</h2> + +<p>Following Kepler and Galileo was a half century +of great astronomical progress along many +lines laid out by the work of the great masters. The +telescope seemed only a toy, but its improvement in +size and quality showed almost inconceivable possibilities +of celestial discoveries.</p> + +<p>Hevelius of Danzig took up the study of the moon, +and his "Selenographia" was finely illustrated by +plates which he not only drew but engraved himself. +Lunar names of mountains, plains, and craters we +owe very largely to him. Also he published among +other works two on comets, the second of which was +published in 1668 and called the "Cometographia," +the first detailed account of all the comets observed +and recorded to date.</p> + +<p>Many were the telescopes turned on the planet +Saturn, and every variety of guess was made as to +the actual shape and physical nature of the weird +appendages discovered by Galileo. The true solution +was finally reached by Huygens, whose mechanical +genius had enabled him to grind and polish larger +and better lenses than his contemporaries; in 1659 +he published the "Systema Saturnium" interpreting +the ring and the cause of its various configurations, +and the first discovery of a Saturnian satellite is +due to him.</p> + +<p>Gascoigne in England about 1640 was the first +to make the important application of the micrometer +<span class="pagenum"><a name="Page_p058" id="Page_p058">[58]</a></span> +to enhance the accuracy of measurement of +small angles in the telescopic field; an invention +made and applied independently many years later +by Huygens in Holland and Auzout and Picard in +France, where the instrument was first regularly +employed as an accessory in the work of an +observatory.</p> + +<p>Another Englishman, Jeremiah Horrocks, was +the first observer of a transit of Venus over the +disk of the sun, in 1639. Horrocks was possessed +of great ability in calculational astronomy also. +This was about the time of the invention of the +pendulum clock by Huygens, which in conjunction +with the later invention of the transit instrument +by Roemer wrought a revolution in the exacting +art of practical astronomy. This was because it +enabled the time to be carried along continuously, +and the revolution of the earth could be utilized +in making precise measures of the position of +sun, moon, and stars. Louis XIV had just founded +the new Observatory at Paris in 1668, and Picard +was the first to establish regular time-observations +there.</p> + +<p>Huygens followed up the motion of the pendulum +in theory as well as practice in his "Horologium +Oscillatorium" (1673), showing the way to measure +the force of gravity, and his study of circular +motion showed the fundamental necessity of some +force directed toward the center in planetary +motions.</p> + +<p>The doctrine of the sphericity of the earth being +no longer in doubt, the great advance in accuracy of +astronomical observation indicated to Willebrord +Snell in Holland the best way to measure an arc +of meridian by triangulation. Picard repeated the +<span class="pagenum"><a name="Page_p059" id="Page_p059">[59]</a></span> +measurements near Paris with even greater accuracy, +and his results were of the utmost significance +to Newton in establishing his law of +gravitation.</p> + +<p>Domenico Cassini, an industrious observer, +voluminous writer, and a strong personality, devised +telescopes of great size, discovered four Saturnian +satellites and the main division in the ring of +Saturn, determined the rotation periods of Mars +and Jupiter, and prepared tables of the eclipses of +Jupiter's satellites. At his suggestion Richer undertook +an expedition to Cayenne in latitude 5 degrees +north, where it was found that the intensity of +gravity was less than at Paris, and his clock therefore +lost time, thus indicating that the earth was +not a perfect sphere as had been thought, but a +spheroid instead.</p> + +<p>The planet Mars passed a near opposition, and +Richer's observations of it from Cayenne, when +combined with those of Cassini and others in +France, gave a new value of the sun's parallax and +distance, really the first actual measurement worth +the name in the history of astronomy.</p> + +<p>To close this era of signal advance in astronomy +we may cite a discovery by Roemer of the first +order: no less than that of the velocity of transmission +of light through space. At the instigation of +Picard, Roemer in studying the motions of Jupiter's +satellites found that the intervals between eclipses +grew less and less as Jupiter and the earth approached +each other, and greater and greater than +the average as the two planets separated farther +and farther. Roemer correctly attributed this difference +to the progressive motion of light and a +rough value of its velocity was calculated, though +<span class="pagenum"><a name="Page_p060" id="Page_p060">[60]</a></span> +not accepted by astronomers generally for more +than a century.</p> + +<p>Why the laws of Kepler should be true, Kepler +himself was unable to say. Nor could anyone else +in that day answer these questions: (1) The planets +move in orbits that are elliptical not circular—why +should they move in an imperfect curve, rather than +the perfect one in which it had always been taught +that they moved? (2) Why should our planet vary +its velocity at all, and travel now fast, now slow; +especially why should the speed so vary that the +line of varying length, joining the planet to the sun, +always passes over areas proportional to the time +of describing them? And (3) Why should there be +any definite relation of the distances of planets from +the sun to their times of revolution about him? +Why should it be exactly as the cube of one to the +square of the other?</p> + +<p>We must remember that the Copernican system +itself was not yet, in the beginning of the +seventeenth century, accepted universally; and +the great minds of that period were most concerned +in overturning the erroneous theory of +Ptolemy.</p> + +<p>The next step in logical order was to find a basic +explanation of the planetary motions, and Descartes +and his theory of vortices are worthy of mention, +among many unsuccessful attempts in this +direction. Descartes was a brilliant French philosopher +and mathematician, but his hypothesis of a +multitude of whirlpools in the ether, while ingenious +in theory, was too vague and indefinite to account for +the planetary motions with any approach to the +precision with which the laws of Kepler represented +them.</p><p><span class="pagenum"><a name="Page_p061" id="Page_p061">[61]</a></span></p> + +<p>Another great astronomer whose labors helped +immensely in preparing the way for the signal discoveries +that were soon to come was Huygens, a +man of versatility as natural philosopher, mechanician, +and astronomical observer. Huygens was born +thirteen years before the death of Galileo, and to +the discovery of the laws of motion by the latter +Huygens added researches on the laws of action of +centrifugal forces. Neither of them, however, appeared +to see the immediate bearing on the great +general problem of celestial motions in its true light, +and it was reserved for another generation, and an +astronomer of another country, to make the one +fundamental discovery that should explain the +whole by a single simple law.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p062" id="Page_p062">[62]</a></span></p> + +<h2><a name="CHAPTER_XIII" id="CHAPTER_XIII"></a>CHAPTER XIII<br /> +<br /> +NEWTON AND MOTION</h2> + +<p>"How is it that you are able to make these great +discoveries?" was once asked of Sir Isaac Newton, +<i>facile princeps</i> of all philosophers, and the discoverer +of the great law of universal gravitation.</p> + +<p>"By perpetually thinking about them," was Newton's +terse and illuminating reply. He had set for +himself the definite problem of Kepler's laws: why +is it that they are true, and is there not some single, +general law that will embody all the circumstances +of the planetary motions?</p> + +<p>Newton was born in 1643, the year after the +death of Galileo. He had a thorough training in +the mathematics of his day, and addressed himself +first to an investigation and definite formulation of +the general laws of motion, which he found to be +three in number, and which he was able to put in +very simple terms. The first one is: Any body, +once it is set in motion, will continue to move forward +in a straight line with a uniform velocity forever, +provided it is acted upon by no force whatever. +In other words, a state of motion is as natural +as a state of rest (rest in relation to things everywhere +adjacent) in which we find all things in +general.</p> + +<p>Here on earth where gravity itself pulls all objects +downward toward the earth, and where resistance +of the air tends to hold a moving body +<span class="pagenum"><a name="Page_p063" id="Page_p063">[63]</a></span> +back and bring it to rest, and where friction from +contact with whatever material substance may be +in its path is perpetually tending to neutralize all +motion—with all three of these forces always at +work to stop a moving body, the truth of this first +and fundamental law of motion was not apparent on +the surface.</p> + +<p>Till Galileo's time everyone had made the mistake +of supposing that some force or other must +be acting continually on every moving body to keep +it in motion. Ptolemy, Copernicus, Kepler, Leonardo +da Vinci—all failed to see the truth of this +law which Newton developed in the immortal +<i>Principia</i>. And at the present day it is not always +easy to accept at first, although the progress of +mechanical science, by reducing friction and resistance, +has produced machines in which motion of +large masses may be kept up indefinitely with the +application of only the merest minimum of force.</p> + +<p>Once a planet is set in motion round the sun, +it would go on forever through frictionless, non-resistant +space; but there must be a central force, +as Huygens saw clearly, to hold it in its orbit. +Otherwise it would at any moment take the direction +of a tangent to the orbit. Here is where Newton's +second law of motion comes in, and he formulated +it with great definiteness. When any force +acts on a moving body, its deviation from a straight +line will be in the direction of the force applied and +proportional to that force.</p> + +<p>In accord with this law, Newton first began to +inquire whether the force of attraction here on +earth, which everyone commonly recognizes as +gravity, drawing all things down toward the center +of the earth, might not extend upward indefinitely. +<span class="pagenum"><a name="Page_p064" id="Page_p064">[64]</a></span> +It is found in operation on the summits of mountain +peaks, and the clouds above them and the rain +falling from them are obviously drawn downward +by the same force. May it not extend outward into +space, even as far as the moon?</p> + +<p>This was an audacious question, but Newton not +only asked, but tried to answer it in the year 1665, +when he was only twenty-three. On the surface of +the earth this attraction is strong enough to draw +a falling body downward through a vertical space +of sixteen feet in a second of time. What ought it +to be at the distance of the moon. The distance of +the moon in Newton's time was better known in +terms of the earth's size than was the size of the +earth itself: the earth's radius was known to be one-sixtieth +of the moon's distance, but the earth's +diameter was thought to be something under 7,000 +miles, so that Newton's first calculations were most +disappointing, and he laid them aside for nearly +twenty years.</p> + +<p>Meanwhile the French astronomers led by Picard +had measured the earth anew, and showed it to be +nearly 8,000 miles in diameter. As soon as Newton +learned of this, he revised his calculations, and +found that by the law of the inverse square the +moon, in one second, should fall away from a tangent +to its orbit one thirty-six hundredth of sixteen +feet.</p> + +<p>This accorded exactly with his original supposition +that the earth's attraction extended to the +moon. So he concluded that the force which makes a +stone fall, or an apple, as the story goes, is the same +force that holds the moon in its orbit, and that this +force diminishes in the exact proportion that the +square of the distance from the earth's center increases. +The moon, indeed, becomes a falling body; +only, as Kingdon Clifford puts it: "She is going so +fast and is so far off that she falls quite around to +the other side of the earth, instead of hitting it; +and so goes on forever."</p><p><span class="pagenum"><a name="Page_p064p1" id="Page_p064p1">[064i]</a></span></p> + +<div class="fig_center" style="width: 394px;"> +<img src="images/p064_1lft.png" width="394" height="454" alt="" /> +<span class="fig_caption">NICHOLAS COPERNICUS</span> +</div> + +<div class="fig_center" style="width: 352px;"> +<img src="images/p064_1rgt.png" width="352" height="455" alt="" /> +<span class="fig_caption">GALILEO GALILEI</span> +</div> + +<p><span class="pagenum"><a name="Page_p064p2" id="Page_p064p2">[53i]</a></span></p> + +<div class="fig_center" style="width: 407px;"> +<img src="images/p064_2lft.png" width="407" height="456" alt="" /> +<span class="fig_caption">JOHANN KEPLER</span> +</div> + +<div class="fig_center" style="width: 415px;"> +<img src="images/p064_2rgt.png" width="415" height="468" alt="" /> +<span class="fig_caption">SIR ISAAC NEWTON</span> +</div> + +<p><span class="pagenum"><a name="Page_p065" id="Page_p065">[65]</a></span></p> + +<p>Newton goes on in the <i>Principia</i> to explain the +extension of gravitation to the other bodies of the +solar system beyond the earth and moon. Clearly +the same gravitation that holds the moon in its +orbit round the earth, must extend outward from +the sun also, and hold all the planets in their orbits +centered about him. Newton demonstrates by calculation +based on Kepler's third law that (1) the +forces drawing the planets toward the sun are inversely +as the squares of their mean distances from +him; and (2) if the force be constantly directed +toward the sun, the radius vector in an elliptic orbit +must pass over equal areas in equal times.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p066" id="Page_p066">[66]</a></span></p> + +<h2><a name="CHAPTER_XIV" id="CHAPTER_XIV"></a>CHAPTER XIV<br /> +<br /> +NEWTON AND GRAVITATION</h2> + +<p>So all of Kepler's laws could be embodied in a +single law of gravitation toward a central body, +whose force of attraction decreases outward in +exact proportion as the square of the distance increases.</p> + +<p>Only one farther step had to be taken, and this +the most complicated of all: he must make all the +bodies of the sky conform to his third law of motion. +This is: Action and reaction are equal, or the +mutual actions of any two bodies are always equal +and oppositely directed. There must be mutual attractions +everywhere: earth for sun as well as sun +for earth, moon for sun and sun for moon, earth for +Venus and Venus for earth, Jupiter for Saturn and +Saturn for Jupiter, and so on.</p> + +<p>The motions of the planets in the undisturbed +ellipses of Kepler must be impossible. As observations +of the planets became more accurate, it was +found that they really did fail to move in exact +accord with Kepler's laws unmodified. Newton +was unable, with the imperfect processes of the +mathematics of his day to ascertain whether the deviations +then known could be accounted for by his +law of gravitation; but he nevertheless formulated +the law with entire precision, as follows:</p> + +<p>Every particle of matter in the universe attracts +every other particle with a force exactly proportioned +<span class="pagenum"><a name="Page_p067" id="Page_p067">[67]</a></span> +to the product of their masses, and inversely +as the square of the distance between their centers.</p> + +<p>The centuries of astronomical research since +Newton's day, however, have verified the great law +with the utmost exactness. Practically every irregularity +of lunar and planetary motion is accounted +for; indeed, the intricacies of the problems +involved, and the nicety of their solution, have led +to the invention of new mathematical processes adequate +to the difficulties encountered.</p> + +<p>And about the middle of the last century, when +Uranus departed from the path laid out for it by +the mathematical astronomers, its orbital deviations +were made the basis of an investigation which +soon led to the assignment of the position where a +great planet could be found that would account for +the unexplained irregularities of the motion of +Uranus. And the immediate discovery of this +planet, Neptune, became the most striking verification +of the Newtonian law that the solar system +could possibly afford.</p> + +<p>The astronomers of still later days investigating +the statelier motions of stellar systems find the +Newtonian law regnant everywhere among the +stars where our most powerful telescopes have as +yet reached. So that Newton's law is known as the +law of Universal Gravitation, and its author is +everywhere held as the greatest scientist of the +ages.</p> + +<p>Newton's <i>Principia</i> may be regarded as the culminating +research of the inductive method, and +further outline of its contents is desirable. It is +divided into three books following certain introductory +sections. The first book treats of the problems +of moving bodies, the solutions being worked +<span class="pagenum"><a name="Page_p068" id="Page_p068">[68]</a></span> +out generally and not with special reference to +astronomy. The second book deals with the motion +of bodies through resistant media, as fluids, and +has very little significance in astronomy. The third +book is the all important one, and applies his general +principles to the case of the actual solar system, +providing a full explanation of the motions of all +the bodies of the system known in his day. Anyone +who critically reads the <i>Principia</i> of Newton +will be forced to conclude that its author was a +genius in the highest sense of the word. The elegance +and thoroughness of the demonstrations, and +the completeness of application of the law of gravitation +are especially impressive.</p> + +<p>The universality of his new law was the feature +to which he gave particular attention. It was clear +to him that the gravitation of a planet, although +it acted as if wholly concentrated at the center, +was nevertheless resident in every one of the particles +of which the planet is composed. Indeed, his +universal law was so formulated as to make every +particle attract every other particle; and an investigation +known as the Cavendish experiment—a research +of great delicacy of manipulation—not only +proves this, but leads also to a measurement of +the earth's mean density, from which we can calculate +approximately how much the earth actually +weighs.</p> + +<p>Another way to attack the same problem is by +measuring the attraction of mountains, as Maskelyne, +Astronomer Royal of Scotland did on Mount +Schehallien in Scotland, which was selected because +of its sheer isolation. The attraction of the mountain +deflected the plumb-lines by measurable +amounts, the volume of the mountain was carefully +<span class="pagenum"><a name="Page_p069" id="Page_p069">[69]</a></span> +ascertained by surveys, and geologists found +out what rocks composed it. So the weight of the +entire mountain became pretty well known, and +combining this with the observed deflection, an independent +value of the earth's weight was found.</p> + +<p>Still other methods have been applied to this +question, and as an average it is found that the +materials composing the earth are about five and +a half times as heavy as water, and the total +weight of the earth is something like six sextillions +of tons.</p> + +<p>What is the true shape of the earth? And does +the earth's turning round on its axis affect this +shape? Newton saw the answer to these questions +in his law of gravitation. A spherical figure followed +as a matter of course from the mutual +attraction of all materials composing the earth, providing +it was at rest, or did not turn round on its +axis. But rotation bulges it at the equator and +draws it in at the poles, by an amount which +calculation shows to be in exact agreement with +the amount ascertained by actual measurement of +the earth itself.</p> + +<p>Another curious effect, not at first apparent, was +that all bodies carried from high latitudes toward +the equator would get lighter and lighter, in consequence +of the centrifugal force of rotation. This +was unexpectedly demonstrated by Richer when +the French Academy sent him south to observe +Mars in 1672. His clock had been regulated exactly +in Paris, and he soon found that it lost time when +set up at Cayenne. The amount of loss was found by +observation, and it was exactly equal to the calculated +effect that the reduction of gravity by centrifugal +action should produce.</p> + +<p><span class="pagenum"><a name="Page_p070" id="Page_p070">[70]</a></span> +Also Newton saw that his law of gravitation +would afford an explanation of the rise and fall of +the tides. The water on the side of the earth toward +the moon, being nearer to the moon, would be more +strongly attracted toward it, and therefore raised +in a tide. And the water on the farther side of the +earth away from the moon, being at a greater distance +than the earth itself, the moon would attract +the earth more strongly than this mass of water, +tending therefore to draw the earth away from the +water, and so raising at the same time a high tide +on the side of the earth away from the moon. As +the earth turns round on its axis, therefore, two +tidal waves continually follow each other at intervals +of about twelve hours.</p> + +<p>The sun, too, joins its gravitating force with that +of the moon, raising tides nearly half as high as +those which the moon produces, because the sun's +vaster mass makes up in large part for its much +greater distance. At first and third quarters of +the moon, the sun acts against the moon, and the +difference of their tide-producing forces gives us +"neap tides"; while at new moon and full, sun and +moon act together, and produce the maximum effect +known as "spring tides."</p> + +<p>Newton passed on to explain, by the action of +gravitation also, the precession of the equinoxes, +a phenomenon of the sky discovered by Hipparchus, +who pretty well ascertained its amount, although +no reason for it had ever been assigned. The plane +of the earth's equator extended to the celestial +sphere marks out the celestial equator, and the two +opposite points where it intersects the plane of the +ecliptic, or the earth's path round the sun, are +called the equinoctial points, or simply the equinoxes. +<span class="pagenum"><a name="Page_p071" id="Page_p071">[71]</a></span> +And precession of the equinoxes is the motion +of these points westward or backward, about 50 +seconds each year, so that a complete revolution +round the ecliptic would take place in about 26,000 +years.</p> + +<p>Newton saw clearly how to explain this: it is +simply due to the attraction of the sun's gravitation +upon the protuberant bulge around the earth's +equator, acting in conjunction with the earth's rotation +on its axis, the effect being very similar to that +often seen in a spinning top, or in a gyroscope. The +moon moving near the ecliptic produces a precessional +effect, as also do the planets to a very slight +degree; and the observed value of precession is the +same as that calculated from gravitation, to a high +degree of precision.</p> + +<p>Newton died in 1727, too early to have witnessed +that complete and triumphant verification of his +law which ultimately has accounted for practically +every inequality in the planetary motions caused by +their mutual attractions. The problems involved are +far beyond the complexity of those which the mathematical +astronomer has to deal with, and the mathematicians +of France deserve the highest credit for +improving the processes of their science so that +obstacles which appeared insuperable were one +after another overcome.</p> + +<p>Newton's method of dealing with these problems +was mainly geometric, and the insufficiency of this +method was apparent. Only when the French +mathematicians began to apply the higher methods +of algebra was progress toward the ultimate goal +assured. D'Alembert and Clairaut for a time were +foremost in these researches, but their places were +soon taken by Lagrange, who wrote the "Mécanique +<span class="pagenum"><a name="Page_p072" id="Page_p072">[72]</a></span> +Analytique," and Laplace, whose "Mécanique +Céleste" is the most celebrated work of all. In large +part these works are the basis of the researches of +subsequent mathematical astronomers who, strictly +speaking, cannot as yet be said to have arrived at +a complete and rigorous solution of all the problems +which the mutual attractions of all the bodies of +the solar system have originated.</p> + +<p>It may well be that even the mathematics of the +present day are incompetent to this purpose. When +the brilliant genius of Sir William Hamilton invented +quaternion analysis and showed the marvelous +facility with which it solved the intricate problems +of physics, there was the expectation that its +application to the higher problems of mathematical +astronomy might effect still greater advances; but +nothing in that direction has so far eventuated. +Some astronomers look for the invention of new +functions with numerical tables bearing perhaps +somewhat the relation to present tables of logarithms, +sines, tangents, and so on, that these tables +do to the simple multiplication table of Pythagoras.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p073" id="Page_p073">[73]</a></span></p> + +<h2><a name="CHAPTER_XV" id="CHAPTER_XV"></a>CHAPTER XV<br /> +<br /> +AFTER NEWTON</h2> + +<p>We have said that practically all the motions in +the solar system have been accounted for by the +Newtonian law of gravitation. It will be of interest +to inquire into the instances that lead to +qualification of this absolute statement.</p> + +<p>One relates to the planet Mercury, whose orbit +or path round the sun is the most elliptical of all +the planetary orbits. This will be explained a little +later.</p> + +<p>The moon has given the mathematical astronomers +more trouble than any other of the celestial +bodies, for one reason because it is nearest to us +and very minute deviations in its motion are therefore +detectible. Halley it was who ascertained two +centuries ago that the moon's motion round the +earth was not uniform, but subject to a slight acceleration +which greatly puzzled Lagrange and Laplace, +because they had proved exactly this sort of +thing to be impossible, unless indeed the body in +question should be acted on by some other force +than gravitation. But Laplace finally traced the +cause to the secular or very slow reduction in the +eccentricity of the earth's own orbit. The sun's +action on the moon was indeed progressively changing +from century to century in such manner as to +accelerate the moon's own motion in its orbit round +the earth.</p> + +<p><span class="pagenum"><a name="Page_p074" id="Page_p074">[74]</a></span> +Adams, the eminent English astronomer, revised +the calculations of Laplace, and found the effect in +question only half as great as Laplace had done; +and for years a great mathematical battle was on +between the greatest of astronomical experts in this +field of research. Adams, in conjunction with Delaunay, +the greatest of the French mathematicians +a half century ago, won the battle in so far as the +mathematical calculations were concerned; but the +moon continues to the present day her slight and +perplexing deviation, as if perhaps our standard +time-keeper, the earth, by its rotation round its axis, +were itself subject to variation. Although many investigations +have been made of the uniformity of +the earth's rotation, no such irregularity has been +detected, and this unexplained variation of the +moon's motion is one of the unsolved problems of +the gravitational astronomer of to-day.</p> + +<p>But we are passing over the most impressive of +all the earlier researches of Lagrange and Laplace, +which concerned the exceedingly slow changes, +technically called the secular variations of the elements +of the planetary orbits. These elements are +geometrical relations which indicate the form of +the orbit, the size of the orbit, and its position +in space; and it was found that none of these relations +or quantities are constant in amount or +direction, but that all, with but one exception, +are subject to very slow, or secular, change, +or oscillation.</p> + +<p>This question assumed an alarming significance +at an early day, particularly as it affected the eccentricity +of the earth's orbit round the sun. Should +it be possible for this element to go on increasing +for indefinite ages, clearly the earth's orbit would +<span class="pagenum"><a name="Page_p075" id="Page_p075">[75]</a></span> +become more and more elliptical, and the sun would +come nearer and nearer at perihelion, and the earth +would drift farther and farther from the sun at +aphelion, until the extremes of temperature would +bring all forms of life on the earth to an end. The +refined and powerful analysis of Lagrange, however, +soon allayed the fears of humanity by accounting +for these slow progressive changes as merely +part of the regular system of mere oscillations, in +entire accord with the operation of the law of gravitation; +and extending throughout the entire planetary +system. Indeed, the periods of these oscillations +were so vast that none of them were shorter +than 50,000 years, while they ranged up to two +million years in length—"great clocks of eternity +which beat ages as ours beat seconds."</p> + +<p>About a century ago, an eminent lecturer on +astronomy told his audience that the problem of +weighing the planets might readily be one that +would seem wholly impossible to solve. To measure +their sizes and distances might well be done, but +actually to ascertain how many tons they weigh—never!</p> + +<p>Yet if a planet is fortunate enough to have one +satellite or more, the astronomer's method of weighing +the planet is exceedingly simple; and all the +major planets have satellites except the two interior +ones, Mercury and Venus. As the satellite +travels round its primary, just as the moon does +round the earth, two elements of its orbit need to +be ascertained, and only two. First, the mean distance +of the satellite from its primary, and second +the time of revolution round it.</p> + +<p>Now it is simply a case of applying Kepler's +third law. First take the cube of the satellite's distance +<span class="pagenum"><a name="Page_p076" id="Page_p076">[76]</a></span> +and divide it by the square of the time of +revolution. Similarly take the cube of the planet's +distance from the sun and divide by the square of +the planet's time of revolution round him. The +proportion, then, of the first quotient to the second +shows the relation of the mass (that is the weight) +of the planet to that of the sun. In the case of +Jupiter, we should find it to be 1,050, in that of +Saturn 3,500, and so on.</p> + +<p>The range of planetary masses, in fact, is very +curious, and is doubtless of much significance in +the cosmogony, with which we deal later. If we +consider the sun and his eight planets, the mass or +weight of each of the nine bodies far exceeds the +combined mass of all the others which are lighter +than itself.</p> + +<p>To illustrate: suppose we take as our unit of +weight the one-billionth part of the sun's weight; +then the planets in the order of their masses will be +Mercury, Mars, Venus, Earth, Uranus, Neptune, +Saturn, and Jupiter. According to their relative +masses, then, Mercury being a five-millionth part +the weight of the sun will be represented by 200; +similarly Venus, a four hundred and twenty-five +thousandth part by 2,350, and so on. Then we have</p> + +<table summary="Planetary Masses"> +<tr> + <td class="text_lf">Mercury</td> + <td> </td> + <td class="text_rt">200</td> +</tr> +<tr> + <td class="text_lf">Mars</td> + <td> </td> + <td class="text_rt brdbt">340</td> +</tr> +<tr> + <td class="text_lf"> Sum of weights of Mercury and Mars</td> + <td> </td> + <td class="text_rt">540</td> +</tr> +<tr> + <td class="text_lf">Venus</td> + <td> </td> + <td class="text_rt brdbt">2,350</td> +</tr> +<tr> + <td class="text_lf"> Sum of weights of Mercury, Mars, and Venus</td> + <td> </td> + <td class="text_rt">2,890</td> +</tr> +<tr> + <td class="text_lf">The Earth</td> + <td> </td> + <td class="text_rt brdbt">3,060</td> +</tr> +<tr> + <td class="text_lf"> Sum of weights of four inner planets</td> + <td> + <span class="pagenum"><a name="Page_p077" id="Page_p077">[77]</a></span></td> + <td class="text_rt">5,950</td> +</tr> +<tr> + <td class="text_lf">Uranus</td> + <td> </td> + <td class="text_rt brdbt">44,250</td> +</tr> +<tr> + <td class="text_lf"> Sum of weights of five planets</td> + <td> </td> + <td class="text_rt">50,200</td> +</tr> +<tr> + <td class="text_lf">Neptune</td> + <td> </td> + <td class="text_rt brdbt">51,600</td> +</tr> +<tr> + <td class="text_lf"> Sum of weights of six planets</td> + <td> </td> + <td class="text_rt">101,800</td> +</tr> +<tr> + <td class="text_lf">Saturn</td> + <td> </td> + <td class="text_rt brdbt">285,580</td> +</tr> +<tr> + <td class="text_lf"> Sum of weights of seven planets</td> + <td> </td> + <td class="text_rt">387,380</td> +</tr> +<tr> + <td class="text_lf">Jupiter</td> + <td> </td> + <td class="text_rt brdbt">954,300</td> +</tr> +<tr> + <td class="text_lf"> Sum of weights of all the planets</td> + <td> </td> + <td class="text_rt">1,341,680</td> +</tr> +<tr> + <td class="text_lf">Mass or weight of the sun</td> + <td class="text_rt" colspan="2">1,000,000,000</td> +</tr> +</table> + +<p>Curious and interesting it is that Saturn is +nearly three times as heavy as the six lighter +planets taken together, Jupiter between two and +three times heavier than all the other planets combined, +while the sun's mass is 750 times that of all +the great planets of his system rolled into one.</p> + +<p>All the foregoing masses, except those of Mercury +and Venus, are pretty accurately known because +they were found by the satellite method just +indicated. Mercury's mass is found by its disturbing +effects on Encke's comet whenever it approaches +very near. The mass of Venus is ascertained by +the perturbations in the orbital motion of the +earth. In such cases the Newtonian law of gravitation +forms the basis of the intricate and tedious +calculations necessary to find out the mass by this +indirect method.</p> + +<p>Its inferiority to the satellite method was strikingly +shown at the Observatory in Washington soon +after the satellites of Mars were discovered in 1877. +The inaccurate mass of that planet, as previously +known by months of computation based upon years +and years of observation, was immediately discarded +<span class="pagenum"><a name="Page_p078" id="Page_p078">[78]</a></span> +in favor of the new mass derived from the +distance and period of the outer satellite by only +a few minutes' calculation.</p> + +<p>In weighing the planets, astronomers always use +the sun as the unit. What then is the sun's own +weight? Obviously the law of gravitation answers +this question, if we compare the sun's attraction +with the earth's at equal distances. First we conceive +of the sun's mass as if all compressed into +a globe the size of the earth, and calculate how far +a body at the surface of this globe would fall in one +second. The relation of this number to 16.1 feet, the +distance a body falls in one second on the actual +earth, is about 330,000, which is therefore the number +of times the sun's weight exceeds that of the +earth.</p> + +<p>A word may be added regarding the force of +gravitation and what it really is. As a matter of +fact Newton did not concern himself in the least +with this inquiry, and says so very definitely. What +he did was to discover the law according to which +gravitation acts everywhere throughout the solar +system. And although many physicists have endeavored +to find out what gravitation really is, its +cause is not yet known. In some manner as yet +mysterious it acts instantaneously over distances +great and small alike, and no substance has been +found which, if we interpose it between two bodies, +has in any degree the effect of interrupting their +gravitational tendency toward each other.</p> + +<p>While the Newtonian law of gravitation has been +accepted as true because it explained and accounted +for all the motions of the heavenly bodies, even including +such motions of the stars as have been subjected +to observation, astronomers have for a long +<span class="pagenum"><a name="Page_p079" id="Page_p079">[79]</a></span> +time recognized that quite possibly the law might +not be absolutely exact in a mathematical sense, +and that deviations from it would surely make their +appearance in time.</p> + +<p>A crude instance of this was suggested about a +century ago, when the planet Uranus was found to +be deviating from the path marked out for it by +Bouvard's tables based on the Newtonian law; and +the theory was advocated by many astronomers +that this law, while operant at the medium distances +from the sun where the planets within +Jupiter and Saturn travel, could not be expected to +hold absolutely true at the vast distance of Uranus +and beyond. The discovery of Neptune in 1846, +however, put an end to all such speculation, and has +universally been regarded as an extraordinary +verification of the law, as indeed it is.</p> + +<p>When, however, Le Verrier investigated the orbit +of Mercury he found an excess of motion in the +perihelion point of the planet's orbit which neither +he nor subsequent investigators have been able to +account for by Newtonian gravitation, pure and +simple. If Newton's theory is absolutely true, the +excess motion of Mercury's perihelion remains a +mystery.</p> + +<p>Only one theory has been advanced to account for +this discrepancy, and that is the Einstein theory +of gravitation. This ingenious speculation was first +propounded in comprehensive form nearly fifteen +years ago, and its author has developed from it +mathematical formulæ which appear to yield results +even more precise than those based on the Newtonian +theory.</p> + +<p>In expressing the difference between the law of +gravitation and his own conception, Einstein says: +<span class="pagenum"><a name="Page_p080" id="Page_p080">[80]</a></span> +"Imagine the earth removed, and in its place suspended +a box as big as a moon or a whole house and +inside a man naturally floating in the center, there +being no force whatever pulling him. Imagine, +further, this box being, by a rope or other contrivance, +suddenly jerked to one side, which is +scientifically termed 'difform motion,' as opposed +to 'uniform motion.' The person would then naturally +reach bottom on the opposite side. The result +would consequently be the same as if he +obeyed Newton's law of gravitation, while, in fact, +there is no gravitation exerted whatever, which +proves that difform motion will in every case produce +the same effects as gravitation…. The term +relativity refers to time and space. According to +Galileo and Newton, time and space were absolute +entities, and the moving systems of the universe +were dependent on this absolute time and space. On +this conception was built the science of mechanics. +The resulting formulas sufficed for all motions of +a slow nature; it was found, however, that they +would not conform to the rapid motions apparent +in electrodynamics…. Briefly the theory of special +relativity discards absolute time and space, and +makes them in every instance relative to moving +systems. By this theory all phenomena in electrodynamics, +as well as mechanics, hitherto irreducible +by the old formulæ, were satisfactorily explained."</p> + +<p>Natural phenomena, then, involving gravitation +and inertia, as in the planetary motions, and +electro-magnetic phenomena, including the motion +of light, are to be regarded as interrelated, and not +independent of one another. And the Einstein +theory would appear to have received a striking +verification in both these fields. On this theory the +<span class="pagenum"><a name="Page_p081" id="Page_p081">[81]</a></span> +Newtonian dynamics fails when the velocities concerned +are a near approach to that of light. The +Newtonian theory, then, is not to be considered as +wrong, but in the light of a first approximation. +Applying the new theory to the case of the motion +of Mercury's perihelion, it is found to account for +the excess quite exactly.</p> + +<p>On the electro-magnetic side, including also the +motion of light, a total eclipse of the sun affords +an especially favorable occasion for applying the +critical test, whether a huge mass like the sun would +or would not deflect toward itself the rays of light +from stars passing close to the edge of its disk, or +limb. A total eclipse of exceptional duration occurred +on May 29, 1919, and the two eclipse parties +sent out by the Royal Society of London and the +Royal Astronomical Society were equipped especially +with apparatus for making this test. Their +stations were one on the east coast of Brazil and the +other on the west coast of Africa.</p> + +<p>Accurate calculation beforehand showed just +where the sun would be among the stars at the time +of the eclipse; so that star plates of this region were +taken in England before the expeditions went out. +Then, during the total eclipse, the same regions were +photographed with the eclipsed sun and the corona +projected against them. To make doubly sure, the +stars were a third time photographed some weeks +after the eclipse, when the sun had moved away +from that particular region.</p> + +<p>Measuring up the three sets of plates, it was +found that an appreciable deflection of the light of +the stars nearest alongside the sun actually exists; +and the amount of it is such as to afford a fair +though not absolutely exact verification of the +<span class="pagenum"><a name="Page_p082" id="Page_p082">[82]</a></span> +theory. The observed deflection may of course be +due to other causes, but the English astronomers +generally regard the near verification as a triumph +for the Einstein theory. Astronomers are already +beginning preparations for a repetition of the +eclipse programme with all possible refinement of +observation, when the next total eclipse of the sun +occurs, September 20, 1922, visible in Australia and +the islands of the Indian Ocean.</p> + +<p>A third test of the theory is perhaps more critical +than either of the others, and this necessitates a +displacement of spectral lines in a gravitational +field toward the red end of the spectrum; but the +experts who have so far made measures for detecting +such displacement disagree as to its actual +existence. The work of St. John at Mt. Wilson is +unfavorable to the theory, as is that of Evershed +of Kodiakanal, who has made repeated tests on the +spectrum of Venus, as well as in the cyanogen +bands of the sun.</p> + +<p>The enthusiastic advocates of the Einstein theory +hold that, as Newton proved the three laws of +Kepler to be special cases of his general law, so +the "universal relativity theory" will enable eventually +the Newtonian law to be deduced from the Einstein +theory. "This is the way we go on in science, +as in everything else," wrote Sir George Airy, +Astronomer Royal; "we have to make out that something +is true; then we find out under certain circumstances +that it is not quite true; and then we +have to consider and find out how the departure can +be explained." Meanwhile, the prudent person +keeps the open mind.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p083" id="Page_p083">[83]</a></span></p> + +<h2><a name="CHAPTER_XVI" id="CHAPTER_XVI"></a>CHAPTER XVI<br /> +<br /> +HALLEY AND HIS COMET</h2> + +<p>Halley is one of the most picturesque characters +in all astronomical history. Next to Newton +himself he was most intimately concerned in giving +the Newtonian law to the world.</p> + +<p>Edmund Halley was born (1656) in stirring +times. Charles I. had just been executed, and it +was the era of Cromwell's Lord Protectorate and +the wars with Spain and Holland. Then followed +(1660) the promising but profligate Charles II. +(who nevertheless founded at Greenwich the greatest +of all observatories when Halley was nineteen), +the frightful ravages of the Black Plague, the +tyrannies of James II., and the Revolution of 1688—all +in the early manhood of Halley, whose scientific +life and works marched with much of the vigor +of the contending personalities of state.</p> + +<p>The telescope had been invented a half century +earlier, and Galileo's discoveries of Jupiter's moons +and the phases of Venus had firmly established the +sun-centered theory of Copernicus.</p> + +<p>The sun's distance, though, was known but +crudely; and why the stars seemed to have no yearly +orbits of their own corresponding to that of the +earth was a puzzle. Newton was well advanced +toward his supreme discovery of the law of universal +gravitation; and the authority of Kepler +taught that comets travel helter-skelter through +<span class="pagenum"><a name="Page_p084" id="Page_p084">[84]</a></span> +space in straight lines past the earth, a perpetual +menace to humanity.</p> + +<p>"Ugly monsters," that comets always were to the +ancient world, the medieval church perpetuated this +misconception so vigorously that even now these +harmless, gauzy visitors from interstellar space +possess a certain "wizard hold upon our imagination." +This entertaining phase of the subject is +excellently treated in President Andrew D. White's +"History of the Doctrine of Comets," in the Papers +of the American Historical Association. Halley's +brilliant comet at its earlier apparitions had been +no exception.</p> + +<p>Halley's father was a wealthy London soap +maker, who took great pride in the growing intellectuality +of his son. Graduating at Queen's College, +Oxford, the latter began his astronomical labors at +twenty by publishing a work on planetary orbits; +and the next year he voyaged to St. Helena to catalogue +the stars of the southern firmament, to +measure the force of terrestrial gravity, and observe +a transit of Mercury over the disk of the sun.</p> + +<p>While clouds seriously interfered with his observations +on that lonely isle, what he saw of the +transit led to his invention of "Halley's method," +which, as applied to the transit of Venus, though +not till long after his death, helped greatly in the +accurate determination of the sun's distance from +the earth. Halley's researches on the proper motions +of the stars of both hemispheres soon made him famous, +and it was said of him, "If any star gets displaced +on the globe, Halley will presently find it out."</p> + +<p>His return to London and election to the Royal +Society (of which he was many years secretary) +added much to his fame, and he was commissioned +<span class="pagenum"><a name="Page_p085" id="Page_p085">[85]</a></span> +by the society to visit Danzig and arbitrate an +astronomical controversy between Hooke and Hevelius, +both his seniors by a generation.</p> + +<p>On the continent he associated with other great +astronomers, especially Cassini, who had already +found three Saturnian moons; and it was then he +observed the great comet of 1680, which led up to +the most famous event of Halley's life.</p> + +<p>The seerlike Seneca may almost be said to have +predicted the advent of Halley, when he wrote +("Quaestiones Naturales," vii): "Some day there +will arise a man who will demonstrate in what region +of the heavens comets pursue their way; why they +travel apart from the planets; and what their sizes +and constitution are. Then posterity will be amazed +that simple things of this sort were not explained +before."</p> + +<p>To Newton it appeared probable that cometary +voyagers through space might have orbits of their +own; and he proved that the comet of 1680 never +swerved from such a path. As it could nowhere +approach within the moon's orbit, clearly threats +of its wrecking the earth and punishing its inhabitants +ought to frighten no more.</p> + +<p>Halley then became intensely interested in +comets, and gathered whatever data concerning the +paths of all these bodies he could find. His first +great discovery was that the comets seen in 1531 by +Apian, and in 1607 by Kepler, traveled round the +sun in identical paths with one he had himself +observed in 1682. A still earlier appearance of +Halley's comet (1456) seems to have given rise to +a popular and long-reiterated myth of a papal bull +excommunicating "the Devil, the Turk, and the +Comet."</p> + +<p><span class="pagenum"><a name="Page_p086" id="Page_p086">[86]</a></span> +No longer room for doubt: so certain was Halley +that all three were one and the same comet, completing +the round of its orbit in about seventy-six +years, that he fearlessly predicted that it would be +seen again in 1758 or 1759. And with equal confidence +he might have foretold its return in 1835 +and 1910; for all three predictions have come true +to the letter.</p> + +<p>Halley's span of existence did not permit his +living to see even the first of these now historic +verifications. But we in our day may emphatically +term the epoch of the third verified return <i>Annus +Halleianus</i>.</p> + +<p>Says Turner, Halley's successor in the Savilian +chair at Oxford to-day: "There can be no more +complete or more sensational proof of a scientific +law, than to predict events by means of it. Halley +was deservedly the first to perform this great service +for Newton's Law of Gravitation, and he would +have rejoiced to think how conspicuous a part England +was to play in the subsequent prediction of +the existence of Neptune."</p> + +<p>Halley rose rapidly among the chief astronomical +figures of his day. But he had little veneration for +mere authority, and the significant veering of his +religious views toward heterodoxy was for years +an obstacle to his advance.</p> + +<p>Still Halley the astronomer was great enough to +question any contemporary dicta that seemed to +rest on authority alone. Everyone called the stars +"fixed" stars; but Halley doubting this, made the +first discovery of a star's individual motion—proper +motion, as astronomers say. To-day, two hundred +years after, every star is considered to be in motion, +and astronomers are ascertaining their real motions +<span class="pagenum"><a name="Page_p087" id="Page_p087">[87]</a></span> +in the celestial spaces to a nicety undreamed of by +even the exacting Halley.</p> + +<p>The moon, of priceless service to the early navigator, +was regarded by all astronomers as endowed +with an average rate of motion round the earth +that did not vary from age to age. But Halley +questioned this too; and on comparing with the +ancient value from Chaldean eclipses, he made another +discovery—the secular acceleration of the +moon's mean motion, as it is technically termed. +This was a colossal discovery in celestial dynamics; +and the reason underlying it lay hidden in Newton's +law for yet another century, till the keener mathematics +of Laplace detected its true origin.</p> + +<hr class="tb" /> + +<p>With Newton, Halley laid down the firm foundations +of celestial mechanics, and they pushed the +science as far as the mathematics of their day +would permit. Halley, however, was not content +with elucidating the motion of bodies nearest the +earth, and pressed to the utmost confines of the +solar system known to him. Here, too, he made a +signal discovery of that mutual disturbance of the +planets in their motion round the sun, called the +great inequality of Jupiter and Saturn.</p> + +<p>Halley's versatile genius attacked all the great +problems of the day. His observation of the sun's +total eclipse in 1715 is the earliest reliable account +of such a phenomenon by a trained astronomer. +He described the corona minutely and was the first +to see that other interesting phenomenon which +only an alert observer can detect, which a great +astronomer of a later day compared to the "ignition +of a fine train of gunpowder," and which has ever +since borne the name of "Baily's beads."</p> + +<p><span class="pagenum"><a name="Page_p088" id="Page_p088">[88]</a></span> +Besides being a great astronomer, Halley was a +man of affairs as well, which Newton, although the +greater mathematician, was not. Without Halley, +Newton's superb discovery might easily have been +lost to the age and nation, for the latter was bent +merely on making discoveries, and on speculative +contemplation of them, with never a thought of +publishing to the world.</p> + +<p>Halley, more practical and businesslike, insisted +on careful writing out and publication. Newton +was then only forty-two, and Halley fully fourteen +years his junior. But the philosophers of that day +were keenly alive to the mystery of Kepler's laws, +and Halley was fully conscious of the grandeur and +far-reaching significance of Newton's great generalization +which embodied all three of Kepler's laws +in one.</p> + +<p>Newton at last yielded, though reluctantly, and +the "Principia" was given to the world, though +wholly at Halley's private charges.</p> + +<p>But Halley was far from being completely engrossed +with the absorbing problems of the sky; +things terrestrial held for years his undivided attention. +Imagine present-day Lords Commissioners +of the Admiralty intrusting a ship of the British +navy to civilian command. Yet such was their confidence +in Halley that he was commissioned as captain +of H. M.'s pink <i>Paramour</i> in 1698, with instructions +to proceed to southern seas for geographical +discoveries, and for improving knowledge of the +longitude problem, and of the variations of the compass. +Trade winds and monsoons, charts of magnetic +variation, tides and surveys of the Channel +coast, and experiments with diving bells were practical +activities that occupied his attention.</p> + +<p><span class="pagenum"><a name="Page_p089" id="Page_p089">[89]</a></span> +Halley in 1720 became Astronomer Royal. He +was the second incumbent of this great office, but +the first to supply the Royal Observatory with instruments +of its own, some of which adorn its walls +even to-day. His long series of lunar observations +and his magnetic researches were of immense +practical value in navigation.</p> + +<p>Halley lived to a ripe old age and left the world +vastly better than he found it. His rise from humblest +obscurity was most remarkable, and he lived +to gratify all the ambitions of his early manhood. +"Of attractive appearance, pleasing manners, and +ready wit," says one of his biographers, "loyal, +generous, and free from self-seeking, he was one +of the most personally engaging men who ever held +the office of Astronomer Royal."</p> + +<p>He died in office at Greenwich in 1742.</p> + +<p>"Halley was buried," says Chambers, "in the +churchyard of St. Margaret's, Lee, not far from +Greenwich, and it has lately been announced that +the Admiralty have decided to repair his tomb at +the public expense, no descendants of his being +known." There is no suitable monument in England +to the memory of one of her greatest scientific men. +In any event the collection and republication of +his epoch-making papers would be welcomed by +astronomers of every nation.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p090" id="Page_p090">[90]</a></span></p> + +<h2><a name="CHAPTER_XVII" id="CHAPTER_XVII"></a>CHAPTER XVII<br /> +<br /> +BRADLEY AND ABERRATION</h2> + +<p>Living at Kew in London early in the 18th century +was an enthusiastic young astronomer, +James Bradley. He is famous chiefly for his accurate +observations of star places which have been invaluable +to astronomers of later epochs in ascertaining +the proper motions of stars.</p> + +<p>The latitude of Bradley's house in Kew was very +nearly the same as the declination of the bright star +Gamma Draconis, so that it passed through his +zenith once every day. Bradley had a zenith sector, +and with this he observed with the greatest care the +zenith distance of Gamma Draconis at every possible +opportunity. This he did by pointing the telescope +on the star and then recording the small angle of its +inclination to a fine plumb line. So accurate were +his measures that he was probably certain of the +star's position to the nearest second of arc.</p> + +<p>What he hoped to find was the star's motion round +a very slight orbit once each year, and due to the +earth's motion in its orbit round the sun. In other +words, he sought to find the star's parallax if it +turned out to be a measurable quantity.</p> + +<p>It is just as well now that his method of observation +proved insufficiently delicate to reveal the parallax +of Gamma Draconis; but his assiduity in observation +led him to an unexpected discovery of greater +moment at that time. What he really found was +<span class="pagenum"><a name="Page_p091" id="Page_p091">[91]</a></span> +that the star had a regular annual orbit; but wholly +different from what he expected, and very much +larger in amount. This result was most puzzling to +Bradley. The law of relative motion would require +that the star's motion in its expected orbit should +be opposite to that of the earth in its annual orbit; +instead of which the star was all the time at right +angles to the earth's motion.</p> + +<p>Bradley was a frequent traveler by boat on the +Thames, and the apparent change in the direction +of the wind when the boat was in motion is said to +have suggested to him what caused the displacement +of Gamma Draconis. The progressive motion of +light had been roughly ascertained by Roemer: let +that be the velocity of the wind. And the earth's +motion in its orbit round the sun, let that be the +speed of the boat. Then as the wind (to an observer +on the moving boat) always seems to come from a +point in advance of the point it actually proceeds +from (to an observer at rest), so the star should be +constantly thrown forward by an angle given by the +relation of the velocity of light to the speed of the +earth in orbital revolution round the sun.</p> + +<p>The apparent places of all stars are affected in +this manner, and this displacement is called the +aberration of light. Astronomers since Bradley's +discovery of aberration in 1726 have devoted a great +deal of attention to this astronomical constant, as it +is called, and the arc value of it is very nearly 20".5. +This means that light travels more than ten thousand +times as fast as the earth in its orbit (186,330 miles +per second as against the earth's 18.5). And we can +ascertain the sun's distance by aberration also because +the exact values of the velocity of light and of +the constant of aberration when properly combined +<span class="pagenum"><a name="Page_p092" id="Page_p092">[92]</a></span> +give the exact orbital speed of the earth; and this +furnishes directly by geometry the radius of the +earth's orbit, that is the distance of the sun.</p> + +<p>In fact, this is one of the more accurate modern +methods of ascertaining the distance of the sun. As +early as 1880 it enabled the writer to calculate the +sun's parallax equal to 8".80, a value absolutely +identical with that adopted by the Paris Conference +of 1896, and now universally accepted as the +standard.</p> + +<p>In whatever part of the sky we observe, every star +is affected by aberration. At the poles of the ecliptic, +23½ degrees from the earth's poles, the annual aberration +orbits of the stars are very small circles, 41" +in diameter. Toward the ecliptic the aberration +orbits become more and more oval, ellipses in fact +of greater and greater eccentricity, but with their +major axes all of the same length, until we reach the +ecliptic itself; and then the ellipse is flattened into +a straight line 41" in length, in which the star travels +forth and back once a year. Exact correspondence +of the aberration ellipses of the stars with the annual +motion of the earth round the sun affords indisputable +proof of this motion, and as every star partakes +of the movement, this proof of our motion +round the sun becomes many million-fold.</p> + +<p>Indeed, if we were to push a little farther the refinement +of our analysis of the effect of aberration +on stellar positions, we could prove also the rotation +of the earth on its axis, because that motion is swift +enough to bear an appreciable ratio to the velocity +of light. Diurnal aberration is the term applied to +this slight effect, and as every star partakes of it, +demonstration of the earth's turning round on its +axis becomes many million-fold also.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p093" id="Page_p093">[93]</a></span></p> + +<h2><a name="CHAPTER_XVIII" id="CHAPTER_XVIII"></a>CHAPTER XVIII<br /> +<br /> +THE TELESCOPE</h2> + +<p>Had anyone told Ptolemy that his earth-centered +system of sun, moon, and stars would ultimately +be overthrown, not by philosophy but by the overwhelming +evidence furnished by a little optical instrument +which so aided the human eye that it could +actually see systems of bodies in revolution round +each other in the sky, he would no doubt have +vehemently denied that any such thing was possible. +To be sure, it took fourteen centuries to bring this +about, and the discovery even then was without +much doubt due to accident.</p> + +<p>Through all this long period when astronomy may +be said to have merely existed, practically without +any forward step or development, its devotees were +unequipped with the sort of instruments which were +requisite to make the advance possible. There were +astrolabes and armillary spheres, with crudely divided +circles, and the excellent work done with them +only shows the genius of many of the early astronomers +who had nothing better to work with. Regarding +star-places made with instruments fixed in +the meridian, Bessel, often called the father of practical +astronomy, used to say that, even if you provided +a bad observer with the best of instruments, +a genius could surpass him with a gun barrel and a +cart wheel.</p> + +<p>Before the days of telescopes, that is, prior to the +seventeenth century, it was not known whether any +<span class="pagenum"><a name="Page_p094" id="Page_p094">[94]</a></span> +of the planets except the earth had a moon or not; +consequently the masses of these planets were but +very imperfectly ascertained; the phases of Mercury +and Venus were merely conjectured; what were the +actual dimensions of the planets could only be +guessed at; the approximate distances of sun, moon, +and planets were little better than guesses; the distances +of the stars were wildly inaccurate; and the +positions of the stars on the celestial sphere, and of +sun, moon, and planets among them were far removed +from modern standards of precision—all because +the telescope had not yet become available as +an optical adjunct to increase the power of the +human eye and enable it to see as if distances were +in considerable measure annihilated.</p> + +<p>Galileo almost universally is said to have been the +inventor of the telescope, but intimate research into +the question would appear to give the honor of that +original invention to another, in another country. +What Galileo deserves the highest praise for, however, +is the reinvention independently of an "optick +tube" by which he could bring distant objects +apparently much nearer to him; and being an astronomer, +he was by universal acknowledgment first +of all men to turn a telescope on the heavenly bodies. +This was in the year 1609, and his first discovery +was the phase of Venus, his second the four Medicean +moons or satellites of Jupiter, discoveries which at +that epoch were of the highest significance in establishing +the truth of the Copernican system beyond +the shadow of doubt.</p> + +<p>But the first telescopes of which we have record +were made, so far as can now be ascertained, in +Holland very early in the 17th century. Metius, a +professor of mathematics, and Jansen and Lipperhey, +<span class="pagenum"><a name="Page_p095" id="Page_p095">[95]</a></span> +who were opticians in Middelburg—all three +are entitled to consideration as claimants of the original +invention of the telescope. But that such an +instrument was pretty well known would appear to +be shown by his government's refusal of a patent to +Lipperhey in 1608; while the officials recognizing the +value of such an instrument for purposes of war, +got him to construct several telescopes and ordered +him to keep the invention a secret.</p> + +<p>Within a year Galileo heard that an instrument +was in use in Holland by which it was possible to +see distant objects as if near at hand. Skilled in +optics as he was, the reinvention was a task neither +long nor difficult for him. One of his first instruments +magnified but three times; still it made a great +sensation in Venice where he exhibited the little +tube to the authorities of that city, in which he first +invented it.</p> + +<p>Galileo's telescope was of the simplest type, with +but two lenses; the one a double convex lens with +which an image of the distant object is formed, the +other a double concave lens, much smaller which +was the eye-lens for examining the image. It is this +simple form of Galilean telescope that is still used +in opera glasses and field glasses, because of the +shorter tube necessary.</p> + +<p>Galileo carried on the construction of telescopes, +all the time improving their quality and enlarging +their power until he built one that magnified thirty +times. What the diameter of the object glass was +we do not know, perhaps two inches or possibly a +little more. Glass of a quality good enough to make +a telescope of cannot have been abundant or even +obtainable except with great difficulty in those early +days.</p> + +<p><span class="pagenum"><a name="Page_p096" id="Page_p096">[96]</a></span> +Other discoveries by this first of celestial observers +were the spots on the sun, the larger mountains +of the moon, the separate stars of which the +Milky Way is composed, and, greatest wonder of all, +the anomalous "handles" (<i>ansæ</i>, he called them) of +Saturn, which we now know as the planet's ring, the +most wonderful of all the bodies in the sky.</p> + +<p>Since Galileo's time, only three centuries past, +the progress in size and improvement in quality +of the telescope have been marvelous. And +this advance would not have been possible except +for, first, the discoveries still kept in large part +secret by the makers of optical glass which have +enabled them to make disks of the largest size; +second, the consummate skill of modern opticians +in fashioning these disks into perfect lenses; and +third, the progress in the mechanical arts and engineering, +by which telescope tubes of many tons' +weight are mounted or poised so delicately that the +thrust of a finger readily swerves them from one +point of the heavens to another.</p> + +<p>As the telescope is the most important of all +astronomical instruments, it is necessary to +understand its construction and adjustment and +how the astronomer uses it. Telescopes are optical +instruments, and nothing but optical parts would be +requisite in making them, if only the optical conditions +of their perfect working could be obtained +without other mechanical accessories.</p> + +<p>In original principle, all telescopes are as simple +as Galileo's; first, an object glass to form the image +of the distant object; second the eyepiece usually +made of two lenses, but really a microscope, to +magnify that image, and working in the same way +that any microscope magnifies an object close at +hand; and third, a tube to hold all the necessary +lenses in the true relative positions.</p> + +<p><span class="pagenum"><a name="Page_p096p1" id="Page_p096p1">[96i]</a></span></p> + +<div class="fig_left" style="width: 425px;"> +<img src="images/p096_1lft.png" width="425" height="569" alt="" /> +<p class="fig_caption center"><span class="smcap">The 100-Inch Hooker Telescope, Largest Reflector +in the World, on Mt. Wilson.</span> (<i>Photo, +Mt. Wilson Solar Observatory.</i>)</p> +</div> + +<div class="fig_right" style="width: 368px;"> +<img src="images/p096_1rgt.png" width="368" height="495" alt="" /> +<span class="fig_caption"><span class="smcap">The Largest Refractor, the 40-Inch Telescope +at Yerkes Observatory. Dome 90 Ft. in Diameter.</span> +(<i>Photo, Yerkes Observatory.</i>)</span> +</div> + +<div style="clear: both;"></div> +<p><span class="pagenum"><a name="Page_p096p2" id="Page_p096p2">[97i]</a></span></p> + +<table style="width: 500px" summary="Mt. Wilson Observatory"> +<tr> +<td rowspan="2"> +<img src="images/p096_2lft.png" width="180" height="633" alt="" title="" /> +</td> +<td> +<img src="images/p096_2tpr.png" width="235" height="475" alt="" title="" /> +</td> +</tr> +<tr> +<td> +<img src="images/p096_2btr.png" width="264" height="305" alt="" title="" /> +</td> +</tr> +<tr><td colspan="2"> +<div class="fig_caption"><span class="smcap">The 150-ft. Tower at the Mt. Wilson Solar Observatory.</span> At the +left is a diagram of tower, telescope and pit. At the upper right is an +exterior view of the tower; below a view looking down into the pit, 75 +ft. deep. (<i>Photo, Mt. Wilson Solar Observatory.</i>)</div> +</td></tr> +</table> + +<p><span class="pagenum"><a name="Page_p097" id="Page_p097">[97]</a></span> +The focal lengths of object glass and eyepiece +will determine just what distance apart the lenses +must be in order to give perfect vision. But it is +quite as important that the axes of all the lenses be +adjusted into one and the same straight line, and +then held there rigidly and permanently. Otherwise +vision with the telescope will be very imperfect +and wholly unsatisfactory. The distance from the +objective, or object glass to its focal point is called +its focal length; and if we divide this by the focal +length of the eyepiece, we shall have the magnifying +power of the telescope. The eyepiece will usually +be made of two lenses, or more, and we use its focal +length considered as a single lens, in getting the +magnifying power. A telescope will generally have +many eyepieces of different focal lengths, so that +it will have a corresponding range of magnifying +powers. The lowest magnifying power will be not +less than four or five diameters for each inch of +aperture of the objective; otherwise the eye will fail +to receive all the light which falls upon the glass. +A 4-inch telescope will therefore have no eyepiece +with a lower magnifying power than about 20 +diameters. The highest magnifying power advantageous +for a glass of this size will be about 250 to +300, the working rule being about 70 diameters to +each inch of aperture, although the theoretical +limit is regarded as 100.</p> + +<p>The reason for a variety of eyepieces with different +magnifying powers soon becomes apparent +on using the telescope. Comets and nebulæ call for +very low powers, while double stars and the planetary +surfaces require the higher powers, provided +<span class="pagenum"><a name="Page_p098" id="Page_p098">[98]</a></span> +the state of the atmosphere at the moment will +allow it. If there is much quivering and unsteadiness, +nothing is gained by trying the higher powers, +because all the waves of unsteadiness are magnified +also in the same proportion, and sharpness of vision, +or fine definition, or "good seeing," as it is called, +becomes impossible. The vibrations and tremors of +the atmosphere are the greatest of all obstacles to +astronomical observation, and the search is always +in order for regions of the world, in deserts or on +high mountains, where the quietest atmosphere is +to be found.</p> + +<p>Quite another power of the telescope is dependent +on its objective solely: its light-gathering power. +Light by which we see a star or planet is admitted +to the retina of the eye through an adjustable aperture +called the pupil. In the dark or at night, the +pupil expands to an average diameter of one-fourth +of an inch. But the object-glass of a telescope, by +focusing the rays from a star, pours into the eye, +almost as a funnel acts with water, all the light +which falls on its larger surface. And as geometry +has settled it for us that areas of surfaces are proportioned +to the squares of their diameters, a two-inch +object glass focuses upon the retina of the eye +64 times as much light as the unassisted eye would +receive. And the great 40-inch objective of the +Yerkes telescope would, theoretically, yield 25,600 +times as much light as the eye alone. But there +would be a noticeable percentage of this lost through +absorption by the glasses of the telescope and +scattering by their surfaces.</p> + +<p>The first makers of telescopes soon encountered +a most discouraging difficulty, because it seemed to +them absolutely insuperable. This is known as +<span class="pagenum"><a name="Page_p099" id="Page_p099">[99]</a></span> +chromatic aberration, or the scattering of light in +a telescope due simply to its color or wave length. +When light passes through a prism, red is refracted +the least and violet the most. Through a lens it is +the same, because a lens may be regarded as an indefinite +system of prisms. The image of a star or +planet, then, formed by a single lens cannot be +optically perfect; instead it will be a confused intermingling +of images of various colors. With low +powers this will not be very troublesome, but great +indistinctness results from the use of high magnifying +powers.</p> + +<p>The early makers and users of telescopes in the +latter part of the seventeenth century found that the +troublesome effects of chromatic aberration could +be much reduced by increasing the focal length of +the objective. This led to what we term engineering +difficulties of a very serious nature, because the +tubes of great length were very awkward in pointing +toward celestial objects, especially near the +zenith, where the air is quietest. And it was next +to impossible to hold an object steadily in the field, +even after all the troubles of getting it there had +been successfully overcome.</p> + +<p>Bianchini and Cassini, Hevelius and Huygens +were among the active observers of that epoch who +built telescopes of extraordinary length, a hundred +feet and upward. One tube is said to have been +built 600 feet in length, but quite certainly it could +never have been used. So-called aerial telescopes +were also constructed, in which the objective was +mounted on top of a tower or a pole, and the eyepiece +moved along near the ground. But it is difficult to +see how anything but fleeting glimpses of the +heavenly bodies could have been obtained with such +<span class="pagenum"><a name="Page_p100" id="Page_p100">[100]</a></span> +contrivances, even if the lenses had been perfect. +Newton indeed, who was expert in optics, gave up +the problem of improving the refracting telescope, +and turned his energies toward the reflector.</p> + +<p>In 1733, half a century after Newton and a century +and a quarter after Galileo, Chester More Hall, +an Englishman, found by experiment that chromatic +aberration could be nearly eliminated by +making the objective of two lenses instead of one, +and the same invention was made independently by +Dollond, an English optician, who took out letters +patent about 1760. So the size of telescopes seemed +to be limited only by the skill of the glassmaker +and the size of disks that he might find it practicable +to produce.</p> + +<p>What Hall and Dollond did was to make the outer +or crown lens of the objective as before, and place +behind it a plano-concave lens of dense flint glass. +This had the effect of neutralizing the chromatic +effect, or color aberration, while at the same time +only part of the refractive effect of the crown lens +was destroyed. This ingenious but costly combination +prepared the way for the great refracting +telescopes of the present day, because it solved, or +seemed to solve, the important problem of getting +the necessary refraction of light rays without harmful +dispersion or decomposition of them.</p> + +<p>Through the 18th century and the first years of +the 19th many telescopes of a size very great for +that day were built, and their success seemed complete. +With large increase in the size of the disks, +however, a new trouble arose, quite inherent in the +glass itself. The two kinds of glass, flint and +crown, do not decompose white light with uniformity, +so that when the so-called achromatic objective +<span class="pagenum"><a name="Page_p101" id="Page_p101">[101]</a></span> +was composed of flint and crown, there was an +effect known as irrationality of dispersion, or +secondary spectrum, which produced a very troublesome +residuum of blue light surrounding the images +of bright objects. This is the most serious defect +of all the great refractors of the day, and effectively +it limits their size to about 60 inches of aperture, +with present types of flint and crown. It is expected +by present experimenters, however, that +further improvements in optical glass will do much +to extend this limit; so that a refracting telescope +of much greater size than any now in existence will +be practicable.</p> + +<p>Improvements in mounting telescopes, too, are +still possible. Within recent years, Hartness, of +Springfield, Vermont, has erected a new and ingenious +type of turret telescope which protects the observer +from wind and cold while his instrument is +outside. It affords exceptional facilities for rapid +and convenient observing, as for variable stars, and +is adaptable to both refractors and reflectors.</p> + +<p>The captivating study of the heavens can of +course be begun with the naked eye alone, but very +moderate optical assistance is a great help and +stimulates. An opera-glass affords such assistance; +a field-glass does still better, and best of all, for +certain purposes, is a modern prism-binocular.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p102" id="Page_p102">[102]</a></span></p> + +<h2><a name="CHAPTER_XIX" id="CHAPTER_XIX"></a>CHAPTER XIX<br /> +<br /> +REFLECTORS—MIRROR TELESCOPES</h2> + +<p>Cherished with the utmost care in the rooms +of the Royal Society of London is a world-famous +telescope, a diminutive reflector made by the +hands of Sir Isaac Newton. We have already mentioned +his connection with the refractor; and how +he abandoned that type of telescope in favor of the +reflecting mirror, or reflector in which the obstacles +to great size appeared to be purely mechanical. By +many, indeed, Newton is regarded as the inventor +of the reflector.</p> + +<p>By the principles of optics, all the rays from a +star that strike a concave mirror will be reflected +to the geometric focal point, provided a section of +that mirror is a parabola. Such a mirror is called +a speculum, and is an alloy of tin, copper, and bismuth. +Its surface takes a very high polish, reflecting +when newly polished nearly 90 per cent of the +light that falls upon it.</p> + +<p>But the focus where the eyepiece must be used is +in front of the mirror, and if the eye were placed +there, the observer's head would intercept all or +much of the light that would otherwise reach the +mirror. Gregory, probably the real inventor of the +reflector, was the first to dodge this difficulty by +perforating the mirror at the center and applying +the eyepiece there, at the back of the speculum; +but it was necessary to first send the rays to that +<span class="pagenum"><a name="Page_p103" id="Page_p103">[103]</a></span> +point by reflection from a second or smaller mirror, +in the optical axis of the speculum. This reflects +the rays backward down the tube to the eyepiece, or +spectroscope, or camera.</p> + +<p>Another English optician, Cassegrain, improved +on this design somewhat by placing the secondary +mirror inside the focus of the speculum, or nearer +to it, so that the tube is shorter. This form is preferable +for many kinds of astronomical work, especially +photography. Herschel sought to do away +with the secondary reflector entirely and save the +loss of light by tilting the speculum slightly, so as +to throw the image at one side of the tube; but this +modification introduces bad definition of the image +and has never been much used.</p> + +<p>A better plan is that of Newton, who placed a +small plane speculum at an angle of 45 degrees in +the optical axis where the secondary mirror of the +Gregory-Cassegrainian type is placed. The rays are +then received by the eyepiece at the side of the upper +end of the tube, the observer looking in at right +angles to the axis. And a modern improvement first +used by Draper is a small rectangular prism in place +of the little plane speculum, effecting a saving of +five to ten per cent of the light.</p> + +<p>It is not easy to say which type of telescope, the +refractor or the reflector, is the more famous. Nor +which is the better or more useful, or the more +likely to lead in the astronomy of the future. When +the successors of Dollond had carried the achromatic +refractor to the limit enforced by the size of +the glass disks they were able to secure, they found +these instruments not so great an improvement +after all. The single-lens telescopes of great focal +length were nearly as good optically, though much +<span class="pagenum"><a name="Page_p104" id="Page_p104">[104]</a></span> +more awkward to handle. But the quality of the +glass obtainable in that day appeared to set an +arbitrary limit to that great amplification of size +and power which progress in observational astronomy +demanded.</p> + +<p>Then came the elder Herschel, best known and +perhaps the greatest of all astronomers. At Bath, +England, music was his profession, especially the +organ. But he was dissatisfied with his little +Gregorian reflector, and being a very clever mechanician +he set out to build a reflector for himself. +It is said that he cast and polished nearly 200 +mirrors, in the course of experiments on the most +highly reflective type of alloys, and the sort of +mechanism that would enable him to give them the +highest polish. In all his work he was ably and +enthusiastically aided by his sister, Caroline Herschel, +most famous of all women astronomers.</p> + +<p>Upward in size of his mirrors he advanced, till +he had a speculum of two feet diameter with a tube +20 feet long. Twelve to fifteen years had elapsed +when in 1781, while testing one of these reflectors +on stars in the constellation Gemini, he made +the first discovery of a planet since the invention +of the telescope—the great planet now known as +Uranus.</p> + +<p>Under the patronage of King George, he advanced +to telescopes of still greater size, his largest being +no less than forty feet in length, with a speculum of +four feet in diameter. Two new satellites of +Saturn were discovered with this giant reflector, +which was dismantled by Sir John Herschel with +appropriate ceremonies, including the singing of an +ode by the Herschel family assembled inside of the +tube, on New Year's Eve, 1839-40.</p> + +<p><span class="pagenum"><a name="Page_p105" id="Page_p105">[105]</a></span> +We have record of but few attempts to improve +the size and definition of great reflectors by the +continental astronomers during this era. In England +and Ireland, however, great progress was +made. About 1860 Lassell built a two-foot reflector, +with which he discovered two new satellites of +Uranus, and which he subsequently set up in the +island of Malta. Ten years later Thomas Grubb +and Son of Dublin constructed a four-foot reflector, +now at the Observatory in Melbourne, Australia. +Calver in conjunction with Common of Ealing, London, +about 1880-95 built several large reflectors, the +largest of five feet diameter, now owned by Harvard +College Observatory; and, rather earlier, Martin of +Paris completed a four-foot reflector.</p> + +<p>The mirrors of these latter instruments were not +made of speculum metal, but of solid glass, which +must be very thick (one-seventh their diameter) in +order to prevent flexure or bending by their own +weight. So sensitive is the optical surface to distortion +that unless a complicated series of levers +and counterpoises is supplied, to support the under +surface of the mirror, the perfection of its optical +figure disappears when the telescope is directed to +objects at different altitudes in the sky. The upper +or outer surface of the glass is the one which receives +the optical polish on a heavy coat of silver +chemically deposited on the polished glass after its +figure has been tested and found satisfactory.</p> + +<p>But far and away the most famous reflecting +telescope of all is the "Leviathan" of Lord Rosse, +built at Birr Castle, Parsonstown, Ireland, about the +middle of the last century. His Lordship made +many ingenious improvements in grinding the +mirror, which was of speculum metal, six feet in +<span class="pagenum"><a name="Page_p106" id="Page_p106">[106]</a></span> +diameter and weighed seven tons. It was ground +to a focal length of fifty-four feet and mounted between +heavy walls of masonry, so that the motion of +the great tube was restricted to a few degrees on +both sides of the meridian. The huge mechanism +was very cumbersome in operation, and photography +was not available in those days; nevertheless +Lord Rosse's telescope made the epochal discovery +of the spiral nebulæ, which no other telescope of +that day could have done.</p> + +<p>In America the reflector has always kept at least +even pace with the refractor. As early as 1830, +Mason and Smith, two students at Yale College, +enthused by Denison Olmsted, built a 12-inch speculum +with which they made unsurpassed observations +of the nebulæ. Dr. Henry Draper, returning from a +visit to Lord Rosse, began about 1865 the construction +of two silver-on-glass reflectors, one of 15 +inches diameter, the other of 28 inches, with which +he did important work for many years in photography +and spectroscopy, and his mirrors are now +the property of Harvard College Observatory. Alvan +Clark and Sons have in later years built a 40-inch +mirror for the Lowell Observatory in Arizona, and +very recently a 6-foot silver-on-glass mirror has +been set up in the Dominion of Canada Astrophysical +Observatory at Victoria, British Columbia, +where it is doing excellent work in the hands of +Plaskett, its designer.</p> + +<p>The huge glass disk for the reflector weighs two +tons, and it must be cast so that there are no internal +strains; otherwise it is liable to burst in +fragments in the process of grinding. It should be +free from air-bubbles, too; so the glass is cast in +one melting, if possible. This disk was made by +<span class="pagenum"><a name="Page_p107" id="Page_p107">[107]</a></span> +the St. Gobain Plate Glass Company, whose works +have been ruthlessly destroyed by the enemy during +the war; but fortunately the great disk had been +shipped from Antwerp only a week before declaration +of hostilities.</p> + +<p>Brashear of Allegheny was intrusted with the +optical parts, which occupied many months of +critical work. The finished mirror is 73 inches in +diameter, its focal length is 30 feet, and its thickness +12 inches. A central hole 10 inches in diameter +makes possible its use as a Gregorian or Cassegrainian +type, as well as Newtonian. The mechanical parts +of this great telescope are by Warner and Swasey of +Cleveland, after the well-known equatorial mounting +of the Melbourne reflector by Grubb of Dublin. +Friction of the polar and declination axes is reduced +by ball bearings. The 66-foot dome has an +opening 15 feet wide and extending six feet beyond +the zenith. All motions of the telescope, dome +shutters, and observing platform are under complete +control by electric motors. Spectroscopic +binaries form one of the special fields of research +with this powerful instrument, and many new binaries +have already been detected.</p> + +<p>The great reflectors designed and constructed by +Ritchey, formerly of Chicago and now of Pasadena, +deserve especial mention. While connected with +the Yerkes Observatory he constructed a two-foot +reflector for that institution, with which he had exceptional +success in photography of the stars and +nebulæ. Later he built a 5-foot reflector, now at +the Carnegie Observatory on Mount Wilson, California, +with which the spiral nebulæ and many +other celestial objects have been especially well +photographed. Ritchey's later years have been +<span class="pagenum"><a name="Page_p108" id="Page_p108">[108]</a></span> +spent on the construction of an even greater mirror, +no less than 100 inches in diameter, which was completed +in 1919, and has already yielded photographic +results dealt with farther on, and far surpassing +anything previously obtained. Theoretically this +huge mirror, if its surface were perfectly reflective +so that it would transmit all the rays falling upon +it, would gather 160,000 times as much light as the +unaided eye alone.</p> + +<p>Whether a 72-inch refractor, should it ever be +constructed, would surpass the 100-inch reflector as +an all-round engine for astronomical research, is a +question that can only be fully answered by building +it and trying the two instruments alongside.</p> + +<p>Probably three-quarters of all the really great +astronomical work in the past has been done by refractors. +They are always ready and convenient +for use, and the optical surfaces rarely require +cleaning and readjustment. With increase of size, +however, the secondary spectrum becomes very +bothersome in the great lenses; and the larger they +are, the more light is lost by absorption on account +of the increasing thickness of the lenses. With the +reflector on the other hand, while there is clearly +a greater range of size, the reflective surface retains +its high polish only a brief period, so that +mere tarnish effectively reduces the aperture; and +the great mirror is more or less ineffective in consequence +of flexure uncompensated by the lever +system that supports the back of the mirror.</p> + +<p>Both types of telescope still have their enthusiastic +devotees; and the next great reflector would +doubtless be a gratifying success, if mounted in some +elevated region of the world, like the Andes of +northern Chile, where the air is exceptionally steady +<span class="pagenum"><a name="Page_p109" id="Page_p109">[109]</a></span> +and the sky very clear a large part of the year. The +highest magnifying powers suitable for work with +such a telescope could then be employed, and new +discoveries added as well as important work done in +extension of lines already begun on the universe of +stars.</p> + +<p>On the authority of Clark, even a six-foot objective +would not necessitate a combined thickness of +its glasses in excess of six inches. Present disks +are vastly superior to the early ones in transparency, +and there is reason to expect still greater improvement. +The engineering troubles incident to +execution of the mechanical side of the scheme need +not stand in the way; they never have, indeed the +astronomer has but just begun to invoke the fertile +resources of the modern engineer. Not long before +his death the younger Clark who had just finished +the great lenses of the 40-inch Yerkes telescope, +ventured this prevision, already in part come true: +"The new astronomy, as well as the old, demands +more power. Problems wait for their solution, and +theories to be substantiated or disproved. The horizon +of science has been greatly broadened within +the last few years, but out upon the borderland I +see the glimmer of new lights that await for their +interpretation, and the great telescopes of the future +must be their interpreters."</p> + +<p>Practically all the great telescopes of the world +have in turn signalized the new accession of power +by some significant astronomical discovery: to +specify, one of Herschel's reflectors first revealed +the planet Uranus; Lord Rosse's "Leviathan" the +spiral nebulæ; the 15-inch Cambridge lens the crape, +or dusky ring of Saturn; the 18½-inch Chicago +refractor the companion of Sirius; the Washington +<span class="pagenum"><a name="Page_p110" id="Page_p110">[110]</a></span> +26-inch telescope the satellites of Mars; the 30-inch +Pulkowa glass the nebulosities of the Pleiades; +and the 36-inch Lick telescope brought to light a +fifth satellite of Jupiter. At the time these discoveries +were made, each of these great telescopes +was the only instrument then in existence with +power enough to have made the discovery possible. +So we may advance to still farther accessions of +power with the expectation that greater discoveries +will continue to gratify our confidence.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p111" id="Page_p111">[111]</a></span></p> + +<h2><a name="CHAPTER_XX" id="CHAPTER_XX"></a>CHAPTER XX<br /> +<br /> +THE STORY OF THE SPECTROSCOPE</h2> + +<p>Sir Isaac Newton ought really to have been +the inventor of the spectroscope, because he +began by analyzing light in the rough with prisms, +was very expert in optics, and was certainly enough +of a philosopher to have laid the foundations of the +science.</p> + +<p>What Newton did was to admit sunlight into a +darkened room through a small round aperture, +then pass the rays through a glass prism and receive +the band of color on a screen. He noticed the +succession of colors correctly—violet, indigo, blue, +green, yellow, orange, red; also that they were not +pure colors, but overlapping bands of color. Apparently +neither he nor any other experimenter for +more than a century went any further, when the +next essential step was taken by Wollaston about +1802 in England. He saw that by receiving the light +through a narrow slit instead of a round hole, he +got a purer spectrum, spectrum being the name +given to the succession of colors into which the +prism splits up or decomposes the original beam of +white sunlight. This seemingly insignificant change, +a narrow slit replacing the round hole, made Wollaston +and not Newton the discoverer of the dark +lines crossing the spectrum at various irregular intervals, +and these singularly neglected lines meant +the basis of a new and most important science.</p> + +<p><span class="pagenum"><a name="Page_p112" id="Page_p112">[112]</a></span> +Even Wollaston, however, passed them by, and +it was Fraunhofer who in 1814-1815 first made a +chart of them. Consequently they are known as +Fraunhofer lines, or dark absorption lines. Sending +the beam of light through a succession of prisms +gives greater dispersion and increases the power of +the spectroscope. The greater the dispersion the +greater the number of absorption lines; and it is the +number and intensity of these lines, with their accurate +position throughout the range of the spectrum +which becomes the basis of spectrum analysis.</p> + +<p>The half century that saw the invention of the +steam engine, photography, the railroad and the +telegraph elapsed without any farther developments +than mere mapping of the fundamental lines, A, B, +C, D, E, F, G, H of the solar spectrum. The moon, +too, was examined and its spectrum found the same, +as was to be expected from sunlight simply reflected.</p> + +<p>Sir John Herschel and other experimenters came +near guessing the significance of the dark lines, but +the problem of unraveling their mystery was finally +solved by Bunsen and Kirchhoff who ascertained +that an incandescent gas emits rays of exactly the +same degree of refrangibility which it absorbs when +white light is passed through it. This great discovery +was at once received as the secure basis of +spectrum analysis, and Kirchhoff in 1858 put in +compact and comprehensive form the three following +principles underlying the theory of the science:</p> + +<p>(1) Solid and liquid bodies, also gases under high +pressure, give when incandescent a continuous +spectrum, that is one with a mere succession of +colors, and neither bright nor dark lines;</p> + +<p>(2) Gases under low pressure give a discontinuous +spectrum, crossed by bright lines whose number and +<span class="pagenum"><a name="Page_p113" id="Page_p113">[113]</a></span> +position in the spectrum differ according to the substances +vaporized;</p> + +<p>(3) When white light passes through a gas, this +medium absorbs or quenches rays of identical wave-length +with those composing its own bright-line +spectrum.</p> + +<p>Clearly then it makes no difference where the light +originates whether it comes from sun or star. Only +it must be bright enough so that we can analyze it +with the spectroscope. But our analysis of sun and +star could not proceed until the chemist had +vaporized in the laboratory all the elements, and +charted their spectra with accuracy. When this had +been done, every substance became at once recognizable +by the number and position of its lines, with +practical certainty.</p> + +<p>How then can we be sure of the chemical and +physical composition of sun and stars? Only by detailed +and critical comparison of their spectra with +the laboratory spectra of elements which chemical +and physical research have supplied. As in the sun, +so in the stars, each of which is encircled by a +gaseous absorptive layer or atmosphere, the light +rays from the self-luminous inner sphere must pass +through this reversing layer, which absorbs light of +exactly the same wave-length as the lines that make +up its own bright line spectrum. Whatever substances +are here found in gaseous condition, the same +will be evident by dark lines in the spectrum of sun +or star, and the position of these dark lines will show, +by coincidence with the position of the laboratory +bright lines, all the substances that are vaporized in +the atmospheres of the self-luminous bodies of the sky.</p> + +<p>Here then originated the science of the new astronomy: +the old astronomy had concerned itself +<span class="pagenum"><a name="Page_p114" id="Page_p114">[114]</a></span> +mainly with positions of the heavenly bodies, <i>where</i> +they are; the new astronomy deals with their chemical +composition and physical constitution, and <i>what</i> +they are. Between 1865 and 1875 the fundamental +application of the basic principles was well advanced +by the researches of Sir William Huggins in England, +of Father Angelo Secchi in Rome, of Jules +Janssen in Paris, and of Dr. Henry Draper in New +York.</p> + +<p>In analyzing the spectrum of the sun, many +thousands of dark absorption lines are found, and +their coincidences with the bright lines of terrestrial +elements show that iron, for instance, is most prominently +identified, with rather more than 2,000 coincidences +of bright and dark lines. Calcium, too, +is indicated by peculiar intensity of its lines, as well +as their great number. Next in order are hydrogen, +nickel and sodium. By prolonged and minute comparison +of the solar spectrum with spectra of terrestrial +elements, something like forty elemental +substances are now known to exist in the sun. Rowland's +splendid photographs of the solar spectrum +have contributed most effectively. About half of +these elements, though not in order of certainty, are +aluminum, cadmium, calcium, carbon, chromium, +cobalt, copper, hydrogen, iron, magnesium, manganese, +nickel, scandium, silicon, silver, sodium, +titanium, vanadium, yttrium, zinc, and zirconium. +Oxygen, too, is pretty surely indicated; but certain +elements abundant on earth, as nitrogen and chlorine, +together with gold, mercury, phosphorus, and +sulphur, are not found in the sun.</p> + +<p>The two brilliant red stars, Aldebaran in Taurus, +and Betelgeuse in Orion, were the first stars whose +chemical constitution was revealed to the eye of man, +<span class="pagenum"><a name="Page_p115" id="Page_p115">[115]</a></span> +and Sir William Huggins of London was the astronomer +who achieved this epoch-making result. Father +Secchi of the Vatican Observatory proceeded at +once with the visual examination of the spectra of +hundreds of the brighter stars, and he was the first +to provide a classification of stellar spectra. There +were four types.</p> + +<p>Secchi's type I is characterized chiefly by the +breadth and intensity of dark hydrogen lines, together +with a faintness or entire absence of metallic +lines. These are bluish or white stars and they are +very abundant, nearly half of all the stars. Vega, +Altair, and numerous other bright stars belong to +this type, and especially Sirius, which gives to the +type the name "Sirians."</p> + +<p>Type II is characterized by a multitude of fine +dark metallic lines, closely resembling the lines of +the solar spectrum. These stars are somewhat +yellowish in tinge like the sun, and from this +similarity of spectra they are called "solars." +Arcturus and Capella are "solars," and on the whole +the solars are rather less numerous than the Sirians. +Stars nearest to the solar system are mostly of this +type, and, according to Kapteyn of Groningen, the +absolute luminous power of first type stars exceeds +that of second type stars seven-fold.</p> + +<p>Secchi's type III is characterized by many dark +bands, well defined on the side toward the blue end +of the spectrum, but shading off toward the red—a +"colonnaded spectrum", as Miss Clerke aptly terms +it. Alpha Herculis, Antares, and Mira, together +with orange and reddish stars and most of the +variable stars, belong in type III.</p> + +<p>Type IV is also characterized by dark bands, often +called "flutings," similar to those of type III, but +<span class="pagenum"><a name="Page_p116" id="Page_p116">[116]</a></span> +reversed as to shading, that is, well defined on the +side toward the red, but fading out toward the blue. +Their atmospheres contain carbon; but they are not +abundant, besides being faint and nearly all blood-red +in tint.</p> + +<p>Following up the brilliant researches of Draper, +who in 1872 obtained the first successful photograph +of a star's spectrum, that of Vega, Pickering of +Harvard supplemented Secchi's classification by +Type V, a spectrum characterized by bright lines. +They, too, are not abundant and are all found near +the middle of the Galaxy. These are usually known +as Wolf-Rayet stars, from the two Paris astronomers +who first investigated their spectra. Type +V stars are a class of objects seemingly apart from +the rest of the stellar universe, and many of the +planetary nebulæ yield the same sort of a spectrum.</p> + +<p>The late Mrs. Anna Palmer Draper, widow of Dr. +Henry Draper, established the Henry Draper Memorial +at Harvard, and investigation of the photographic +spectra of all the brighter stars of the +entire heavens has been prosecuted on a comprehensive +scale, those of the northern hemisphere at +Cambridge, and of the southern at Arequipa, Peru. +These researches have led to a broad reclassification +of the stars into eight distinct groups, a work +of exceptional magnitude begun by the late Mrs. +Fleming and recently completed by Miss Annie +Cannon, who classified the photographic spectra of +more than 230,000 stars on the new system, as follows:—</p> + +<p>The letters O, B, A, F, G, K, M, N represent a +continuous gradation in the supposed order of stellar +evolution, and farther subdivision is indicated by +tenths, G5K meaning a type half way between G and +<span class="pagenum"><a name="Page_p117" id="Page_p117">[117]</a></span> +K, and usually written G5 simply. B2 would indicate +a type between B and A, but nearer to B than +A, and so on. On this system, the spectrum of a +star in the earliest stages of its evolution is made +up of diffuse bright bands on a faint continuous +background. As these bands become fewer and +narrower, very faint absorption lines begin to appear, +first the helium lines, followed by several series +of hydrogen lines. On the disappearance of the +bright bands, the spectrum becomes wholly absorptive +bands and lines. Then comes a very great +increase in intensity of the true hydrogen spectrum, +with wide and much diffused lines, and few if any +other lines. Then the H and K calcium lines and +other lines peculiar to the sun become more and +more intense. Then the hydrogen lines go through +their long decline. The calcium spectrum becomes +intense, and later the spectrum becomes quite like +that of the sun with a great wealth of lines. Following +this stage the spectrum shortens from the +ultra violet, the hydrogen lines fade out still farther, +and bands due to metallic compounds make their appearance, +the entire spectrum finally resembling that +of sun spots. To designate these types rather more +categorically:—</p> + +<p>Type O—bright bands on a faint continuous background, +with five subdivisions, Oa, Ob, Oc, Od, Oe, +according to the varying width and intensity of the +bands.</p> + +<p>Type B—the Orion type, or helium type, with additional +lines of origin unknown as yet, but without +any of the bright bands of type O.</p> + +<p>Type A—the Sirian type, the regular Balmer +series of hydrogen lines being very intense, with +a few other lines not conspicuously marked.</p> + +<p><span class="pagenum"><a name="Page_p118" id="Page_p118">[118]</a></span> +Type F—the calcium type, hydrogen lines less +strongly marked, but with the narrow calcium lines +H and K very intense.</p> + +<p>Type G—the solar type, with multitudes of metallic +lines.</p> + +<p>Type K—in some respects similar to G, but with +the hydrogen lines fading out, and the metallic lines +relatively more prominent.</p> + +<p>Type M—spectrum with peculiar flutings due to +titanium oxide, with subdivisions Ma and Mb, and +the variable stars of long period, with a few bright +hydrogen lines additional, in a separate class Md.</p> + +<p>Type N—similar to M, in that both are pronouncedly +reddish, but with characteristic flutings +probably indicating carbon compounds.</p> + +<p>The Draper classification being based on photographic +spectra, and the original Secchi classification +being visual, the relation of the two systems is +approximately as follows:</p> + +<table summary="Secchi Types"> +<tr> + <td>Secchi Type</td> + <td class="text_rt">I</td> + <td class="text_lf">includes Draper B & A</td> +</tr> +<tr> + <td> </td> + <td class="text_rt">II</td> + <td class="text_lf">includes Draper F, G & K</td> +</tr> +<tr> + <td> </td> + <td class="text_rt">III</td> + <td class="text_lf">includes Draper M</td> +</tr> +<tr> + <td> </td> + <td class="text_rt">IV</td> + <td class="text_lf">includes Draper N</td> +</tr> +</table> + +<p>Pickering's marked success in organization and +execution of this great programme was due to his +adoption of the "slitless spectroscope," which made +it possible to photograph stellar spectra in vast +numbers on a single plate. The first observers of +stellar spectra placed the spectroscope beyond the +focus of the telescope with which it was used, thereby +limiting the examination to but one star at a +time. In the slitless spectroscope, a large prism is +mounted in front of the objective (of short focus), +so that the star's rays pass through it first, and then +are brought to the same focus on the photographic +<span class="pagenum"><a name="Page_p119" id="Page_p119">[119]</a></span> +plate, for all the stars within the field of view, sometimes +many thousand in number. This arrangement +provides great advantages in the comparison +and classification of stellar spectra.</p> + +<p>When spectroscopic methods were first introduced +into astronomy, there was no expectation that +the field of the old or so-called exact astronomy +would be invaded. Physicists were sometimes +jocularly greeted among astronomers as "ribbon +men," and no one even dreamed that their researches +were one day to advance to equal recognition with +results derived from micrometer, meridian circle, +and heliometer.</p> + +<p>The first step in this direction was taken in 1868 +by Sir William Huggins of London, who noticed +small displacements in the lines of spectra of very +bright stars. In fact the whole spectrum appeared +to be shifted; in the case of Sirius it was shifted +toward the red, while the whole spectrum of Arcturus +was shifted by three times this amount toward +the violet end of the spectrum. The reason was not +difficult to assign.</p> + +<p>As early as 1842 Doppler had enunciated the +principle that when we are approaching or are approached +by a body which is emitting regular vibrations, +then the number of waves we receive in a +second is increased, and their wave-length correspondingly +diminished; and just the reverse of this +occurs when the distance of the vibrating body is +increasing. It is the same with light as with sound, +and everyone has noticed how the pitch of a locomotive +whistle suddenly rises as it passes, and falls +as suddenly on retreating from us. So Huggins +drew the immediate inference that the distance between +the earth and Sirius was increasing at the +<span class="pagenum"><a name="Page_p120" id="Page_p120">[120]</a></span> +rate of nearly twenty miles per second, while Arcturus +was nearing us with a velocity of sixty miles +per second.</p> + +<p>These pioneer observations of motions in the line +of sight, or radial velocities as they are now called, +led directly to the acceptance of the high value of +spectroscopic work as an adjunct of exact astronomy +in stellar research. Nor has it been found +wanting in application to a great variety of exact +problems in the solar system which would have been +wholly impossible to solve without it.</p> + +<p>Foremost is the sun, of course, because of the +overplus of light. Young early measured the displacement +of lines in the spectra of the prominences, +and found velocities sometimes exceeding 250 miles +per second. Many astronomers, Dunér among them, +investigated the rotation of the sun by the spectroscopic +method. The sun's east limb is coming toward +us, while the west is going from us; and by +measuring the sum of the displacements, the rate of +rotation has been calculated, not only at the sun's +equator but at many solar latitudes also, both north +and south. As was to be expected, these results +agree well with the sun's rotation as found by the +transits of sun spots in the lower latitudes where +they make their appearance.</p> + +<p>Bélopolsky has applied the same method to the +rotation of the planet Venus, and Keeler, by measuring +the displacement of lines in the spectrum of +Saturn, on opposite sides of the ring, provided a +brilliant observational proof of the physical constitution +of the rings; because he showed that the +inner ring traveled round more swiftly than the +outer one, thus demonstrating that the ring could +not be solid, but must be composed of multitudes of +<span class="pagenum"><a name="Page_p121" id="Page_p121">[121]</a></span> +small particles traveling around the ball of Saturn, +much as if they were satellites. Indeed, Keeler ascertained +the velocity of their orbital motion and +found that in each case it agreed exactly with that +required by the Keplerian law.</p> + +<p>Even the filmy corona of the sun was investigated +in similar fashion by Deslandres at the total eclipse +of 1893, and he found that it rotates bodily with the +sun. But the complete vindication of the spectroscopic +method as an adjunct of the old astronomy +came with its application to measurement of the +distance of the sun. The method is very interesting +and was first suggested by Campbell in 1892. +Spectrum-line measurements have become very accurate +with the introduction of dry-plate photography, +and ecliptic stars were spectrographed, toward +and from which the earth is traveling by its +orbital motion round the sun. By accurate measurement +of these displacements, the orbital velocity of +the earth is calculated; and as we know the exact +length of the year, or a complete period, the length +of the orbit itself in miles becomes known, and thus, +by simple mensuration, the length of the radius of +the orbit—which is the distance of the sun.</p> + +<p>If we pass from sun to star, the triumph of the +spectroscope has been everywhere complete and significant. +As the spectroscopic survey of the stars +grew toward completeness, it became evident that +the swarming hosts of the stellar universe are in +constant motion through space, not only athwart +the line of vision as their proper motions had long +disclosed, but some stars are swiftly moving toward +our solar system and others as swiftly from it.</p> + +<p>Fixed stars, strictly speaking—there are no such. +All are in relative motion. Exact astronomy by discussion +<span class="pagenum"><a name="Page_p122" id="Page_p122">[122]</a></span> +of the proper motions had assigned a region +of the sky toward which the sun and planets are +moving. Spectrography soon verified this direction +not only, but gave a determination of the velocity +of our motion of twelve miles per second in +a direction approximately that of the constellation +Lyra. From corresponding radial velocities, we +draw the ready conclusion that certain groups or +clusters of stars are actually connected in space and +moving as related systems, as in the Pleiades and +Ursa Major.</p> + +<p>Rather more than a quarter century ago, the +spectroscope came to the assistance of the telescope +in helping to solve the intricate problem of stellar +distribution. Kapteyn, by combining the proper +motions of certain stars with their classification in +the Draper catalogue of stellar spectra, drew the +conclusion that, as stars having very small proper +motions show a condensation toward the Galaxy, the +stars composing this girdle are mostly of the Sirian +type, and are at vast distances from the solar system. +The proper motion of a star near to us will +ordinarily be large, and, in the case of solar stars, +the larger their proper motion the greater their +number. So it would appear that the solar stars +are aggregated round the sun himself, and this conclusion +is greatly strengthened by the fact that of +stars whose distances and spectral type are both +ascertained, seven of the eight nearest to us are +solar stars.</p> + +<p>In 1889 the spectroscope achieved an unexpected +triumph by enabling the late Professor Pickering to +make the first discovery of a spectroscopic double, or +binary star, a type of object now quite abundant. +Unlike the visual binary systems whose periods are +<span class="pagenum"><a name="Page_p123" id="Page_p123">[123]</a></span> +years in length, the spectroscopic binaries have short +periods, reckoned in some cases in days, or hours +even. If the orbit of a very close binary is seen +edge on, the light of the two stars will coalesce twice +in every revolution. Halfway between these points +there are two times when the two stars will be moving, +one toward the earth and the other from it. At +all times the light of the star, in so far as the telescope +shows it, proceeds from a single object.</p> + +<p>Now photograph the star's spectrum at each of +the four critical points above indicated: in the first +pair the lines are sharply defined and single, because +at conjunction the stars are simply moving athwart +the line of sight, while at the intermediate points +the lines are double. Doppler's principle completely +accounts for this: the light from the receding companion +is giving lines displaced toward the red, while +the approaching companion yields lines displaced +toward the violet. Mizar, the double star at the +bend of the handle in the Great Dipper was the first +star to yield this peculiar type of spectrum, and the +period of its invisible companion is about 52 days. +The relative velocity of the components is 100 miles +a second, and applying Newton's law we find its +mass exceeds that of the sun forty-fold. Capella has +been found to be a spectroscopic binary; also the +pole star. Spectroscopic binaries have relatively +short periods, one of the shortest known being only +35 hours in length. It is in the constellation Scorpio. +Beta Aurigæ is another whose lines double on +alternate nights, giving a period of four days; and +the combined mass of both stars is more than twice +that of the sun. The catalogue of spectroscopic +binaries is constantly enlarging; but thousands +doubtless exist that can never be discovered by this +<span class="pagenum"><a name="Page_p124" id="Page_p124">[124]</a></span> +method, as is evident if their orbits are perpendicular +to the line of sight or nearly so. The history +of the spectroscopic binaries is one of the most interesting +chapters in astronomy, and affords a marvelous +confirmation of the prediction of Bessel who +first wrote of "the astronomy of the invisible."</p> + +<p>Find a star's distance by the spectroscope? Impossible, +everyone would have said, even a very few +years ago. Now, however, the thing is done, and +with increasing accuracy.</p> + +<p>Adams of Mount Wilson has found, after protracted +investigation, that the relative intensity of +certain spectral lines varies according to the absolute +brightness of a star; indeed, so close is the correspondence +that the spectroscopic observations are +employed to provide in certain cases a good determination +of the absolute magnitude, and therefore of +the distance. To test this relation, the spectroscopic +parallaxes have been compared with the measured +parallaxes in numerous instances, and an excellent +agreement is shown. This new method is adding +extensively to our knowledge of stellar luminosities +and distances, and even the vast distances of globular +clusters and spiral nebulæ are becoming known.</p> + +<p>In fact, but few departments of the old astronomy +are left which the new astronomy has not invaded, +and this latest triumph of the spectroscope in determining +accurately the distances of even the remotest +stars is enthusiastically welcomed by advocates +of the old and new astronomy alike.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p125" id="Page_p125">[125]</a></span></p> + +<h2><a name="CHAPTER_XXI" id="CHAPTER_XXI"></a>CHAPTER XXI<br /> +<br /> +THE STORY OF ASTRONOMICAL PHOTOGRAPHY</h2> + +<p>The most powerful ally of both telescope and spectroscope +is photography. Without it the marvelous +researches carried on with both these types +of instrument would have been essentially impossible. +Even the great telescopes of Herschel and +Lord Rosse, notwithstanding their splendid record +as optical instruments, might have achieved vastly +more had photography been developed in their time +to the point where the astronomer could have employed +its wonderful capabilities as he does to-day. +And, with the spectroscope, it is hardly too much to +say that no investigator ever observes visually with +that instrument any more: practically every spectrum +is made a matter of photographic record first. +The observing, or nowadays the measuring, is all +done afterward.</p> + +<p>All telescopes and cameras are alike, in that each +must form or have formed within it an image by +means of a lens or mirror. In the telescope the eye +sees the fleeting image, in the camera the process of +registering the image on a plate or film is known as +photography. Daguerre first invented the process +(silver film on a copper plate) in 1839. The year +following it was first employed on the moon, in 1850 +the first star was photographed, in 1851 the first +total eclipse of the sun; all by the primitive daguerreotype +<span class="pagenum"><a name="Page_p126" id="Page_p126">[126]</a></span> +process, which, notwithstanding its awkwardness +and the great length of exposure required, +was found to possess many advantages for astronomical +work.</p> + +<p>About the middle of the last century the wet plate +process, so called because the sensitized collodion +film must be kept moist during exposure, came into +general use, and the astronomers of that period were +not slow to avail themselves of the advantages of +a more sensitive process, which in 1872, in the +skillful hands of Henry Draper, produced the first +spectrum of a star. In 1880 a nebula was first +photographed, and in 1881 a comet.</p> + +<p>Before this time, however, the new dry-plate process +had been developed to the point where astronomers +began to avail of its greater convenience and +increased sensitiveness, even in spite of the coarseness +of grain of the film. Forty years of dry-plate +service have brought a wealth of advantages scarcely +dreamed of in the beginning, and nearly every department +of astronomical research has been enhanced +thereby, while many entirely new photographic +methods of investigation have been worked +out.</p> + +<p>Continued improvement in photographic processes +has provided the possibility of pictures of +fainter and fainter celestial objects, and all the +larger telescopes have photographed stars and +nebulæ of such exceeding faintness that the human +eye, even if applied to the same instrument, would +never be able to see them. This is because the eye, +in ten or twelve seconds of keen watching, becomes +fatigued and must be rested, whereas the action of +very faint light rays is cumulative on the highly +sensitive film; so that a continuous exposure of many +<span class="pagenum"><a name="Page_p127" id="Page_p127">[127]</a></span> +hours' duration becomes readily visible to the eye +on development. So a supersensitive dry plate will +often record many thousand stars in a region where +the naked eye can see but one.</p> + +<p>Perhaps the greatest amplification of photography +has taken place at the Harvard Observatory under +Pickering, where a library of many hundred thousand +plates has accumulated; and at Groningen, Holland, +where Kapteyn has established an astronomical +laboratory without instruments except such as +are necessary to measure photographic plates, whenever +and wherever taken. So it is possible to +select the clearest of skies, all over the world, for +exposure of the plates, and bring back the photographs +for expert discussion.</p> + +<p>Of course the sun was the celestial body first +photographed, and its surpassing brilliance necessitates +reduction of exposure to a minimum. In +moments of exceptional steadiness of the atmosphere, +a very high degree of magnification of the +solar surface on the photographic plate is permitted, +and the details in formation, development, and ending +of sun spots are faithfully registered. Nevertheless, +it cannot be said that photography has yet +entirely replaced the eye in this work, and careful +drawings of sun spots at critical stages in their life +are capable of registering fine detail which the plate +has so far been unable to record. Janssen of Paris +took photographs of the solar photosphere so highly +magnified that the granulation or willow-leaf structure +of the surface was clearly visible, and its variations +traceable from hour to hour.</p> + +<p>The advantages of sun spot photography in ascertaining +the sun's rotation, keeping count of the +spots, and in a permanent record for measurement +<span class="pagenum"><a name="Page_p128" id="Page_p128">[128]</a></span> +of position of the sun's axis and the spot zones, are +obvious. In direct portrayal of the sun's corona +during total eclipses, photography has offered superior +advantages over visual sketching, in the form +and exact location of the coronal streamers; but the +extraordinary differences of intensity between the +inner corona and its outlying extensions are such +that halation renders a complete picture on a single +plate practically impossible. The filamentous detail +of the inner corona, and the faintest outlying extensions +or streamers, the eye must still reveal directly.</p> + +<p>In solar spectrum photography, research has been +especially benefited; indeed, exact registry of the +multitudinous lines was quite impossible without it. +Photographic maps of the spectrum by Thollon, +McClean and Rowland are so complete and accurate +that no visual charts can approach them. Rowland's +great photographic map of the solar spectrum +spread out into a band about forty feet in length; +and in the infra-red, Langley's spectrobolometer extended +the invisible heat spectrum photographically +to many times that length. At the other end of the +spectrum, special photographic processes have extended +the ultra-violet spectrum far beyond the +ocular limit, to a point where it is abruptly cut off by +absorption of the earth's atmosphere. On the same +plate with certain regions of the sun's spectrum, the +spectra of terrestrial metals are photographed side +by side, and exact coincidences of lines show that +about forty elemental substances known to terrestrial +chemistry are vaporized in the sun.</p> + +<p><span class="pagenum"><a name="Page_p128p1" id="Page_p128p1">[128i]</a></span></p> + +<div class="fig_center" style="width: 645px;"> +<img src="images/p128_1top.png" width="645" height="510" alt="" /> +<span class="fig_caption"><span class="smcap">A View of the 100-foot Dome in Which the Largest Telescope +in the World is Housed.</span> (<i>Courtesy, Mt. Wilson Solar Observatory.</i>)</span> +</div> + +<div class="fig_center" style="width: 641px;"> +<img src="images/p128_1bot.png" width="641" height="519" alt="" /> +<span class="fig_caption"><span class="smcap">Mount Chimborazo, Near the Equator.</span> An observatory located on +this mountain would make it possible to study the phenomena of +northern and southern skies from the same point. (<i>Courtesy, Pan-American +Union.</i>)</span> +</div> + +<p><span class="pagenum"><a name="Page_p128p2" id="Page_p128p2">[129i]</a></span></p> + +<div class="fig_center" style="width: 652px;"> +<img src="images/p128_2top.png" width="652" height="514" alt="" /> +<div class="fig_caption"><span class="smcap">Lick Observatory, on the Summit of Mt. Hamilton, About +Twenty-Five Miles S. W. of San Jose, California.</span> It contains +the famous Lick telescope, a 36-inch refractor.</div> +</div> + +<div class="fig_center" style="width: 640px;"> +<img src="images/p128_2bot.png" width="640" height="499" alt="" /> +<div class="fig_caption"><span class="smcap">Near View of the Eye-End of the Yerkes Telescope.</span> The eyepiece +is removed and its place taken by a photographic plate.</div> +</div> + + +<p><span class="pagenum"><a name="Page_p129" id="Page_p129">[129]</a></span> +Young was the first to photograph a solar prominence +in 1870, and twenty years later Deslandres +of Paris and Hale of Chicago independently invented +the spectroheliograph, by which the chromosphere +and prominences of the sun, as well as the +disk of the sun itself, are all photographed by monochromatic +light on a single plate. Hale has developed +this instrument almost to the limit, first +at the Yerkes Observatory of the University of +Chicago, and more recently at the Mount Wilson +Observatory of the Carnegie Institution, where spectroheliograms +of marvelous perfection are daily +taken. It was with this instrument that Hale discovered +the effect of an electro-magnetic field in sun +spots which has revolutionized solar theories, a research +impossible to conceive of without the aid of +photography.</p> + +<p>When we apply Doppler's principle, photography +becomes doubly advantageous, whether we determine, +as Dunér did and more recently Adams, the +sun's own rotation and find it to vary in different +solar latitudes, the equator going fastest; or apply +the method to the sun's corona at the east and west +limbs of the sun, which Deslandres in 1893 +proved to be rotating bodily with the sun, because +of the measured displacement of spectral lines +of the corona in juxtaposition on the photographic +plate.</p> + +<p>In the solar astronomy of measurement, too, photography +has been helpfully utilized, as in registering +the transits of Mercury over the sun's disk, for +correcting the tables of the planet's orbital motion; +and most prominently in the action taken by the +principal governments of the world in sending out +expeditions to observe the transits of Venus in 1874 +and 1882, for the purpose of determining the parallax +of Venus and so the distance of the earth from +the sun.</p> + +<p><span class="pagenum"><a name="Page_p130" id="Page_p130">[130]</a></span> +In our studies of the moon, photography has almost +completely superseded ocular work during the +past sixty years. Rutherfurd and Draper of New +York about 1865 obtained very excellent lunar photographs +with wet plates, which were unexcelled for +nearly half a century. The Harvard, Lick, and Paris +Observatories have published pretty complete photographic +atlases of the moon, and the best negatives +of these series show nearly everything that the eye +can discern, except under unusual circumstances. +Later lunar photography was taken up at the Yerkes +Observatory, and exceptionally fine photographs on +a large scale were obtained with the 40-inch refractor, +using a color screen. More recently the +60-inch and 100-inch mirrors of the Mount Wilson +Observatory have taken a series of photographs of +the moon far surpassing everything previously done, +as was to be expected from the unique combination +of a tranquil mountain atmosphere with the extraordinary +optical power of the instruments, and a +special adaptation of photographic methods. During +lunar eclipses, Pickering has made a photographic +search for a possible satellite of the moon, +occultations of stars by the moon have been recorded +by photography, and Russell of Princeton has shown +how the position of the moon among the stars can +be determined by the aid of photography with a high +order of precision.</p> + +<p>The story of planetary photography is on the +whole disappointing. Much has been done, but there +is much that is within reach, or ought to be, that +remains undone. From Mercury nothing ought perhaps +to be expected. On many of the photographs +of the transit of Venus, especially those taken under +the writer's direction at the Lick Observatory in +<span class="pagenum"><a name="Page_p131" id="Page_p131">[131]</a></span> +1882, we have unmistakable evidence of the planet's +atmosphere. Here again the wet plate process, although +more clumsy, demonstrated its superiority +over the dry process used by other expeditions.</p> + +<p>In spectroscopy, Bélopolsky has sought to determine +the period of rotation of Venus on her axis. At +the Lowell Observatory, Douglass succeeded in photographing +the faint zodiacal light, and very successful +photographs of Mars were taken at this institution +as early as 1905 by Slipher. Two years later +these were much improved upon by the writer's expedition +to the Andes of Chile, when 12,000 exposures +of Mars were made, many of them showing the +principal <i>canali</i>, and other prominent features of +the planet's disk. At subsequent oppositions of the +planet, Barnard at the Yerkes Observatory and the +Mount Wilson observers have far surpassed all these +photographs.</p> + +<p>For future oppositions a more sensitive film is +highly desired, in connection with instruments possessing +greater light-gathering power, so permitting +a briefer exposure that will be less influenced by irregularities +and defects of the atmosphere. The +spectrum of Mars is of course that of sunlight, very +much reduced, and modified to a slight extent by its +passing twice through the atmosphere of Mars. +What amount of aqueous vapor that atmosphere may +contain is a question that can be answered only by +critical comparison of the Martian spectrum with +the spectrum of the moon, and photography affords +the only method by which this can be done.</p> + +<p>Many are the ways in which photography has +aided research on the asteroid group. Since 1891 +more than 600 of them have been discovered by photography, +and it is many times easier to find the +<span class="pagenum"><a name="Page_p132" id="Page_p132">[132]</a></span> +new object on the photographic plate than to detect +it in the sky as was formerly done by means of star +charts. The planet by its motion during the exposure +of the plate produces a trail, whereas the surrounding +stars are all round dots or images. Or by +moving the plate slightly during exposure, as in +Metcalf's ingenious method, we may catch the +planet at that point where it will give a nearly +circular image, and thus be quite as easy to detect, +because all the stars on the same plate will then +be trails.</p> + +<p>Photographic photometry of the asteroids has revealed +marked variations in their light, due perhaps +to irregularities of figure. On account of their faint +light, the asteroids are especially suited, as Mars is +not, to exact photography for ascertaining their +parallax, and from this the sun's distance when the +asteroid's distance has been found. Many asteroids +have been utilized in this way, in particular Eros +(433). In 1931 it approaches the earth within +13 million miles, when the photographic method +will doubtless give the sun's distance with the utmost +accuracy.</p> + +<p>Photographs of Jupiter have been very successfully +taken at the Yerkes and Lowell Observatories +and elsewhere, but the great depth of the planet's +atmosphere is highly absorptive, so that the impression +is very weak in the neighborhood of the +limb, if the exposure is correctly timed for the center +of the disk. The striking detail of the belts, however, +is excellently shown. Wood of Baltimore has +obtained excellent results by monochromatic photography +of Jupiter and Saturn with the 60-inch reflector +on Mount Wilson. Jupiter's satellites have +not been neglected photographically, and Pickering +<span class="pagenum"><a name="Page_p133" id="Page_p133">[133]</a></span> +has observed hundreds of the eclipses of the satellites +by a sort of cinematographic method of repeated +exposures, around the time of disappearance +and reappearance by eclipse. The newest outer +satellites of Jupiter were all discovered by photography, +and it is extremely doubtful if they would +have been found otherwise.</p> + +<p>Saturn has long been a favorite object with the +astronomical photographer, and there are many +fine pictures in spite of his yellowish light, relatively +weak photographically. The marvelous ring +system with the Cassini division, the oblateness of +the ball, the occasional markings on it—all are well +shown in the best photographs; but the call is for +more light and a more sensitive photographic process. +Pickering's ninth satellite (Phœbe) was discovered +by photography, one of the faintest moons +in the solar system. Like the faint outer moons +of Jupiter, few existing telescopes are powerful +enough to show it. Its orbit has been found from +photographic observations, and its position is +checked up from time to time by photography.</p> + +<p>But the crowning achievement of spectrum photography +in the Saturnian system is Keeler's application +of Doppler's principle in determining the +rate of orbital motion of particles in different zones +of the rings, thereby establishing the Maxwellian +theory of the constitution of the rings beyond the +possibility of doubt. For Uranus and Neptune +photography has availed but little, except to negative +the existence of additional satellites of these +planets, which doubtless would have been discovered +by the thorough photographic search which has +been made for them by W. H. Pickering without +result.</p> + +<p><span class="pagenum"><a name="Page_p134" id="Page_p134">[134]</a></span> +As with the asteroids, so with comets: several of +these bodies have been discovered by photography; +none more spectacular than the Egyptian comet of +May 17th, 1882, which impressed itself on the plates +of the corona of that date. Withdrawal of the sun's +light by total eclipse made the comet visible, and it +had never been seen before, nor is it known whether +it will ever return. In cometary photography, much +the same difficulties are present as in photographing +the corona: if the plate is exposed long enough to +get the faint extensions of the tail, the fine filaments +of the coma or head are obliterated by halation and +overexposure.</p> + +<p>No one has had greater success in this work than +Barnard, whose photographs of comets, particularly +at the Lick Observatory, are numerous and unexcelled. +His photographs of the Brooks Comet of +1893 revealed rapid and violent changes in the tail, +as if shattered by encounter with meteors; and the +tail of Halley's comet in 1910 showed the rapid propagation +of luminous waves down the tail, similar to +phenomena sometimes seen in streamers of the +aurora. Draper obtained the first photograph of a +comet's spectrum in 1881, disclosing an identity +with hydrocarbons burning in a Bunsen flame, also +bands in the violet due to carbon compounds. The +photographic spectra of subsequent comets have +shown bright lines due to sodium and the vapor of +iron and magnesium.</p> + +<p>Even the elusive meteor has been caught by +photography, first by Wolf in 1891, who was exposing +a plate on stars in the Milky Way. On developing +it, he found a fine, dark nearly uniform +line crossing it, due to the accidental flight across +the field of a meteor of varying brightness. Since +<span class="pagenum"><a name="Page_p135" id="Page_p135">[135]</a></span> +then meteor trails have been repeatedly photographed, +and even the trail spectra of meteors have +been registered on the Harvard plates. At Yale +in 1894 Elkin employed a unique apparatus for +securing photographic trails of meteors: six photographic +cameras mounted at different angles on a +long polar axis driven by clockwork, the whole +arranged so as to cover a large area of the sky where +meteors were expected.</p> + +<p>When we pass from the solar system to the stellar +universe the advantages of photography and the amplification +of research due to its employment as +accessory in nearly every line of investigation are +enormous. So extensively has photography been +introduced that plates, and to a slight extent films, +are now almost exclusively used in securing original +records. Regrettably so in case of the nebulæ, because +the numerous photographs of the brighter +nebulæ taken since 1880 when Draper got the first +photograph of the nebula of Orion, are as a rule not +comparable with each other. Differences of instruments, +of plates, of exposure, and development—all +have occasioned differences in portrayal of a nebula +which do not exist. When we consider faithful +accuracy of portrayal of the nebulæ for purposes of +critical comparison from age to age, many of our +nebular photographs of the past forty years, fine +as they are and marvelous as they are, must fail +to serve the purpose of revealing progressive +changes in nebular features in the future.</p> + +<p>Roberts and Common in England were among the +first to obtain nebular photographs with extraordinary +detail, also the brothers Henry of Paris. As +early as 1888 Roberts revealed the true nature of +the great nebula in Andromeda, which had never +<span class="pagenum"><a name="Page_p136" id="Page_p136">[136]</a></span> +been suspected of being spiral; and Keeler and +Perrine at the Lick Observatory pushed the photographic +discovery of spiral nebulæ so far that their +estimates fill the sky with many hundred thousands +of these objects.</p> + +<p>In the southern hemisphere the 24-inch Bruce +telescope of Harvard College Observatory has +obtained many very remarkable photographs of +nebulæ, particularly in the vicinity of Eta Carinæ. +But the great reflectors of the Mount Wilson Observatory, +on account of their exceptional location and +extraordinary power, have surpassed all others in +the photographic portrayal of these objects, especially +of the spiral nebulæ which appear to show all +stages in transition from nebula to star. No less +remarkable are the photographs of such wonderful +clusters as Omega Centauri, a perfect visual representation +of which is wholly impossible. Intercomparison +of the photographs of clusters has afforded +Bailey of Harvard, Shapley of Mount Wilson and +others the opportunity of discovery that hundreds +of the component stars are variable.</p> + +<p>What is the longest photographic exposure ever +made? At the Cape of Good Hope, under the direction +of the late Sir David Gill, exposures on nebulæ +were made, utilizing the best part of several nights, +and totaling as high as seventeen, or even twenty-three +hours. But the Mount Wilson observers have +far surpassed this duration. To study the rotation +and radial velocity of the central part of the nebula +of Andromeda, an exposure of no less than 79 hours' +total duration was made on the exceedingly faint +spectrum, and even that record has since been +exceeded. The eye cannot be removed from the +guiding star for a moment while the exposure +<span class="pagenum"><a name="Page_p137" id="Page_p137">[137]</a></span> +is in progress, and this tedious piece of work was +rewarded by determining the velocity of the center +of the nucleus as a motion of approach at the rate +of 316 kilometers per second.</p> + +<p>But when the stars, their magnitudes and their +special peculiarities are to be investigated <i>en masse</i>, +photography provides the facile means for researches +that would scarcely have been dreamed of +without it. The international photographic chart of +the entire heavens, in progress at twenty observatories +since 1887, the photographic charts of the +northern heavens at Harvard and of the southern sky +at Cape Town, the manifold investigations that have +led up to the Harvard photometry, and the unparalleled +photographic researches of the Henry Draper +Memorial, enabling the spectra of many hundred +thousand stars to be examined and classified—all +this is but a part of the astronomical work in stellar +fields that photography has rendered possible.</p> + +<p>Then there are the stellar parallaxes, now observed +for many stars at once photographically, +when formerly only one star's parallax could be +measured at a time and with the eye at the telescope. +And photo-electric photometry, measuring smaller +differences of light than any other method, and providing +more accurate light-curves of the variable +stars. And perhaps most remarkable of all, the +radial velocity work on both stars and nebulæ, giving +us the distance of whole classes of stars, discovering +large numbers of spectroscopic binaries +and checking up the motion of the solar system +toward Lyra within a fraction of a mile per +second.</p> + +<p>All told, photography has been the most potent +adjunct in astronomical research, and it is impossible +<span class="pagenum"><a name="Page_p138" id="Page_p138">[138]</a></span> +to predict the future with more powerful +apparatus and photographic processes of higher +sensitiveness. The field of research is almost +boundless, and the possibilities practically without +limit.</p> + +<p>What would Herschel have done with £100,000—and +photography!</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p139" id="Page_p139">[139]</a></span></p> + +<h2><a name="CHAPTER_XXII" id="CHAPTER_XXII"></a>CHAPTER XXII<br /> +<br /> +MOUNTAIN OBSERVATORIES</h2> + +<p>The century that has elapsed since the time of +Sir William Herschel, known as the father of +the new or descriptive astronomy, has witnessed all +the advances of the science that have been made +possible by adopting the photographic method of +making the record, instead of depending upon the +human eye. Only one eye can be looking at the +eyepiece at a time: the photograph can be studied +by a thousand eyes.</p> + +<p>At mountain elevations telescopes are now extensively +employed, and there the camera is of +especial and additional value, because the photograph +taken on the mountain can be brought down +for the expert to study, at ease and in the comfort +of a lower elevation. We shall next trace the movement +that has led the astronomer to seek the summits +of mountains for his observatories, and the +photographer to follow him.</p> + +<p>Not only did the genius of Newton discover the +law of universal gravitation, and make the first experiments +in optics essential to the invention of the +spectroscope, but he was the real originator also of +the modern movement for the occupation of mountain +elevations for astronomical observatories. His +keen mind followed a ray of light all the way from +its celestial source to the eye of the observer, and +analyzed the causes of indistinct and imperfect +vision.</p> + +<p><span class="pagenum"><a name="Page_p140" id="Page_p140">[140]</a></span> +Endeavoring to improve on the telescope as +Galileo and his followers had left it, he found such +inherent difficulties in glass itself that he abandoned +the refracting type of telescope for the reflector, to +the construction of which he devoted many years. +But he soon found out, what every astronomer and +optician knew to their keen regret, that a telescope, +no matter how perfectly the skill of the optician's +hand may make it, cannot perform perfectly unless +it has an optically perfect atmosphere to look +through.</p> + +<p>So Newton conceived the idea of a mountain +observatory, on the summit of which, as he thought, +the air would be not only cloudless, but so steady +and equable that the rays of light from the heavenly +bodies might reach the eye undisturbed by atmospheric +tremors and quiverings which are almost +always present in the lower strata of the great +ocean of air that surrounds our planet.</p> + +<p>This is the way Newton puts the question in his +treatise on <i>Opticks</i>—he says: "The Air through +which we look upon the Stars, is in a perpetual +Tremor; as may be seen by the tremulous Motion +of Shadows cast from high Towers, and by the +twinkling of the fix'd stars…. The only remedy +is a most serene and quiet Air, such as may perhaps +be found on the tops of the highest Mountains above +the grosser Clouds."</p> + +<p>Newton's suggestion is that the <i>highest</i> mountains +may afford the best conditions for tranquillity; +and it is an interesting coincidence that the summits +of the highest mountains, about 30,000 feet in elevation, +are at about the same level where the turbulence +of the atmosphere most likely ceases, according +to the indications of recent meteorological research. +<span class="pagenum"><a name="Page_p141" id="Page_p141">[141]</a></span> +These heights are far above any elevations permanently +occupied as yet, but a good beginning has +been made and results of great value have already +been reached.</p> + +<p>Curiously, investigation of mountain peaks and +their suitability for this purpose was not undertaken +till nearly two centuries after Newton, when +Piazzi Smyth in 1856 organized his expedition to +the summit of a mountain of quite moderate elevation, +and published his "Teneriffe: an Astronomer's +Experiment." Teneriffe is an accessible peak of +about 10,000 feet, on an island of the Canaries off +the African coast, where Smyth fancied that conditions +of equability would exist; and on reaching +the summit with his apparatus and spending a few +days and nights there, he was not disappointed. +Could he have reached an elevation of 13,000 feet, +he would have had fully one-third of all the atmosphere +in weight below him, and that the most turbulent +portion of all. Nevertheless, the gain in +steadiness of the atmosphere, providing "better seeing," +as the astronomer's expression is, even at +10,000 feet, was most encouraging, and led to attempts +on other peaks by other astronomers, a few +of whom we shall mention.</p> + +<p>Davidson, an observer of the United States Coast +Survey, with a broad experience of many years in +mountain observing, investigated the summit of the +Sierra Nevada mountains as early as 1872, at an +elevation of 7,200 feet. His especial object was to +make an accurate comparison between elevated stations +at different heights. He found the seeing +excellent, especially on the sun; but the excessive +snowfall at his station, 45 feet annually, was a condition +very adverse to permanent occupation.</p> + +<p><span class="pagenum"><a name="Page_p142" id="Page_p142">[142]</a></span> +In the summer of 1872, Young spent several +weeks at Sherman, Wyoming, at an elevation exceeding +8,300 feet. He carried with him the 9.4-inch +telescope of Dartmouth College, where he was +then professor, and this was the first expedition on +which a large glass was used by a very skillful +observer at great elevation. He found the number +of good days and nights small, but the sky was exceedingly +favorable when clear. Many 7th magnitude +stars could be detected with the naked eye. +Young's observations at Sherman were mainly +spectroscopic, however, and they demonstrated the +immense advantage of a high-level station, far above +the dust and haze of the lower atmosphere. He +pronounced the 9.4-inch glass at 8,000 feet the full +equivalent of a 12-inch at sea level.</p> + +<p>Mont Blanc of 15,000 feet elevation was another +summit where the veteran Janssen of Paris maintained +a station for many years; but the continental +conditions of atmospheric moisture and circulation +were not favorable on the whole. Janssen was +mainly interested in the sun, and the daylight seeing +is rarely benefited, owing to the strong upward +currents of warm air set in motion by the sun itself.</p> + +<p>Mountains in the beautiful climate of California +were among the earliest investigated, and when in +1874 the trustees of Mr. James Lick's estate were +charged with equipping an observatory with the +most powerful telescope in existence, they wisely +located on the summit of Mount Hamilton. It is +4,300 feet above sea level, and Burnham and other +astronomers made critical tests of the steadiness of +vision there by observing double stars, which afford +perhaps the best means of comparing the optical +quality of the atmosphere of one region with another. +<span class="pagenum"><a name="Page_p143" id="Page_p143">[143]</a></span> +The writer was fortunate in having charge +of the observations of the transit of Venus in 1882 +on the mountain, when the Observatory was in +process of construction, and the quality of the photographs +obtained on that occasion demonstrated anew +the excellence of the site. Particularly at night, for +about nine months of the year, the seeing is exceptionally +good, especially when fog banks rolling +in from the Pacific, cover the valleys below like a +blanket, preventing harmful radiation from the soil +below.</p> + +<p>The great telescope mounted in 1888, a 36-inch +refractor by Alvan Clark, has fulfilled every expectation +of its projectors, and justified the selection +of the site in every particular. The elevation, although +moderate, is still high enough to secure very +marked advantage in clearness and steadiness of +the air, and at the same time not so high that the +health and activities of the observers are appreciably +affected by the thinner air of the summit. This +telescope is known the world over for the monumental +contributions to science made by the able +astronomers who have worked with it: among them +Barnard who discovered the fifth satellite of Jupiter +in 1892; Burnham, Hussey, and Aitken, who have +discovered and measured thousands of close double +stars; Keeler, who spent many faithful years on the +summit; and Campbell, the present director, whose +spectroscopic researches on stellar movements have +added greatly to our knowledge of the structure of +the universe. Among the many lines of research +now in progress at the Lick Observatory and in the +D. O. Mills Observatory at Santiago, Chile, are the +discoveries of stars whose velocities in space are not +constant, but variable with the spectral type of the +<span class="pagenum"><a name="Page_p144" id="Page_p144">[144]</a></span> +star. Mr. Lick's bequest for the Observatory was +about $700,000. So ably has this scientific trust +been administered that he might well have endowed +it with his entire estate, exceeding $4,000,000.</p> + +<p>Another California mountain that was early investigated +is Mount Whitney. Its summit elevation +is nearly 15,000 feet, and in 1881 Langley made +its ascent for the purpose of measuring the solar +constant. He found conditions much more favorable +than on Mount Etna, Sicily—elevation about +10,000 feet—which he had visited the year before. +But the height of Mount Whitney was such as to +occasion him much inconvenience from mountain +sickness, an ailment which is most distressing and +due partly to lack of oxygen and partly to mere +diminution of mechanical pressure. Mount Whitney +was also visited many years after by Campbell for +investigating the spectrum of Mars in comparison +with that of the moon. Langley found on Mount +Whitney an excellent station lower down, at about +12,000 feet elevation; and by equipping the two stations +with like apparatus for measuring the solar +heat, he obtained very important data on the selective +absorption of the atmosphere.</p> + +<p>Returning from the transit of Venus in 1882, +Copeland of Edinburgh visited several sites in the +Andes of Peru, ascending on the railway from Mollendo. +Vincocaya was one of the highest, something +over 14,000 feet elevation. His report was most +enthusiastic, not only as to clearness and transparency +of the atmosphere, but also as to its steadiness, +which for planetary and double star observations +is almost as important. Copeland's investigation +of this region of the Andes has led many other +astronomers to make critical tests in the same +<span class="pagenum"><a name="Page_p145" id="Page_p145">[145]</a></span> +general region. Climatic conditions are particularly +favorable, and the sites for high-level research +are among the best known, the atmosphere being +not only clear a large part of the year, but in certain +favored spots exceedingly steady.</p> + +<p>In 1887 the writer ascended the summit of +Fujiyama, Japan, 12,400 feet elevation. The early +September conditions as to steadiness of atmosphere +were extraordinarily fine, but the mountain is +covered by cloud many months in each year. There +is a saddle on the inside of the crater that would +form an ideal location for a high-level observatory. +This expedition was undertaken at the request of the +late Professor Pickering, director of Harvard College +Observatory, which had recently received a bequest +from Uriah A. Boyden, amounting to nearly a +quarter of a million dollars, to "establish and maintain, +in conjunction with others, an astronomical +observatory on some mountain peak."</p> + +<p>Great elevations were systematically investigated +in Colorado and California, the Chilean desert of +Atacama was visited, and a temporary station established +at Chosica, Peru, elevation about 5,000 +feet. Atmospheric conditions becoming unfavorable, +a permanent station was established in 1891 at +Arequipa, Peru, elevation 8,000 feet, which has been +maintained as an annex to the Harvard Observatory +ever since. The cloud conditions have been +on the whole less favorable than was expected, but +the steadiness of the air has been very satisfactory. +In addition to planetary researches conducted there +in the earlier years by W. H. Pickering, many large +programs of stellar research have been executed, +especially relating to the magnitudes and spectra of +the stars. In conjunction with the home observatory +<span class="pagenum"><a name="Page_p146" id="Page_p146">[146]</a></span> +in the northern hemisphere, this afforded a +vast advantage in embracing all the stars of the +entire heavens, on a scale not attempted elsewhere. +The Bruce photographic telescope of 24-inch aperture +has been employed for many years at Arequipa, +and with it the plates were taken which enabled +Pickering to discover the ninth satellite of Saturn +(Phœbe), and the splendid photographs of southern +globular clusters in which Bailey has found +numerous variable stars of very short periods—very +faint objects, but none the less interesting, and +of much significance in modern study of the evolution +and structure of the stellar universe. The +crowning research of the observatory is the Henry +Draper catalogue of stellar spectra, now in process +of publication, which is of the first order of importance +in statistical studies of stellar distribution +with reference to spectral type, and in studying the +relation of parallax and distance, proper motion, +radial velocity and its variation to the spectral +characteristics of the stars.</p> + +<p>Perrine of Cordova is now establishing on Sierra +Chica about twenty-five miles southwest of Cordova, +a great reflecting telescope comparable in size +with the instruments of the northern hemisphere, +for investigation of the southern nebulæ and +clusters, and motions of the stars. The elevation +of this new Argentine observatory will be 4,000 feet +above sea level.</p> + +<p>Another observatory at mountain elevation and +in a highly favorable climate is the Lowell Observatory, +located at about 7,000 feet elevation at Flagstaff, +Arizona. Many localities were visited and +the atmosphere tested especially for steadiness, an +optical quality very essential for research on the +<span class="pagenum"><a name="Page_p147" id="Page_p147">[147]</a></span> +planetary surfaces. Mexico was one of these stations, +but local air currents and changes of temperature +there were such that good seeing was far from +prevalent, as had been expected. At Flagstaff, on +the other hand, conditions have been pretty uniformly +good, and an enormous amount of work on +the planet Mars has been accumulated and published. +The first successful photographs of this +planet were taken there in 1905, and Jupiter, +Saturn, the zodiacal light and many other test +objects have been photographed, which demonstrates +the excellence of the site for astronomical +research. Within recent years spectrum research +by Slipher, especially on the nebulæ, has been added +to the program, and the rotation and radial velocities +of many nebulæ have been determined.</p> + +<p>On Mount Wilson, near Pasadena, California, at +an elevation of nearly 6,000 feet, is the Carnegie +Solar Observatory, founded and equipped under the +direction of Professor George E. Hale, as a department +of the Carnegie Institution of Washington, of +which Dr. John Campbell Merriam is President. The +climatology of the region was carefully investigated +and tests of the seeing made by Hussey and others. +Although equipped primarily for study of the sun, +the program of the observatory has been widely +amplified to include the stars and nebulæ. The instrumental +equipment is unique in many respects. +To avoid the harmful effect of unsteadiness of air +strata close to the ground a tower 150 feet high was +erected, with a dome surmounting it and covering +a cœlostat with mirror for reflecting the sun's rays +vertically downward. Underneath the tower a dry +well was excavated to a depth equal to ½ the height +of the tower above it. In the subterranean chamber +<span class="pagenum"><a name="Page_p148" id="Page_p148">[148]</a></span> +is the spectroheliograph of exceptional size and +power. The sun's original image is nearly 17 inches +in diameter on the plate, and the solar chromosphere +and prominences, together with the photosphere and +faculæ, are all recorded by monochromatic light.</p> + +<p>Connected with the observatory on Mount Wilson +are the laboratories, offices and instrument shops in +Pasadena, 16 miles distant, where the remarkable +apparatus for use on the mountain is constructed. +A reflecting telescope with silver-on-glass mirror +60 inches in diameter was first built by Ritchey and +thoroughly tested by stellar photographs. Also the +northern spiral nebulæ were photographed, exhibiting +an extraordinary wealth of detail in apparent +star formation. The success of this instrument +paved the way for one similar in design, but with a +mirror 100 inches in diameter, provided by gift of +the late John D. Hooker of Los Angeles. The telescope +was completed in 1919. Notwithstanding its +huge size and enormous weight, the mounting is +very successful, as well as the mirror. Mercurial +bearings counterbalance the weight of the polar +axis in large part. This great telescope, by far the +largest and most powerful ever constructed, is now +employed on a program of research in which its +vast light-gathering power will be utilized to the +full. Under the skillful management of Hale and +his enthusiastic and capable colleagues, the confines +of the stellar heavens will be enormously extended, +and secrets of evolution of the universe and of its +structure no doubt revealed.</p> + +<p>In all the mountain stations hitherto established, +as the Lick Observatory at 4,000 feet, the Mount +Wilson Observatory at 6,000 feet, the Lowell Observatory +at 7,000 feet, the Harvard Observatory at 8,000 +<span class="pagenum"><a name="Page_p149" id="Page_p149">[149]</a></span> +feet; and Teneriffe and Etna at 10,000, Fujiyama +at 12,000, Pike's Peak at 14,000, Mont Blanc and +Mount Whitney at 15,000, the researches that have +been carried on have fully demonstrated the vast +advantage of increased elevation in localities where +climatological conditions as well as elevation are +favorable. Nevertheless, only one-half of the extreme +altitude contemplated by Sir Isaac Newton +has yet been attained.</p> + +<p>Can the greater heights be reached and permanently +occupied? Geographically and astronomically +the most favorably located mountain for a great +observatory is Mount Chimborazo in Ecuador. Its +elevation is 22,000 feet, and it was ascended by Edward +Whymper in 1880. Situated very nearly on +the earth's equator, almost the entire sidereal +heavens are visible from this single station, and all +the planets are favored by circumzenith conditions +when passing the meridian. No other mountain in +the world approaches Chimborazo in this respect. +But the summit is perpetually snow-capped, exceedingly +inaccessible, and the defect of barometric +pressure would make life impossible up there in the +open.</p> + +<p>Only one method of occupation appears to be feasible. +The permanent snow line is at about 16,000 +feet, where excellent water power is available. By +tunneling into the mountain at this point, and diagonally +upward to the summit, permanent occupation +could be accomplished, at a cost not to exceed +one million dollars.</p> + +<p>The rooms of the summit observatory would need +to be built as steel caissons, and supplied with compressed +air at sea-level tension. The practicability +of this plan was demonstrated by the writer in +<span class="pagenum"><a name="Page_p150" id="Page_p150">[150]</a></span> +September, 1907, at Cerro de Pasco, Peru. A steel +caisson was carried up to an elevation exceeding +14,000 feet. Patients suffering acutely with mountain +sickness were placed inside this caisson, and on +restoring the atmospheric pressure within it artificially +all unfavorable symptoms—headache, high respiration +and accelerated pulse—disappeared. There +was every indication that if persons liable to this +uncomfortable complaint were brought up to this +elevation, or indeed any attainable elevation, under +unreduced pressure, the symptoms of mountain +sickness would be unknown. Comfortable occupation +of the highest mountain summits was thereby +assured.</p> + +<p>The working of astronomical instruments from +within air-tight compartments does not present any +insurmountable difficulties, either mechanical or +physical. Since the time these experiments were +made, the Guayaquil-Quito railway has been constructed +over a saddle of Chimborazo, at an elevation +of 12,000 feet; and only six miles of railway +would need to be built from this station to the point +where the tunnel would enter the mountain.</p> + +<p>Only by the execution of some such plan as this +can astronomers hope to overcome the baleful effects +of an ever mobile atmosphere, and secure the advantages +contemplated by Sir Isaac Newton in that +tranquillity of atmosphere, which he conceived as +perpetually surrounding the summits of the highest +mountains.</p> + +<p>In Russell's theory of the progressive development +of the stars, from the giant class to the dwarf, an +element of verification from observation is lacking, +because hitherto no certain method of measuring +the very minute angular diameters of the stars has +<span class="pagenum"><a name="Page_p151" id="Page_p151">[151]</a></span> +been successfully applied. The apparent surface +brightness corresponding to each spectral type is +pretty well known, and by dividing it into the total +apparent brightness, we have the angular area subtended +by the star, quite independent of the star's +distance. This makes it easy to estimate the angular +diameter of a star, and Betelgeuse is the one +which has the greatest angular diameter of all +whose distances we know. Antares is next in order +of angular diameter, 0".043, Aldebaran 0".022, +Arcturus 0".020, Pollux 0".013, and Sirius only +0".007.</p> + +<p>Can these theoretical estimates be verified by +observation? Clearly it is of the utmost importance +and the exceedingly difficult inquiry has been +undertaken with the 100-inch reflector on Mount +Wilson, employing the method of the interferometer +developed by Michelson and described later on, an +instrument undoubtedly capable of measuring much +smaller angles than can be measured by any other +known method. Unquestionably the interference +of atmospheric waves, or in other words what astronomers +call "poor seeing," will ultimately set the +limit to what can be accomplished. "But even if," +says Eddington, "we have to send special expeditions +to the top of one of the highest mountains in +the world, the attack on this far-reaching problem +must not be allowed to languish."</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p152" id="Page_p152">[152]</a></span></p> + +<h2><a name="CHAPTER_XXIII" id="CHAPTER_XXIII"></a>CHAPTER XXIII<br /> +<br /> +THE PROGRAM OF A GREAT OBSERVATORY</h2> + +<p>The Mount Wilson Observatory has now been in +operation about fifteen years. The novelty in +construction of its instruments, the investigations +undertaken with them and the discoveries made, the +interpretation of celestial phenomena by laboratory +experiment, and the recent addition to its equipment +of a telescope 100 inches in diameter, surpassing all +others in power, directs especial attention to the extensive +activities of this institution, whose budget +now exceeds a million dollars annually. Results are +only achieved by a carefully elaborated program, +such as the following, for which the reader is mainly +indebted to Dr. Hale, the director of the observatory, +who gives a very clear idea of the trend of +present-day research on the magnetic nature of the +sun, and the structure and evolution of the sidereal +universe.</p> + +<p>The purpose of the observatory, as defined at its +inception, was to undertake a general study of +stellar evolution, laying especial emphasis upon the +study of the sun, considered as a typical star; +physical researches on stars and nebulæ; and the +interpretation of solar and stellar phenomena by +laboratory experiments. Recognizing that the development +of new instruments and methods afforded +the most promising means of progress, well-equipped +machine shops and optical shops were provided +with this end in view.</p> + +<p><span class="pagenum"><a name="Page_p153" id="Page_p153">[153]</a></span> +The original program of the observatory has +been much modified and extended by the independent +and striking discovery by Campbell and Kapteyn +of an important relationship between stellar +speed and spectral type; the demonstration by +Hertzsprung and Russell of the existence of giant +and dwarf stars; the successful application of +the 60-inch reflector by Van Maanen to the measurement +of minute parallaxes of stars and nebulæ; the +important developments of Shapley's investigation +of globular star clusters; the possibilities of research +resulting from Seares's studies in stellar photometry; +and the remarkable means of attack developed +by Adams through the method of spectroscopic +parallaxes.</p> + +<p>By this method the absolute magnitude, and hence +the distance of a star is accurately determined from +estimates of the relative intensities of certain lines +in stellar spectra. Attention was first directed toward +lines of this character in 1906, when it was +inferred that the weakening of some lines in the +spectra of sun spots and the strengthening of others +was the result of reduced temperature of the spot +vapors. On testing this hypothesis by laboratory +experiments, it was fully verified.</p> + +<p>Subsequently Adams, who had thus become familiar +with these lines and their variability, studied +them extensively in the spectra of other stars. In +this way was discovered the dependence of their +relative intensities on the star's absolute magnitude, +so providing the powerful method of spectroscopic +parallaxes.</p> + +<p>This method, giving the absolute magnitude as +well as the distance of every star (excepting those +of the earliest type) whose spectrum is photographed, +<span class="pagenum"><a name="Page_p154" id="Page_p154">[154]</a></span> +is no less important from the evolutional +than from the structural point of view.</p> + +<p>Investigations in solar physics which formerly +held chief place in the research program have +developed along unexpected lines. It could not be +foreseen at the outset that solar magnetic phenomena +might become a subject of inquiry, demanding +special instrumental facilities, and throwing light +on the complex question of the nature of the sun +spots and other solar problems of long standing. It +is obvious that these researches, together with those +on the solar rotation and the motions of the solar +atmosphere, developed by Adams and St. John, must +be carried to their logical conclusion, if they are to +be utilized to the fullest in interpreting stellar and +nebular phenomena.</p> + +<p>The discovery of solar magnetism, like many +other Mount Wilson results, was the direct outcome +of a long series of instrumental developments. The +progressive improvement and advance in size of the +tools of research was absolutely necessary. Hale's +first spectroheliograph at Kenwood in 1890 was attached +to a 12-inch refractor, and the solar image +was but two inches in diameter. It was soon found +that a larger solar image was essential, and a spectrograph +of much greater linear dispersion; in fact, +the spectrograph must be made the prime element +in the combination, and the telescope so designed +as to serve as a necessary auxiliary.</p> + +<p>Accordingly, successive steps have led through +spectrographs of 18 and 30 feet dimension to a +vertical spectrograph 75 feet in focal length. The +telescope is the 150 feet tower telescope, giving a +solar image of 16.5 inches in diameter. Its spectrograph +is massive in construction, and by extending +<span class="pagenum"><a name="Page_p155" id="Page_p155">[155]</a></span> +deep into the earth, it enjoys the stability and constancy +of temperature required for the most exacting +work.</p> + +<p>Another direct outgrowth of the work of sun-spot +spectra is a study of the spectra of red stars, where +the chemistry of these coolest regions of the sun is +partially duplicated. The combination of titanium +and oxygen, and the significant changes of line intensity +already observed in both instances, and also +in the electric furnace at reduced temperatures, give +indication of what may be expected to result from +an attack on the spectra of the red stars with more +powerful instrumental means, which is now provided +by the 100-inch telescope and its large stellar +spectrograph.</p> + +<p>Other elements in the design of the 100-inch +Hooker telescope have the same general object in +view—that of developing and applying in astronomical +practice the effective research methods suggested +by recent advances in physics. Fresh possibilities +of progress are constantly arising, and these +are utilized as rapidly as circumstances permit.</p> + +<p>The policy of undertaking the interpretations of +celestial phenomena by laboratory experiments, an +important element in the initial organization of +Mount Wilson, has certainly been justified by its +results. Indeed, the development of many of the +chief solar investigations would have been impossible +without the aid of special laboratory studies, +going hand in hand with the astronomical observations. +So indispensable are such researches, and so +great is the promise of their extension, that the +time has now come for advancing the laboratory +work from an accessory feature to full equality with +the major factors in the work of the observatory. +<span class="pagenum"><a name="Page_p156" id="Page_p156">[156]</a></span> +Accordingly a new instrument now under installation +is an extremely powerful electro-magnet, designed +by Anderson for the extension of researches +on the Zeeman effect, and for other related investigations. +Within the large and uniform field of this +magnet, which is built in the form of a solenoid, a +special electric furnace, designed for this purpose +by King, is used for the study of the inverse Zeeman +effect at various angles with the lines of force. +This will provide the means of interpreting certain +remarkable anomalies in the magnetic phenomena +of sun spots.</p> + +<p>The 100-inch telescope is now in regular use. All +the tests so far applied show that it greatly surpasses +the 60-inch telescope in every class of work. +For many months most of the observations and photographs +have been made with the Cassegrain combination +of mirrors, giving an equivalent focal +length of 134 feet and involving three reflections of +light. The 100-inch telescope is found to give nearly +2.8 times as much light as the 60-inch telescope, and +therefore extends the scope of the instrument to all +the stars an entire magnitude fainter. This is a +very important gain for research on the faint +globular clusters, as well as the small and faint +spiral and planetary nebulæ, providing a much +larger scale for these objects and sufficient light at +the same time. Photographs of the moon and many +other less critical tests have been made with very +satisfactory results. Those of the moon appear to +be decidedly superior in definition to any previously +taken with other instruments.</p> + +<p>Another investigation is of great importance in +the light of recent advances in theoretical dynamics. +Darwin, in his fundamental researches on the dynamics +<span class="pagenum"><a name="Page_p157" id="Page_p157">[157]</a></span> +of rotating masses, dealt with incompressible +matter, which assumes the well-known pear-shaped +figure, and may ultimately separate into two +bodies. Roche on the other hand discussed the evolution +of a highly compressible mass, which finally +acquires a lens-shaped form and ejects matter at +its periphery. Both of these are extreme cases. +Jeans has recently dealt with intermediate cases, +such as are actually encountered in stars and +nebulæ. He finds that when the density is less +than about one-fourth that of water, a lens-shaped +figure will be produced with sharp edges, as depicted +by Roche. Matter thrown off at opposite +points on the periphery, under the influence of small +tidal forces from neighboring masses, may take the +form of two symmetric filaments, though it is not +yet entirely clear how these may attain the characteristic +configuration of spiral nebulæ. The preliminary +results of Van Maanen indicate motion +outward along the arms, in harmony with Jeans's +views.</p> + +<p>Jeans further discusses the evolution of the arms, +which will break up into nuclei (of the order of +mass of the sun) if they are sufficiently massive, but +will diffuse away if their gravitational attraction is +small. The mass of our solar system is apparently +not great enough, according to Jeans, to account for +its formation in this way. As is apparent, these investigations +lead to conclusions very different from +those derived by Chamberlin and Moulton from the +planetesimal hypothesis.</p> + +<p>This is a critical study of spiral nebulæ for which +the 100-inch telescope is of all instruments in existence +the best suited. The spectra of the spirals +must be studied, as well as the motions of the matter +<span class="pagenum"><a name="Page_p158" id="Page_p158">[158]</a></span> +composing the arms. Their parallaxes, too, must be +ascertained. A photographic campaign including +spiral nebulæ of various types will settle the question +of internal motions. The large scale of the +spiral nebulæ at the principal focus of the Hooker +telescope, and the experience gained in the measurement +of nebular nuclei for parallax determination, +will help greatly in this research. A multiple-slit +spectrograph, already applied at Mount Wilson, will +be employed, not only on spiral nebulæ whose plane +is directed toward us, but also on those whose plane +lies at an angle sufficient to permit both components +of motion to be measured by the two methods.</p> + +<p>In dealing with problems of structure and motion +in the Galactic system, the 100-inch telescope offers +especial advantages, because of its vast light-gathering +power. Studies of radial velocities of the stars +have hitherto been necessarily confined to the +brighter stars, for the most part even to those visible +to the naked eye. While some of these are very +distant, most of the stars whose radial velocities are +known belong to a very limited group, perhaps constituting +a distinct cluster of which the sun is a +member, but in any event of insignificant proportions +when contrasted with the Galaxy. Current +spectrographic work with the 60-inch telescope includes +stars of the eighth magnitude, and some even +fainter. But while the 60-inch has enabled Adams +to measure the distances of many remote stars by +his new spectroscopic method, and to double the +known extent (so far as spectroscopic evidence is +concerned) of the star streams of Kapteyn, a much +greater advance into space is necessary to find out +the community of motion among the stars comprising +the Galactic system. The Hooker telescope will +<span class="pagenum"><a name="Page_p159" id="Page_p159">[159]</a></span> +enable us to determine accurate radial velocities +to stars of the eleventh magnitude, which doubtless +truly represent the Galaxy.</p> + +<p>In order to secure a maximum return within a +reasonable period of time, the stars in the selected +areas of Kapteyn will be given the preference, because +of the vast amount of work already done, relating +to their positions, proper motions, and visual +and photographic magnitudes. Such consideration +as spectral type, the known directions of star-streaming, +and the position of the chosen regions +with reference to the plane of the Galaxy are given +adequate weight, and it is of fundamental importance +that the method of spectroscopic parallaxes +will permit dwarf stars to be distinguished from +stars that are in the giant class, but rendered faint +by their much greater distance. In addition to these +problems, the stellar spectrograms will provide rich +material for study of the relationship between +stellar mass and speed, and the nature of giant +stars and dwarf stars.</p> + +<p>Shapley's recent studies of globular clusters have +indicated the significance of these objects in both +evolutional and structural problems, and the possibility +of determining their parallaxes by a number +of independent methods is of prime importance, both +in its bearing on the structure of the universe and +because it permits a host of apparent magnitudes to +be at once transformed into absolute magnitudes. +Here the advantage of the Hooker telescope is two-fold: +at its 134-foot focus the increased scale of +the crowded clusters makes it possible to select +separate stars for spectrum photography (which +could not be done with the 60-inch where the images +were commingled); and the great gain in light is +<span class="pagenum"><a name="Page_p160" id="Page_p160">[160]</a></span> +such that the spectra of stars to the 14th magnitude +have been photographed in less than an hour.</p> + +<p>Faint globular clusters, then, will comprise a large +part of the early program with the 100-inch telescope: +the faintest possible stars in them must be detected +and their magnitudes and colors measured; +spectral types must be determined, and the radial +velocities of individual stars and of clusters as a +whole; spectroscopic evidence of possible axial rotation +of globular clusters must be searched for; and +the method of spectroscopic parallaxes, as well as +other methods, must be applied to ascertaining the +distances of these clusters.</p> + +<p>The possibility of dealing with many problems +relating to the distribution and evolution of the +faintest stars depends upon the establishment of +photographic and photovisual magnitude scales. +Below the twelfth magnitude, the only existing scale +of standard visual or photovisual magnitudes is the +Mount Wilson sequence, already extended by Seares +to magnitude 17.5 with the 60-inch telescope.</p> + +<p>Extension of this scale to even fainter magnitudes, +and its application to the faintest stars within +its range is an important task for this great telescope, +as it will doubtless bring within range hundreds +of millions of stars that are beyond the reach +of the 60-inch. The giants among them will form for +us the outer boundary of the Galactic system, while +the dwarfs will be of almost equal interest from the +evolutional standpoint. The photometric program +of the 100-inch, then, will deal with such questions +as the condensation of the fainter stars toward the +Galactic plane, the color of the most distant stars, +and the final settlement of the long inquiry regarding +the possible absorption of light in space.</p> + +<p><span class="pagenum"><a name="Page_p160p1" id="Page_p160p1">[160i]</a></span></p> + +<div class="fig_center" style="width: 646px;"> +<img src="images/p160_1.png" width="646" height="501" alt="" /> +<div class="fig_caption"><span class="smcap">Great Sun-Spot Group, August 8, 1917.</span> The disk in the lower left corner represents the +comparative size of the earth. (<i>Photo, Mt. Wilson Solar Observatory.</i>)</div> +</div> + +<p><span class="pagenum"><a name="Page_p160p2" id="Page_p160p2">[161i]</a></span></p> + +<div class="fig_center" style="width: 641px;"> +<img src="images/p160_2top.png" width="641" height="522" alt="" /> +<div class="fig_caption"><span class="smcap">The Sun's Disk.</span> The view shows the "rice grain" structure of the +photosphere and brilliant calcium flocculi. (<i>Photo, Yerkes Observatory.</i>)</div> +</div> + +<div class="fig_center" style="width: 651px;"> +<img src="images/p160_2bot.png" width="651" height="519" alt="" /> +<div class="fig_caption"><span class="smcap">The Lunar Surface Visible During a Total Eclipse of the Moon, +February 8, 1906.</span> (<i>Photo, Yerkes Observatory.</i>)</div> +</div> + +<p><span class="pagenum"><a name="Page_p161" id="Page_p161">[161]</a></span> +Another research of exceptional promise will be +undertaken, which is of great importance in a general +study of stellar evolution; and that is the determination +of the spectral-energy curves of stars +of various classes, for the purpose of measuring +their surface temperatures. A very few of the +nebulæ are found to be variable, and their peculiarities +need investigation, also special problems of +variable stars and temporary stars, and the spectra +of the components of close double stars which are +beyond the power of all other instruments to +photograph.</p> + +<p>Such a program of research conveys an excellent +idea of many of the great problems that are +under investigation by astronomers to-day, and gives +some notion of the instrumental means requisite in +executing comprehensive plans of this character. +It will not escape notice that the climax of instrumental +development attained at Mount Wilson has +only been made possible by an unbroken chain of +progress, link by link, each antecedent link being +necessary to the successful forging of its following +one. In very large part, and certainly indispensable +to these instrumental advances, has the art of working +in glass and metals been the mainstay of research. +As we review the history of astronomical +progress, from Galileo's time to our own, the consummate +genius of the artisan and his deft handiwork +compel our admiration almost equally with the +keen intelligence of the astronomer who uses these +powerful engines of his own devising to wrest the +secrets of nature from the heavens.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p162" id="Page_p162">[162]</a></span></p> + +<h2><a name="CHAPTER_XXIV" id="CHAPTER_XXIV"></a>CHAPTER XXIV<br /> +<br /> +OUR SOLAR SYSTEM</h2> + +<p>Now let us go upward in imagination, far, far beyond +the tops of the highest mountains, beyond +the moon and sun, and outward in space until we +reach a point in the northern heavens millions and +millions of miles away, directly above and equally +distant from all points in the ecliptic, or path in +which our earth travels yearly round the sun. Then +we should have that sort of comprehensive view of +the solar system which is necessary if we are to +visualize as a whole the working of the vast machine, +and the motions, sizes, and distances of all the bodies +that comprise it. Of such stupendous mechanism our +earth is part.</p> + +<p>Or in lieu of this, let us attempt to get in mind a +picture of the solar system by means of Sir William +Herschel's apt illustration: "Choose any well-leveled +field. On it place a globe two feet in diameter. This +will represent the sun; Mercury will be represented +by a grain of mustard seed on the circumference of +a circle 164 feet in diameter for its orbit; Venus, +a pea on a circle of 284 feet in diameter; the Earth +also a pea, on a circle of 430 feet; Mars a rather +larger pin's head on a circle of 654 feet; the asteroids, +grains of sand in orbits of 1,000 to 1,200 feet; +Jupiter, a moderate sized orange in a circle of nearly +half a mile across; Saturn, a small orange on a +circle of four-fifths of a mile; Uranus, a full-sized +<span class="pagenum"><a name="Page_p163" id="Page_p163">[163]</a></span> +cherry or small plum upon the circumference of a +circle more than a mile and a half; and finally Neptune, +a good-sized plum on a circle about two miles +and a half in diameter…. To imitate the motions of +the planets in the above mentioned orbits, Mercury +must describe its own diameter in 41 seconds; Venus +in 4 minutes, 14 seconds; the Earth in 7 minutes; +Mars in 4 minutes 48 seconds; Jupiter in 2 minutes +56 seconds; Saturn in 3 minutes 13 seconds; Uranus +in 2 minutes 16 seconds; and Neptune in 3 minutes +30 seconds."</p> + +<p>Now, let us look earthward from our imaginary +station near the north pole of the ecliptic. All +these planetary bodies would be seen to be traveling +eastward round the sun, that is, in a counter-clockwise +direction, or contrary to the motions of the +hands of a timepiece. Their orbits or paths of motion +are very nearly circular, and the sun is practically +at the center of all of them except Mercury and +Mars; of Venus and Neptune, almost at the absolute +center. The planes of all their orbits are very nearly +the same as that of the ecliptic, or plane in which +the earth moves. These and many other resemblances +and characteristics suggest a uniformity of +origin which comports with the idea of a family, and +so the whole is spoken of as the solar system, or +the sun and his family of planets.</p> + +<p>In addition to the nine bodies already specified, +the solar system comprises a great variety of other +and lesser bodies; no less than twenty-six moons or +satellites tributary to the planets and traveling +round them in various periods as the moon does +round our earth. Then between the orbits of Mars +and Jupiter are many thousands of asteroids, so +called, or minor planets (about 1,000 of them have +<span class="pagenum"><a name="Page_p164" id="Page_p164">[164]</a></span> +actually been discovered, and their paths accurately +calculated). And at all sorts of angles with the +planetary orbits are the paths of hundreds of comets, +delicate filmy bodies of a wholly different constitution +from the planets, and which now and then blaze +forth in the sky, their tails appearing much like the +beam of a searchlight, and compelling for the time +the attention of everybody. Connected with the +comets and doubtless originally parts of them are +uncounted millions of millions of meteors, which +for the time become a part of the solar system, their +minute masses being attracted to the planets, upon +which they fall, those hitting the earth being visible +to us as familiar shooting stars.</p> + +<p>We next follow the story of astronomy through +the solar system, beginning with the sun itself and +proceeding outward through his family of planets, +now much more numerous and vastly more extended +than it was to the ancient world, or indeed till +within a century and a half of our own day.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p165" id="Page_p165">[165]</a></span></p> + +<h2><a name="CHAPTER_XXV" id="CHAPTER_XXV"></a>CHAPTER XXV<br /> +<br /> +THE SUN AND OBSERVING IT</h2> + +<p>As lord of day, king of the heavens, mankind +in the ancient world adored the sun. By their +researches into the epoch of the Assyrians, Hittites, +Phœnicians and other early peoples now passed +from earth, archæologists have unearthed many +monuments that evidence the veneration in which +the early peoples who inhabited Egypt and Asia +Minor many thousand years ago held the sun. A +striking example is found in the architecture of +early Egyptian temples, on the lintels of which are +carved representations of the winged globe or the +winged solar disk, and there is a bare possibility that +the wings of the globe were suggested by a type of +the solar corona as glimpsed by the ancients.</p> + +<p>Little knew they about the distance and size of +the sun; but the effects of his light and heat upon all +vegetal and animal life were obvious to them. Doubtless +this formed the basis for their worship of the +sun. Occasional huge spots must have been visible +to the naked eye, and the sun's corona was seen at +rare intervals. Plutarch and Philostratus describe +it very much as we see it to-day.</p> + +<p>How completely dependent mankind is upon the +sun and its powerful radiations, only the science +of the present day can tell us. By means of the sun's +heat the forests of early geologic ages were enabled +to wrest carbon from the atmosphere and store it in +<span class="pagenum"><a name="Page_p166" id="Page_p166">[166]</a></span> +forms later converted by nature's chemistry into +peat and coal. Through processes but imperfectly +understood, the varying forms of vegetable life are +empowered to conserve, from air and soil, nitrogen +and other substances suitable for and essential to the +life maintenance of animal creatures. Breezes that +bring rain and purify the air; the energy of water +held under storage in stream and dam and fall; +trade winds facilitating commerce between the continents; +oceanic currents modifying coastal climates; +the violence of tornado, typhoon and water-spout, +together with other manifestations of natural +forces—all can be traced back to their origin in the +tremendous heating power of the solar rays. In +everything material the sun is our constant and +bountiful benefactor. If his light and heat were +withdrawn, practically every form of human activity +on this planet would come to an early end.</p> + +<p>How far away is the sun? What is the size of the +sun? These are questions that astronomers of the +present day can answer with accuracy.</p> + +<p>So closely do they know the sun's distance that +it is employed as their yardstick of the sky, or +unit of celestial measurement. Many methods have +been utilized in ascertaining the distance of the sun, +and the remarkable agreement among them all is +very extraordinary. Some of them depend upon +pure geometry, and the basic measure which we +make from the earth is not the distance of the sun +directly; but we find out how far away Venus is +during a transit of Venus, for example, or how far +away Mars is or some of the asteroids are at their +closer oppositions. Then it is possible to calculate +how far away the sun is, because one measurement +of distance in the solar system affords us the scale +<span class="pagenum"><a name="Page_p167" id="Page_p167">[167]</a></span> +on which the whole structure is built. But perhaps +the simplest method of getting the sun's distance is +by the velocity of light, 186,300 miles a second. From +eclipses of Jupiter's moons we know that light takes +8 minutes 20 seconds to pass from sun to earth. +So that the sun's distance is the simple product of +the two, or 93 millions of miles.</p> + +<p>Once this fundamental unit is established, we have +a firm basis on which to build up our knowledge of +the distances, the sizes and motions of the heavenly +bodies, especially those that comprise the solar system. +We can at once ascertain the size of the sun, +which we do by measuring the angle which it fills, +that is, the sun's apparent diameter. Finding this +to be something over a half a degree in arc, the processes +of elementary trigonometry tell us that the +sun's globe is 865,000 miles in diameter. For nearly +a century this has been accurately measured with +the greatest care, and diameters taken in every +direction are found to be equal and invariably the +same. So we conclude that the sun is a perfect +sphere, and so far as our instruments can inform +us, its actual diameter is not subject to appreciable +change.</p> + +<p>The vastness of the sun's volume commands our +attention. As his diameter is 110 times that of the +earth, his mere size or volume is 110×110×110 or +1,300 thousand times that of the earth, because the +volumes of spheres are in proportion as the cubes +of their diameters. If the materials that compose +the sun were as heavy as those that make up the +earth, it would take 1,300 thousand earths to weigh +as much as the sun does. But by a method which we +need not detail here, the sun's actual weight or +mass is found to be only 300 thousand (more nearly +<span class="pagenum"><a name="Page_p168" id="Page_p168">[168]</a></span> +330,000), times greater than the earth's. So we must +infer that, bulk for bulk, the component materials +of the sun are about one-fourth lighter than those +of the earth, that is, about one and one-half times +as dense as water.</p> + +<p>To look at this in another way: it is known that +a body falling freely toward the earth from outer +space would acquire a speed of seven miles a second, +whereas if it were to fall toward the sun instead, +the velocity would be 383 miles a second on reaching +his surface. If all the other bodies of the solar +system, that is, the earth and moon, all the planets +and their satellites, the comets and all were to be +fused together in a single globe, it would weigh only +one-seven hundred and fiftieth as much as the sun +does.</p> + +<p>At the surface, however, the disproportion of +gravity is not so great, because of the sun's vast +size: it is only about twenty-eight times greater on +the sun than on the earth; and instead of a body +falling 16 feet the first second as here, it would fall +444 feet there. Pendulums of clocks on the sun +would swing five times for every tick here, and an +athlete's running high jump would be scaled down +to three inches.</p> + +<p>Let us next inquire into the amount of the sun's +light and heat, and the enormously high temperature +of a body whose heat is so intense even at the vast +distance at which we are from it. The intensity +of its brightness is such that we have no artificial +source of light that we can readily compare it with. +In the sky the next object in brightness is the full +moon, but that gives less than the half-millionth +part as much light as the sun. The standard candle +used in physics gives so little light in comparison +<span class="pagenum"><a name="Page_p169" id="Page_p169">[169]</a></span> +that we have to use an enormous number to express +the quantity of light that the sun gives.</p> + +<p>A sperm candle burning 120 grains hourly is the +standard, and if we compare this with the sun when +overhead, and allow for the light absorbed by the atmosphere, +we get the number 1575 with twenty-four +ciphers following it, to express the candlepower of +the sun's light. If we interpose the intense calcium +light or an electric arc light between the eye and +the sun, these artificial sources will look like black +spots on the disk. Indeed, the sun is nearly four +times brighter than the "crater," or brightest part +of the electric arc. The late Professor Langley at +a steel works in Pennsylvania once compared direct +sunlight with the dazzling stream of molten metal +from a Bessemer converter; but bright as it was, sunlight +was found to be five thousand times brighter.</p> + +<p>Equally enormous is the heat of the sun. Our intensest +sources of artificial heat do not exceed 4,000 +degrees Fahrenheit, but the temperature at the sun's +surface is probably not less than 16,000 degrees F. +One square meter of his surface radiates enough +heat to generate 100,000 horsepower continuously. +At our vast distance of 93 millions of miles, the sun's +heat received by the earth is still powerful enough to +melt annually a layer of ice on the earth more than +a hundred feet in thickness. If the solar heat that +strikes the deck of a tropical steamship could be +fully utilized in propelling it, the speed would reach +at least ten knots.</p> + +<p>Many attempts have been made in tropical and +sub-tropical climates to utilize the sun's heat directly +for power, and Ericsson in Sweden, Mouchot in +France, and Shuman in Egypt have built successful +and efficient solar engines. Necessary intermission +<span class="pagenum"><a name="Page_p170" id="Page_p170">[170]</a></span> +of their power at night, as well as on cloudy days, +will preclude their industrial introduction until +present fuels have advanced very greatly in cost. +All regions of the sun's disk radiate heat uniformly, +and the sun's own atmosphere absorbs so much that +we should receive 1.7 times more heat if it were removed. +So far as is known, solar light and heat are +radiated equally in all directions, so that only a very +minute fraction of the total amount ever reaches the +earth, that is, 1 2200 millionth part of the whole. +Indeed all the planets and other bodies of the solar +system together receive only one one hundred millionth +part; the vast remainder is, so far as we +know, effectively wasted. It is transformed, but +what becomes of it, and whether it ever reappears in +any other form, we cannot say.</p> + +<p>How is this inconceivably vast output of energy +maintained practically invariable throughout the +centuries? Many theories have been advanced, but +only one has received nearly universal assent, that +of secular contraction of the sun's huge mass upon +itself. Shrinkage means evolution of heat; and it is +found by calculation that if the sun were to contract +its diameter by shrinking only two-hundred and fifty +feet per year, the entire output of solar heat might +thus be accounted for. So distant is the sun and so +slow this rate of contraction that centuries must +elapse before we could verify the theory by actual +measurements. Meanwhile, the progress of physical +research on the structure and elemental properties +of matter has brought to light the existence of +highly active internal forces which are doubtless +intimately concerned in the enormous output of +radiant energy, though the mechanism of its maintenance +is as yet known only in part.</p> + +<p><span class="pagenum"><a name="Page_p171" id="Page_p171">[171]</a></span> +Abbot, from many years' observations of the +solar constant, at Washington, on Mount Wilson, +and in Algeria, finds certain evidence of fluctuation +in the solar heat received by the earth. It cannot +be a local phenomenon due to disturbances in our +atmosphere, but must originate in causes entirely +extraneous to the earth. Interposition of meteoric +dust might conceivably account for it, but there is +sufficient evidence to show that the changes must be +attributed to the sun itself. The sun, then, is a variable +star; and it has not only a period connected +with the periodicity of the sun spots, but also an +irregular, nonperiodic variation during a cycle of a +week or ten days, though sometimes longer, and occasioning +irregular fluctuations of two to ten per +cent of the total radiation. Radiation is found to +increase with the spottedness.</p> + +<p>Attempts have been made on the basis of the contraction +theory to find out the past history of the sun +and to predict its future. Probably 20 to 50 millions +of years in the past represents the life of the sun +much as it is at present; and if solar radiation in +the future is maintained substantially as now, the +sun will have shrunk to one-half its present diameter +in the next five million years.</p> + +<p>So far then as heat and light from the sun are +concerned, the sun may continue to support life on +the earth not to exceed ten million years in the +future. But the sun's own existence, independently +of the orbs of the system dependent upon it, might +continue for indefinite millions of aeons before it +would ever become a cold dead globe; indeed, in +the present state of science, we cannot be sure +that it is destined to reach that condition within +calculable time.</p> + +<p><span class="pagenum"><a name="Page_p172" id="Page_p172">[172]</a></span> +A few words on observing the sun, an object much +neglected by amateurs. On account of the intense +light, a very slight degree of optical power is sufficient. +Indeed a piece of window glass, smoked in a +candle flame with uniform graduation from end to +end, will be found worth while in a beginner's daily +observation of the sun. The glass should be smoked +densely enough at one end so that the sunlight as +seen through it will not dazzle the eye on the clearest +days. At the other end of the glass, the degree of +smoke film should not be quite so dense, so that +the sun can be examined on hazy, foggy or partly +cloudy days. An occasional naked-eye spot will reward +the patient observer.</p> + +<p>If a small spyglass, opera glass or field glass is +at hand, excellent views of the sun may be had by +mounting the glass so that it can be held steadily +pointed on the sun, and then viewing the disk by projection +on a white card or sheet of paper. Care +must be taken to get a good focus on the projected +image, and then the faculæ, or whitish spots, or +mottling nearer the sun's edge will usually be well +seen. By moving the card farther away from the +eyepiece, a larger disk may be obtained, in effect +a higher degree of magnification. But care must be +used not to increase it too much. Keep direct sunlight +outside the tube from falling on the card where +the image is being examined. This is conveniently +done by cutting a large hole, the size of the brass +cell of the object glass, through a sheet of corrugated +strawboard, and slipping this on over the cell. In +this way the spots on the sun can be examined with +ease and safety to the eye.</p> + +<p>For large instruments a special type of eyepiece +is provided known as a helioscope, which disposes of +<span class="pagenum"><a name="Page_p173" id="Page_p173">[173]</a></span> +the intense heat rays that are harmful to the eye. +Frequent examination of the eyepiece should be +made and the eyepiece cooled if necessary. That +part of the sun's surface under observation is known +as the photosphere, that is, the part which radiates +light. If the atmosphere admits the use of high +magnifying powers, the structure of the photosphere +will be found more and more interesting the higher +the power employed. It is an irregularly mottled +surface showing a species of rice-grain structure +under fairly high magnification. These grains are +grouped irregularly and are about 500 miles across. +Under fine conditions of vision they may be subdivided +into granules. The faculæ, or white spots, +are sometimes elevations above the general solar +level; they have occasionally been seen projecting +outside the limb, or edge of the disk.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p174" id="Page_p174">[174]</a></span></p> + +<h2><a name="CHAPTER_XXVI" id="CHAPTER_XXVI"></a>CHAPTER XXVI<br /> +<br /> +SUN SPOTS AND PROMINENCES</h2> + +<p>Dark spots of a deep bluish black will often be +seen on the photosphere of the sun. Sometimes +single, though generally in groups, the larger ones +will have a dark center, called the umbra, surrounded +by the very irregular penumbra which is +darker near its outer edge and much brighter apparently +on its inner edge where it joins on the +umbra. The penumbra often shows a species of +thatch-work structure, and systematic sketches of +sun spots by observers skilled in drawing are greatly +to be desired, because photography has not yet +reached the stage where it is possible to compete +with visual observation in the matter of fine detail. +The spots themselves nearly always appear like depressions +in the photosphere, and on repeated +occasions they have been seen as actual notches +when on the edge of the sun.</p> + +<p>Many spots, however, are not depressions: some +appear to be actual elevations, with the umbra perhaps +a central depression, like the crater in the +general elevation of a volcano. Spots are sometimes +of enormous size. The largest on record was +seen in 1858; it was nearly 150,000 miles in breadth, +and covered a considerable proportion of the whole +visible hemisphere of the sun. A spot must be +nearly 30,000 miles across in order to be seen with +the naked eye.</p> + +<p><span class="pagenum"><a name="Page_p175" id="Page_p175">[175]</a></span> +In their beginning, development, and end, each +spot or group of spots appears to be a law unto +itself. Sometimes in a few hours they will form, +though generally it is a question of days and even +weeks. Very soon after their formation is complete, +tonguelike encroachments of the penumbra +appear to force their way across the umbra, and this +splitting up of the central spot usually goes on quite +rapidly. Sun spots in violent disturbance are rarely +observed. As the sun turns round on his axis, the +spots will often be carried across the disk from the +center to the edge, when they become very much +foreshortened. The sun's period of rotation is 28 +days, so that if a spot lasts more than two weeks +without breaking up, it may reappear on the eastern +limb of the sun after having disappeared at the +western edge. Two or three months is an average +duration for a spot; the longest on record lasted +through 18 months in 1840-41.</p> + +<p>The position of the sun's axis is well known, its +equator being tilted about 7 degrees to the ecliptic, +and the spots are distributed in zones north and +south of the equator, extending as far as 30 degrees +of solar latitude. In very high latitudes spots are +never seen; they are most abundant in about latitude +15 degrees both north and south, and rather more +numerous in the northern than in the southern +hemisphere of the sun. Recent research at Mount +Wilson makes the sun a great magnet; and its +magnetic axis is inclined at an angle of 6 degrees +to the axis of rotation, around which it revolves in +32 days.</p> + +<p>There is a most interesting periodicity of the +spots on the sun, for months will sometimes elapse +with spots in abundance and visible every day, while +<span class="pagenum"><a name="Page_p176" id="Page_p176">[176]</a></span> +at other periods, days and even weeks will elapse +without a single spot being seen. There is a well +recognized period of eleven and one-tenth years, the +reason underlying which is not, however, known. +After passing through the minimum of spottedness, +they begin to break out again first in latitudes of +25 degrees-30 degrees, rather suddenly, and on +both sides of the equator, and they move toward +the equator as their number and individual size +decrease.</p> + +<p>The last observed epoch of maximum spot activity +on the sun was passed in 1917.</p> + +<p>Many attempts have been made to ascertain the +cause of the periodicity of sun spots, but the real +cause is not yet known. If the spots are eruptional +in character, the forces held in check during seasons +of few spots may well break out in period. The +brighter streaks and mottlings known as faculæ are +probably elevations above the general photosphere, +and seem to be crusts of luminous matter, often incandescent +calcium, protruding through from the +lower levels. Generally the faculæ are numerous +around the dark spots, and absorption of the sun's +light by his own atmosphere affords a darker background +for them, with better visibility nearer the +rim of the solar disk. The spectroheliograph reveals +vast zones of faculæ otherwise invisible, +related to the sun-spot zones proper on both sides +of the equator.</p> + +<p>In some intimate way the magnetism of sun and +earth are so related that outbreaks of solar spots +are accompanied with disturbances of electrical and +other instruments on the earth; also the aurora +borealis is seen with greater frequency during +periods when many spots are visible.</p> + +<p><span class="pagenum"><a name="Page_p177" id="Page_p177">[177]</a></span> +Within very recent years the discovery of a magnetic +field in sun spots has been made by Hale with +powerful instruments of his own design. Sun spots +had never been investigated before with adequate +instrumental means. He recognized the necessity +of having a spectroscope that would record the +widened lines of sun-spot spectra, and the strengthened +and weakened lines on a large scale. Certain +changes in relative intensity were traced to a reduced +temperature of the spot vapors by comparison +with photographs of the spectrum of iron and +other metallic vapors in an electric arc at different +temperatures. Here the work of the laboratory was +essential. Sun spots were thus found to be regions +of reduced temperature in the solar atmosphere. +Chemical unions were thus possible, and thousands +of faint lines in spot-spectra were measured and +identified as band lines due to chemical compounds. +Thus the chemical changes at work in sun-spot +vapors were recognized.</p> + +<p>Then followed the highly significant investigations +of solar vortices and magnetic fields. Improvements +in photographic methods had revealed immense +vortices surrounding sun spots in the higher +part of the hydrogen atmosphere; and this led to the +hypothesis that a sun spot is a solar storm, resembling +a terrestrial tornado, and in which the hot +vapors whirling at high velocity are cooled by expansion. +This would account for the observed intensity +changes of the spectrum lines and the presence +of chemical compounds. The vortex hypothesis suggested +an explanation of the widening of many spot +lines, and the doubling or trebling of some of them. +As it is known that electrons are emitted by hot +bodies, they must be present in vast numbers in the +<span class="pagenum"><a name="Page_p178" id="Page_p178">[178]</a></span> +sun; and positive or negative electrons, if caught +and whirled in a vortex, would produce a magnetic +field.</p> + +<p>Zeeman in 1896 had discovered that the lines in +the spectrum of a luminous vapor in a magnetic +field are widened, or even split into several components +if the field is strong enough. Characteristic +effects of polarization appear also. The new apparatus +of the observatory in conjunction with experiments +in the laboratory immediately provided +evidence that proved the existence of magnetic fields +in sun spots, and strengthened the view that the +spots are caused by electric vortices.</p> + +<p>Extended investigations have led Hale to the conclusion +that the sun itself is a magnet, with its poles +situated at or near the poles of rotation. In this respect +the sun resembles the earth, which has long +been known to be a magnet. The sun's axial rotation +permits investigation of the magnetic phenomena +of all parts of its surface, so that ultimately +the exact position of the sun's magnetic poles and the +intensity of the field at different levels in the solar +atmosphere will be ascertained. Schuster is of the +opinion that not only the sun and earth, but every +star, and perhaps every rotating body, becomes a +magnet by virtue of its rotation. Hale is confident +that the 100-inch reflector will permit the test for +magnetism to be applied to a few of the stars.</p> + +<p>The sun can be observed at Mount Wilson on at +least nine-tenths of all the days in the year, and a +daily record of the polarities of all spots with the +150-foot tower telescope is a part of the routine. A +method has been devised for classifying sun spots on +the basis of their magnetic properties, and more +than a thousand spots have already been so classified. +<span class="pagenum"><a name="Page_p179" id="Page_p179">[179]</a></span> +About 60 per cent of all sun spots are found +to be binary groups, the single or multiple members +of which are of opposite magnetic polarity. Unipolar +spots are very seldom observed without some +indication of the characteristics of bipolar groups. +These are usually exhibited in the form of flocculi +following the spot. The bipolar spot seems to be +the dominant type, and the unipolar type a variant +of it.</p> + +<p>Although devised for quite another purpose, that +of photographing the hydrogen prominences on the +limb of the sun, the spectroheliograph has contributed +very effectively to many departments of +solar research. The prominences are dull reddish +cloudlets that were first seen during total eclipses +of the sun. Probably Vassenius, a Swedish astronomer, +during the total eclipse of 1733, made the +earliest record of them, as pinkish clouds quite detached +from the edge of the moon; and in that day, +when it had not yet been proved that the moon was +without atmosphere, he naturally thought they belonged +to the moon, not the sun. Undoubtedly Ulloa, +a Spanish admiral, also saw the prominences in observing +the total eclipse of 1778; but they seem to +have attracted little attention till 1842, when a very +important total eclipse was central throughout +Europe, and observed with great care by many of +the eminent astronomers of all countries.</p> + +<p>So different did the prominences appear to different +eyes, and so many were the theories as to +what they were, that no general consensus of opinion +was reached, and some thought them no part of +either sun or moon, but a mere mirage or optical +illusion. But at the return of this eclipse in 1860, +photography was employed so as to demonstrate beyond +<span class="pagenum"><a name="Page_p180" id="Page_p180">[180]</a></span> +a shadow of doubt the real existence and true +solar character of the prominences. By the slow +progress of the moon across the sun and the prominences +on the edge, a unique series of photographs +by De la Rue showed the moon's edge gradually +cutting off the prominences piecemeal on one side +of the sun, and equally gradually uncovering them +on the opposite side.</p> + +<p>The prominences, then, were known to be real +phenomena of the sun, some of them disconnectedly +floating in his atmosphere, as if clouds. Their forms +did not vary rapidly, they were very abundant, and +their light was so rich in rays of great photographic +intensity that many were caught on the plate which +the eye failed to see; they appeared at every part +of the sun's limb and their height above it indicated +that they must be many thousand miles in actual +dimension. What they were, however, remained an +entire mystery, and no one even thought it possible +to find out what their chemical constitution might +be or to measure the speed with which they moved.</p> + +<p>A few years later came the great Indian eclipse +(August 28, 1868), at that date the longest total +eclipse ever observed. Janssen of France and many +others went out to India to witness it. Fortunately +the prominences were very brilliant and this led +Janssen to believe it would be possible for him to +see them the day after the eclipse was over. By +modifying the adjustment of his apparatus suitably +and changing its relation to the sun's edge, he found +that hydrogen is the main constituent in the light of +the prominences. In addition to this he was able +to trace out the shapes of the prominences, and even +measure their dimensions. His station in India was +at Guntoor, many weeks by post from home; so that +<span class="pagenum"><a name="Page_p181" id="Page_p181">[181]</a></span> +his account of this important discovery reached the +Paris Academy of Sciences for communication with +another from the late Sir Norman Lockyer of England, +announcing a like discovery, wholly independently.</p> + +<p>The principle is simply this, and admirably stated +by Young: "Under ordinary circumstances the prominences +are invisible, for the same reason as the +stars in the daytime: they are hidden by the intense +light reflected from the particles of our own atmosphere +near the sun's place in the sky; and if we +could only sufficiently weaken this aerial illumination, +without at the same time weakening <i>their</i> +light, the end would be gained. And the spectroscope +accomplishes this very thing. Since the air-light +is reflected sunshine, it of course presents the +same spectrum as sunlight, a continuous band of +color crossed by dark lines. Now, this sort of +spectrum is greatly weakened by every increase of +dispersive power, because the light is spread out into +a longer ribbon and made to cover a more extended +area. On the other hand, a spectrum of bright lines +undergoes no such weakening by an increase in the +dispersive power of the spectroscope. The bright +lines are only more widely separated—not in the +least diffused or shorn of their brightness."</p> + +<p>Simultaneous announcement of this great discovery, +by astronomers of different nations, working +in widely separate regions of the earth, led to +the striking of a gold medal by the French Government +in honor of both astronomers and bearing their +united effigies. Ever since the famous Indian eclipse +of 1868, it has not been necessary to wait for a total +eclipse in order to observe the solar prominences, +but every observer provided with suitable apparatus +<span class="pagenum"><a name="Page_p182" id="Page_p182">[182]</a></span> +has been able to observe them in full sunlight whenever +desired, and the charting of them is part of the +daily routine at several observatories in different +parts of the world. So vast has been the accumulation +of data about them that we know their numbers +to fluctuate with the spots on the sun; and their distribution +over the sun's surface resembles in a way +that of the spots.</p> + +<p>While the spots and protuberances are most numerous +around solar latitude 20 degrees both north +and south, the prominences do not disappear above +latitude 35 to 40 degrees, as the spots do, but from +latitude 60 degrees they increase in number to about +75 degrees, and are occasionally observed even at +the sun's poles. Faculæ and prominences are more +closely related than the sun spots and prominences. +There are wide variations in both magnitude and +type of the prominences. Heights above the sun's +limb of a few thousand miles are very common, and +they rarely reach elevations as great as 100,000 +miles, though a very occasional one reaches even +greater heights.</p> + +<p>Classification of the prominences divides them +into two broad types, the quiescent and the eruptive. +The former are for the most part hydrogen, and +the latter metallic. The quiescent prominences resemble +closely the stratus and cirrus type of terrestrial +clouds, and are frequently of enormous extent +along the sun's edge. They are relatively long-lived, +persisting sometimes for days without much change. +The eruptive prominences are more brilliant, changing +their form and brightness rapidly. Often they +appear as brilliant spikes or jets, reaching altitudes +that average about 25,000 miles. Rarely seen near +the sun's poles, they are much more numerous nearer +<span class="pagenum"><a name="Page_p183" id="Page_p183">[183]</a></span> +the sun spots. Speed of motion of their filaments +sometimes exceeds one hundred miles a second, and +the changing variety of shapes of the eruptive prominences +is most interesting. Oftentimes they change +so rapidly that only photography can do them justice.</p> + +<p>Prominence photography began with Young a +half century ago, who obtained the first successful +impression on a microscope slide with a sensitized +film of collodion; as was necessary in the earlier +wet-plate process of photography, which required +exposures so long that little progress was effected +for about twenty years. Then it was taken up by +Deslandres of Paris and Hale of Chicago independently, +both of whom succeeded in devising a complex +type of apparatus known as the spectroheliograph, +by which all the prominences surrounding +the entire limb of the sun can be photographed at +any time by light of a single wave-length, together +with the disk of the sun on the same negative.</p> + +<p>The prominences appear to be intimately connected +with a gaseous envelope surrounding the solar +photosphere, in which sodium and magnesium are +present as well as hydrogen. The depth of the +chromosphere is usually between 5,000 and 10,000 +miles, and its existence was first made out during +the total solar eclipses of 1605 and 1706, when it appeared +as an irregular rose-tinted fringe, though not +at the time recognized as belonging to the sun.</p> + +<p>The constitution of the sun and its envelopes are +still under discussion, and no complete theory of the +sun has yet been advanced which commands the +widest acceptance. Of the interior of the sun we can +only surmise that it is composed of gases which, because +of intense heat and compression, are in a state +unfamiliar on earth and impossible to reproduce in +<span class="pagenum"><a name="Page_p184" id="Page_p184">[184]</a></span> +our laboratories. Their consistency may be that of +melted pitch or tar.</p> + +<p>Surrounding the main body of the sun are a series +of layers, shells, or atmospheres. Outside of all and +very irregular in structure, indeed probably not a +solar atmosphere at all, is the solar corona, parts +of which behave much as if it were an atmosphere, +but it appears to be bound up in some way with the +sun's radiation. It has streamers that vary with the +sun-spot period, but its constitution and function are +very imperfectly known, because it has never been +seen or photographed except at rare intervals on +occasion of total eclipses of the sun.</p> + +<p>Beneath the corona we meet the projecting prominences, +to which parts of the corona are certainly +related, and beneath them the first true layer or +atmosphere of the sun known as the chromosphere, +its average depth being about one-hundredth part of +the sun's diameter. Beneath the chromosphere is +the layer of the sun from which emanates the light +by which we see it, called the photosphere. It appears +to be composed of filaments due to the condensation +of metallic vapors, and it is the outer +extremities of these filaments which are seen as +the granular structures everywhere covering the +disk of the sun. Their light shines through the +chromosphere and the spots are ruptures in this +envelope.</p> + +<p>Between photosphere and chromosphere is a very +thin envelope, probably not over 700 miles in thickness, +called the reversing layer. It is this relatively +thin shell that is responsible for the absorption +which produces the dark lines in the spectrum of +the sun. Under normal conditions the filaments of +the photosphere are radial, that is vertical on the +<span class="pagenum"><a name="Page_p185" id="Page_p185">[185]</a></span> +sun; but whenever eruptions take place, as during +the occurrence of spots, the adjacent filaments are +violently swept out of their normal vertical lines +and these displaced columns then form what we view +as the spot's penumbra. From the outer surface of +the sun's chromosphere rise in eruptive columns +vapors of hydrogen and the various metals of which +the sun is composed. These and the spots would +naturally occur in periods just as we see them.</p> + +<p>We have said that the sun is composed of a mass +of highly heated or incandescent vapors or gases, +whose compression on account of gravity must +render their physical condition quite different from +any gaseous forms known on the earth or which we +can reproduce here. As the result of more than +half a century of studious observation of the sun +and mapping of its spectrum in every part, and diligent +comparison with the spectra of all known chemical +elements on the earth, we find that the sun contains +no elements not already found here, but that +a great preponderance of elements known to earth +are found in the sun.</p> + +<p>The intensity of their spectral lines is one prominent +indication of the presence of elements in the +sun, and the number of coincidences of spectral +lines is another. Iron, nickel, calcium, manganese, +sodium, cobalt, and carbon are among the elements +most strongly identified. A few of the rarer terrestrial +elements are of doubtful existence in the sun, +and a very few, as gold, bismuth, antimony, and +sulphur are not found there, and the existence of +oxygen in the sun is regarded by some experts as +doubtful. But if the whole earth were vaporized by +heat, probably its spectrum would resemble that of +the sun very closely.</p> + +<p><span class="pagenum"><a name="Page_p186" id="Page_p186">[186]</a></span> +What are the effects of the sun, and sun spots in +particular, on our weather? Is the influence of their +periodicity potent or negligible? If we investigate +conditions pertaining to terrestrial magnetism, as +fluctuations of the magnetic needle, and the frequency +of auroræ, there is no occasion for doubt of +the sun's direct influence, although we are not able +to say just how that influence becomes potent. If, +however, we look into questions of temperature, +barometric pressure, rainfall, cyclones, crops, and +consequent financial conditions, we find fully as much +evidence against solar influence as for it. The slight +variations of the sun's light and heat due to the +presence or absence of sun spots can scarcely be +sensible, and much longer periods of closer observation +are necessary before such questions can be +finally decided. The slighter such influences are, if +they actually exist, and the more veiled they are by +other influences more or less powerful, the more difficult +it is to discover their effects with certainty.</p> + +<p>The importance of solar radiation in the prediction +of terrestrial weather has long been recognized, +but until very recently no practical application has +been made. The Smithsonian Astrophysical Observatory +at Washington, under the direction of Dr. +Abbot, has for many years carried on at a number +of stations a series of determinations of the constant +of solar radiation by the spectro-bolometric method +originated by Langley. A new station in Calama, +Chile, has recently been inaugurated, at which the +solar constant is worked out each day, and telegraphed +to the Argentine weather service, where +it is employed in forecasting for the day.</p> + +<p>Abbot's new method of solar constant determination +is based on the fact that atmospheric transparency +<span class="pagenum"><a name="Page_p187" id="Page_p187">[187]</a></span> +varies oppositely to the variations of brightness +of the sky. Increase of haziness presents more +reflecting surface to scatter the solar rays indirectly +to the earth. Of course it presents also additional +surface to obstruct the direct rays from the sun. By +measuring the brightness of the sky near the sun, +it becomes possible to infer the coefficients of atmospheric +transmission at all wave lengths. The +direct observations and the complete deduction of +the solar constant for the day can all be completed +within two or three hours.</p> + +<p>Clayton of Buenos Aires has now employed these +results in the Argentine weather predictions for two +years, and the introduction of this new element in +forecasting has brought about a pronounced gain in +the value of the predictions. Its adoption by the +weather bureaus of other nations will doubtless +come in due time, and the new method take a firmly +established rank in practical meteorology.</p> + +<p>Abbot's observations many years ago first called +attention to the variability of the solar constant +through a range of several per cent both from year +to year, and in irregular short periods of weeks or +even days. Abbot considers this the more likely +explanation than that atmospheric changes should +take place simultaneously all over the earth. The +sun is but a star, the stars that are irregularly +variable in light and heat are numerous, and the +sun itself appears to be one of these.</p> + +<p>Especially important to the agricultural and vineyard +interests of Argentina is the question of precipitation, +and Clayton finds this very dependent on +solar radiation. At epochs of practically stationary +solar intensity, there is little or no precipitation; +but quite generally he finds that great decrease of +<span class="pagenum"><a name="Page_p188" id="Page_p188">[188]</a></span> +solar radiation is followed in from three to five days +by heavy precipitation. Direct temperature effects +are also traced in Buenos Aires and other South +American cities, lagging from two to three days +behind the observed solar fluctuations.</p> + +<p>The station at Calama yields about 250 determinations +of the solar constant each year, and the Mount +Wilson station about half that number. They are +the only stations of this character at present in existence, +and others should be established in widely +separated and cloudless regions, as Egypt, southern +California and Australia. Uniformity in the +methods of observing would be highly desirable, +and the Smithsonian Institution has perfected the +details of common control of such stations which it +is expected may be established at an early day.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p189" id="Page_p189">[189]</a></span></p> + +<h2><a name="CHAPTER_XXVII" id="CHAPTER_XXVII"></a>CHAPTER XXVII<br /> +<br /> +THE INNER PLANETS</h2> + +<h3>VULCAN</h3> + +<p>About the middle of the last century, Le Verrier, +a great French astronomer, having added the +planet Neptune beyond the outside confines of the +solar system, sought evidence of a lesser planet traveling +round the sun within the orbit of Mercury. +For many years close watch was kept on the sun in +the hope of discovering such a body in the act of +passing across the disk, or in transit, as it is +technically termed. Lescarbault, a French physician, +announced that he had actually seen such a +planet, Vulcan it was called, passing over the sun +in 1859. Total eclipses of the sun would afford the +best opportunity for seeing such a body, and on +several such occasions astronomers thought they had +found it. But the signal advantages of photography +have been applied so often to this search, and always +unsuccessfully, that the existence of Vulcan, or the +intramercurian planet, is now regarded as mythical.</p> + + +<h3>MERCURY</h3> + +<p>This planet is an elusive body that very few, even +astronomers, have ever seen. It is not very bright, +has a rapid motion and never retreats far from the +sun, so that it was a puzzle to the ancients who saw +it, sometimes in the twilight after sunset and again +in the twilight of dawn. When following the sun +<span class="pagenum"><a name="Page_p190" id="Page_p190">[190]</a></span> +down in the west, in March or April, Mercury is +likely to be best seen; twinkling rather violently and +nearly as bright as a star of the first magnitude.</p> + +<p>Very little is to be seen on the minute disk of this +planet, except that it goes through all the phases of +the moon—crescent, gibbous, full, gibbous, crescent. +Whether Mercury turns round on its axis or not, +cannot be said to be known, because the markings +that are suspected on its surface are too indefinite +to permit exact observation. More than likely the +planet presents always the same side or face to the +sun, so that it turns round on its axis once, while +traveling once around the sun in its orbit. Mercury's +day and year would therefore be equal in length. +Nor have we much evidence on the question of an +atmosphere surrounding Mercury; probably it is +very thin, if indeed there is any at all. When Mercury +comes directly between us and the sun, crossing +in transit, the edge of the planet as projected +against the sun is very sharply defined, and this +would indicate an absence of atmosphere on Mercury.</p> + +<p>Transits of Mercury can occur in May and +November only: there was one on November 7, 1914, +and there will be one on May 7, 1924. The latter +will be nearly eight hours in length, which is almost +the limit. Mercury's distance from the sun averages +36 million miles, the diameter of the planet is 3,000 +miles, and his orbital speed is 30 miles per second, +the swiftest of all the planets. No moon of Mercury +is known to exist, although many times diligently +searched for, especially during transits of the planet.</p> + + +<h3>VENUS</h3> + +<p>Brightest of all the planets, and the most beautiful +of all is Venus. Its path is next outside the orbit +<span class="pagenum"><a name="Page_p191" id="Page_p191">[191]</a></span> +of Mercury, but within that of the earth, so that it +partakes of all the phases of the moon. Like Mercury +it sometimes passes exactly between us and +the sun, a rare phenomenon which is known as a +transit of Venus.</p> + +<p>Being without telescopes, the ancients knew nothing +about these occurrences, but they were puzzled +for centuries over the appearance of the planet in +the west after sunset, when they called it Hesperus, +and in early dawn in the east when they gave it the +name Phosphorus.</p> + +<p>Venus is known to be girdled with an atmosphere +denser than ours, and it seems to be always filled +with dense clouds. It is the reflection of sunlight +from this perpetually cloudy exterior which gives +Venus her singular radiance. So brilliant is she +that even full daylight is not strong enough to overpower +her rays; and she may often be seen glistening +in the clear blue daytime sky, if one knows pretty +nearly in what direction to look for her.</p> + +<p>Venus is 67 million miles from the sun, and as +our own distance is 93 million miles, this planet +can come within 26 million miles of the earth. It +is therefore at times our nearest known neighbor in +space, excepting only the Moon and Eros, one of the +erratic little planets that travel round the sun between +Mars and Jupiter. Also possibly a comet +might come much nearer.</p> + +<p>Astronomers always take advantage of this nearness +of Venus to us, if a transit across the sun takes +place; because it affords an excellent method of finding +out what the distance of the sun is from the +Earth. A pair of these transits happens about once +a century, there were transits in 1874 and 1882, and +the next pair occur in 2004 and 2012. In actual size, +<span class="pagenum"><a name="Page_p192" id="Page_p192">[192]</a></span> +Venus is almost as large a planet as our own, being +7,700 miles in diameter, as compared with 7,920 +for the earth. Her velocity in her orbit is twenty-two +miles per second, and she travels all the way +round the sun in seven and one half months or 225 +days.</p> + +<p>Venus from her striking brilliancy always leads +the novice to expect to see great things on applying +the telescope. But aside from a brilliant disk, now +a slender crescent, now half full like the moon at +quarter, and again gibbous as the moon is between +quarter and full, the telescope reveals but little. +There is pretty good evidence that the markings +thought to have been seen on the planet's surface +are illusory, and so it is wholly uncertain in what +direction the planet's axis lies; also there is great +uncertainty about the length of the day on Venus, +or the period of turning round on its axis. Probably +it is the same in length as the planet's year.</p> + +<p>Once when Venus passed very close to the sun, +just barely escaping a transit, Lyman of Yale University +caught sight of it by hiding the sun behind +a tall building or church spire. The dark side of +Venus was turned toward us and he could not of +course see that. But the planet was clearly there, +completely encircled by a narrow delicate luminous +ring, which was due to sunlight shining through the +atmosphere that surrounds the planet. Similar +ring effects were seen by observers of the transits +of Venus in 1874 and 1882; and from all their observations +it is concluded that Venus has an atmosphere +probably at least twice as dense and extensive +as that which encircles the earth. Spurious +satellites of Venus are many, but no real moon is +known to attend this planet.</p> + +<p><span class="pagenum"><a name="Page_p192p1" id="Page_p192p1">[192i]</a></span></p> + +<div class="fig_center" style="width: 458px;"> +<img src="images/p192_1.png" width="458" height="589" alt="" /> +<div class="fig_caption"><span class="smcap">The Surface of the Moon in the Region of Copernicus.</span> Photograph +made with the Hooker 100-inch reflecting telescope. (<i>Photo, Mt. Wilson +Solar Observatory.</i>)</div> +</div> + +<p><span class="pagenum"><a name="Page_p192p2" id="Page_p192p2">[193i]</a></span></p> + +<div class="fig_center" style="width: 456px;"> +<img src="images/p192_2.png" width="456" height="590" alt="" /> +<div class="fig_caption"><span class="smcap">A View of the South Central Portion of the Moon at Last +Quarter.</span> (<i>Photo, Mt. Wilson Solar Observatory.</i>)</div> +</div> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p193" id="Page_p193">[193]</a></span></p> + +<h2><a name="CHAPTER_XXVIII" id="CHAPTER_XXVIII"></a>CHAPTER XXVIII<br /> +<br /> +THE MOON AND HER SURFACE</h2> + +<p>As the sun has always reigned as king of day, so +is the moon queen of night. Observation of her +phases, now waxing, now waning, with her stately +motion always eastward among the stars, began +with the earliest ages. Often when near the full +she must have been seen herself eclipsed, and much +more rarely the occurrence of total eclipses of the +sun are certain to have suggested the moon's intervention +between earth and sun, shutting off the sunlight +completely, because these eclipses never took +place except when the moon was in the same part +of the sky with the sun.</p> + +<p>If we watch the nightly march of the moon, we +shall find that she travels over her own breadth in +about an hour's time. By using a telescope on the +stars just eastward or to the left of her, she will now +and then be seen to pass between us and a star—on +very rare occasions a planet—extinguishing its light +with great suddenness, the most nearly instantaneous +of all phenomena in nature. Draw a line connecting +the cusps, or horns of the lunar crescent, and +then a line eastward at right angles to this, and it +will show the direction of the moon's own motion in +its orbit round the earth quite accurately.</p> + +<p>As the phase advances, note the inside edge of the +advancing crescent: this will be quite rough and +jagged, compared to the outside edge which is the +<span class="pagenum"><a name="Page_p194" id="Page_p194">[194]</a></span> +moon's real contour and relatively very smooth. The +position of the inside curve will change from night +to night, and it marks the line of sunrise on the +moon during the fortnight elapsing between new +moon and full; while from full through last quarter +and back to new moon, this advancing line marks +the region of sunset on the moon. The general shape +of this line is never a circle but always elliptical, +and astronomers call it the terminator. All along +the terminator, sunlight strikes the lunar surface +at a small angle, whether near sunrise or sunset; +so that owing to the mountains and other high +masses of the moon's surface, the terminator +is always a more or less jagged and irregular +line.</p> + +<p>Onward from new moon toward full the horns of +the crescent are always turned upward or eastward. +When the general line of the terminator becomes +a straight line from cusp to cusp, the moon is said +to have reached first quarter or quadrature. Onward +toward full the terminator will be seen to bend +the other way, and in about a week's time it will have +merged itself with the moon's limb. The moon is +then said to be full. Afterward the phase phenomena +recur in the reverse order, with third quarter +midway between full and new moon again; the phase +of the moon being called gibbous all the way from +first quarter to third quarter, except when exactly +full.</p> + +<p>As we know that the moon is, like the earth, a +nonluminous body, and shines only by virtue of the +sunlight falling upon it, clearly an entire half of the +moon's globe must be perpetually illumined by sunlight. +The varying phases then are due simply to +that part of the illuminated hemisphere which is +<span class="pagenum"><a name="Page_p195" id="Page_p195">[195]</a></span> +turned toward us. New moon is entirely invisible +because the sunward hemisphere is turned wholly +away from us, while at full moon we see the lunar +disk complete because we are on the same side of +the moon that the sun is and practically in line with +both sun and moon.</p> + +<p>If we could visit the moon, we should see the earth +in exactly complementary phase. At new moon here +we should be enjoying full earth there, and full moon +here would be coincident with new or dark earth +there. The narrow crescent of new moon here would +be the period of gibbous earth there; and it is the +reflection of sunlight from this gibbous earth which +illuminates the part of the moon but faintly seen +at this time, popularly known as the "old moon in +the new moon's arms." Its greater visibility at some +times than at others is due to greater prevalence of +clouded area in the reflecting regions of the earth +turned toward the moon, and the higher reflective +power of clouds than that possessed by mere land +and water.</p> + +<p>As the moon goes all the way round the sky every +month, the same as the sun does in a year, and +travels in nearly the same path, clearly it must also +go north and south every month as the sun does. +So in midsummer when the sun runs high upon the +meridian, we expect to find full moons running low, +and likewise in midwinter the full moon always runs +high, as almost everyone has sometimes or other +noticed.</p> + +<p>This eastward or true orbital motion of the moon +is responsible for another relation which soon comes +to light when we begin to observe the moon; and +that is the later hour of rising or setting each night. +Our clock time is regulated by the sun, which also +<span class="pagenum"><a name="Page_p196" id="Page_p196">[196]</a></span> +is moving eastward about 1° daily, or twice its own +breadth. So the moon's eastward gain on the sun +amounts to about 12 degrees daily, and one degree +being equal to 4 minutes, the retarded time of moonrise +or moonset each day amounts to very nearly 50 +minutes on the average; though sometimes the delay +will be less than a half hour and at other times +it will exceed an hour and a quarter. The season +of least retardation of rising of the full moon is in +the autumn, and so the moon that falls in late September +or October is known as the Harvest moon, +and the next succeeding full moon is called the +Hunter's moon.</p> + +<p>Lunation is a term sometimes given to the moon's +period from any definite phase round to the same +phase again. Its length is the true period of the +moon's revolution once around the earth, from the +sun all the way round till it overtakes the sun again. +The synodic period is another name for lunation, +and its true length is 29 and one-half days, or very +accurately 29 d. 12 h. 44 m. 2.7 s. as calculated by +astronomers with great exactness from many thousand +revolutions of the moon. But if we want the +true period of the moon round the earth as referred +to a star, it is much shorter than this, amounting to +only 27 days and nearly one-third. This is called +the moon's sidereal period of revolution, because it +is the time elapsed while she is traveling eastward +from a given star around to coincidence with the +same star again.</p> + +<p>If we study the moon's path in the sky more +critically, we shall find that it does not quite follow +the ecliptic, or the sun's path, but that twice each +month she deviates from the ecliptic, once to the +north and once to the south of it, by roughly ten +<span class="pagenum"><a name="Page_p197" id="Page_p197">[197]</a></span> +times her own breadth. More accurately this angle +is 5°8'40", an almost invariable quantity, and it is +therefore known as an astronomical constant, or the +inclination of the moon's orbit to the ecliptic. So +the moon's orbit must intersect the ecliptic, and +as both are great circles in the sky, the points of intersection +are known as the moon's nodes, one ascending +and the other descending, and the nodes are +180 degrees apart.</p> + +<p>The figure of the moon's orbit is not circular, although +it deviates only slightly from that form. But +like the paths of all other satellites round their +primary planets, and of the planets themselves round +the sun, the moon's orbit is also an ellipse. The distance +of the moon's center from the earth's center +is therefore perpetually changing; the point of +nearest approach is called perigee, and that of farthest +recession, apogee.</p> + +<p>The moon's distance from the earth is easier +and simpler to be ascertained than that of any other +heavenly body, because it is the nearest. An outline +of the method of finding this distance is not +difficult to present; and it resembles in every particular +the method a surveyor uses to find the distance +of some inaccessible point which he cannot measure +directly. Up and down a stream, for example, he +measures the length of a line, and from each end of +it he measures the angle between the other end of +the line and the object on the opposite side of the +stream whose distance he wishes to find out. Then +he applies the science of trigonometry to these three +measures, two of angles and one the length of the +side or base included between them, and a few minutes' +calculation gives the distance of the inaccessible +object from either end of the base line.</p> + +<p><span class="pagenum"><a name="Page_p198" id="Page_p198">[198]</a></span> +Now in like manner, to transfer the process to the +sky, let the two ends of the base be represented by +two astronomical observatories, for example, Greenwich +in the northern hemisphere and Cape Town in +the southern. The base line is the chord or straight +line through the earth connecting the two observatories, +and we know the length of this line pretty accurately, +because we know the size of the earth. The +angles measured are somewhat different from those +in the terrestrial example, but the process amounts +to the same thing because the astronomers at the +two observatories measure the angular distance of +the center of the moon from the zenith, each using +his own zenith at the same time; and the same +science of trigonometry enables them to figure out +the length of any side of the triangles involved. The +side which belongs to both triangles is the distance +from the center of the earth to the center of the +moon, and the average of many hundred measures +of this gives 238,800 miles, or about ten times the +distance round the equator of the earth.</p> + +<p>We have said that the orbit in which the moon +travels round the earth is practically a circle, but +the earth's center is found not at the center of +this orbit, but set to one side, or eccentrically, so +that the distance spanning the centers of the two +bodies is sometimes as small as 221,610 miles at +perigee, and 252,970 miles at apogee. The moon's +speed in this orbit averages rather more than half +a mile every second of time—more accurately 3,350 +feet a second, or 2,290 miles per hour.</p> + +<p>Once the moon's distance is known, its size or +diameter is easy to ascertain. An angular measure +is necessary, of course, that of the angle which +the disk of the moon fills as seen from the earth. +<span class="pagenum"><a name="Page_p199" id="Page_p199">[199]</a></span> +There are many types of astronomical instruments +with which this angle can be measured, and its +value is something more than half a degree (31' 7"). +The moon's actual diameter figures out from this +2,163 miles; and it would therefore require nearly +fifty moons merged in one to make a ball the size +of the earth.</p> + +<p>Still, no other planet has a satellite as large in +proportion to its primary as the moon is in relation +to the earth. But the materials that compose +the moon have less than two-thirds the +average density of those that make up the earth, +so that eighty-one moons fused together would be +necessary to equal the mass or weight of the earth. +If we figure out the force of attraction of the moon +for bodies on its surface, we find it equals about +one-sixth that of the earth. Athletes could perform +some astounding feats there—miracles of high +jump and hammer-throw.</p> + +<p>Our interest in the moon's physical characteristics +never wanes. Her nearness to us has always fascinated +astronomer and layman alike. Early users +of the telescope were readily led into error regarding +the general characteristics of the lunar surface; +and it is easy to see why they thought the smooth +level planes must be seas, and gave them names to +that effect which persist to-day, as Mare Crisium, +Mare Serenitatis and so on. We may be sure that +no water exists on the moon's surface, although +some astronomers think that solid water, as ice or +snow, may still exist there at a temperature too low +for appreciable evaporation.</p> + +<p>Perhaps water, seas, and oceans were once there, +but their secular dissemination and loss as vapor +have gone on through the millions of millions of +<span class="pagenum"><a name="Page_p200" id="Page_p200">[200]</a></span> +years till even the moon's atmosphere appears to +have vanished completely. At least there is much +better evidence of absence of atmosphere on the +moon than of its presence—not enough at any rate +to equal a thousandth part of the barometric pressure +that we have at the earth's surface. Frequent +observations of stars passing behind the moon +in occultation have satisfied astronomers on this +point.</p> + +<p>We often say of the brilliant full moon, it is as +bright as day. The photometer or instrument for +accurate comparison of lights, their amount and intensity, +tells a different story. Indeed, if the entire +dome of the sky were filled with full moons, we +should be receiving only one-eighth of the light the +sun gives us, and it would require more than 600,000 +average full moons to equal the light radiation +of the sun. Heat from the moon, however, is quite +different. Early attempts to measure it detected +none at all, but with modern instruments there is +little trouble in detecting heat from the moon, +though measurement of it is not easy.</p> + +<p>Much of the moon's heat is sun heat, directly reflected +from the moon, as sunlight is, but most of it +is due to radiation of solar heat previously absorbed +by the materials of the lunar surface. The actual +temperature of the moon's surface suffers great +variation. A fortnight's perpetual shining of the +sun upon the lunar rocks would certainly heat them +above the temperature of boiling water, if the moon +had an atmosphere to conserve and store this heat; +but the entire absence of such an air blanket probably +permits the sun's heat to be radiated away nearly +as fast as it is received, leaving the temperature at +the surface always very low.</p> + +<p><span class="pagenum"><a name="Page_p201" id="Page_p201">[201]</a></span> +What physical influences the moon really has upon +the earth must be very slight, barring the tides. +But there is little hope of getting people generally +to take that view, because the moon appears to be +the planet of the people, and opinion that the moon +controls the weather, for instance, amounts with +them to practical certainty. More than likely all +these notions are but legitimate survivals of superstition +and astrology. In addition to the tides, our +magnetic observatories reveal slight disturbances +with the swinging of the moon from apogee to perigee +and back; but long series of weather observations +have been faithfully interrogated, with negative +or contradictory results. If one believes that +the moon's changes affect the weather, it is easy to +remember coincidences, and pass over the many +times when no change has taken place. The moon +changes pretty frequently anyhow. As Young well +puts it: "A change of the moon necessarily occurs +about once a week…. <i>All</i> changes, of the weather +for instance, must therefore occur within three and +three-fourth days of a change of the moon, and fifty +per cent of them ought to occur within forty-six +hours of a change, even if there were no causal connection +whatever."</p> + +<p>When we turn to the strongly diversified surface +of the moon itself, we find much to rivet the attention, +even with slender optical aid. Everyone +wants to know how near the telescope, the biggest +possible telescope, brings the moon to us. That will +depend on many things, first of all on the magnifying +power of the eyepiece employed on the telescope, +and eyepieces are changed on telescopes just as they +are on microscopes, though not for the same reasons. +The theoretical limit of the power of a telescope is +<span class="pagenum"><a name="Page_p202" id="Page_p202">[202]</a></span> +usually considered as 100 for each inch of diameter +or aperture of the object glass.</p> + +<p>A 40-inch telescope, as that of the Yerkes +Observatory, the largest refracting telescope in +existence, should bear a magnifying power not to +exceed 4,000. But this limit is practically never +reached, one-half of it or fifty to the inch of aperture +being a good working limit of power, even under +exceptional conditions of steadiness of atmosphere. +If we reduce the effective distance of the moon from +240,000 miles to 100 miles, that is about the utmost +that can be expected. But even at that distance we +can make out only landscape details, nothing whatever +like buildings or the works of intelligence.</p> + +<p>The larger relations of light and shade, so +obvious to the naked eye on the moon, vanish on +looking at it with the telescope, but we are at once +captivated by the novel character of the surface +and the seemingly great variety of detail that is +clearly visible. As soon as the new moon comes +out in the west, one may begin to gaze with interest +and watch the terminator or sunrise line +gradually steal over the roughened surface, bringing +new and striking craters into view each night. +Around the time of quarter moon, or a little past +it, is one of the best times for telescopic views of +the moon, because the huge craters, Tycho and +Copernicus, are then in fine illumination. Close +to the phase of full moon is never a good time, +because there are no shadows of the rough surface +then, and its entire structure seems to be quite +flat and uninteresting, except for the streaks or +rills which radiate from Tycho in every direction, +and are the only lunar features that are best seen +near full.</p> + +<p><span class="pagenum"><a name="Page_p203" id="Page_p203">[203]</a></span> +In a broad, general way, the moon's surface, if +compared with the earth's, differs in having no +water. Our extensive oceans are replaced there by +smooth, level plains which were at first thought to +be seas and so named. There are ten or twelve of +them in all. Then we find mountain ranges, so +numerous on the earth, relatively few on the moon. +Those that exist are named, in part, for terrestrial +mountain ranges, as the Alps, Caucasus, and the +Apennines.</p> + +<p>But the nearly circular crater, a relatively rare +formation on the earth, is seen dotted all over the +moon in every size, from a fraction of a mile in +diameter up to sixty, seventy, and in extreme cases +a hundred miles. No mere description of plains +and mountains and craters affords an adequate +idea of the moon's surface as it actually is; a +telescopic view is necessary, or some of the modern +photographs which give an even better notion of +the moon than any telescopic view. Many of the +lunar craters are without doubt volcanic in origin, +others seem to be ruins of molten lakes. Many +thousands of the smaller ones appear as if formed +by a violent pelting of the surface when semi-plastic, +perhaps by enormous showers of meteoric +matter. More than 30,000 craters cover the half +of the lunar surface visible from the earth, and +hundreds of them are named for philosophers and +astronomers.</p> + +<p>Measurement of the height of lunar mountains +has been made in numerous instances, especially +when their shadows fall on plains or surfaces that +are nearly level, so that the length of the shadow +can be measured. In general, the height of lunar +peaks is greater than that of terrestrial peaks, +<span class="pagenum"><a name="Page_p204" id="Page_p204">[204]</a></span> +owing probably to the lesser surface gravity on +the moon. About forty lunar peaks are higher +than Mont Blanc.</p> + +<p>Most astronomers regard it as certain that no +changes ever take place on the moon; probably no +very conspicuous changes ever do. Some, however, +have made out a fair case for comparatively recent +changes in surface detail. Extreme caution is necessary +in drawing conclusions, because the varying +changes of illumination from one phase to another +are themselves sufficient to cause the appearance +of change. At intervals of a double lunation, equal +to fifty-nine days, one and one-half hours, the +terminator goes very nearly through the same +objects, so that the circumstances of illumination +are comparable. In Mare Serenitatis the little +crater named Linné was announced to have disappeared +about a half century ago; subsequently it +became visible again and other minor changes were +reported, perhaps due to falling in of the walls of +the crater.</p> + +<p>If one were to visit the moon, he must needs +take air and water along with him, as well as +other sustenance. No atmosphere means no diffused +light; we could see nothing unless the sun's +direct rays were shining upon it. Anyone stepping +into the shadow of a lunar crag would become +wholly invisible. No sound, however loud, could be +heard; sound in fact would become impossible. A +rock might roll down the wall of a lunar crater, +but there would be no noise; though we should +know what had happened by the tremor produced. +So slight is gravity there that a good ball player +might bat a baseball half a mile or more. Looking +upward, all the stars would be appreciably brighter +<span class="pagenum"><a name="Page_p205" id="Page_p205">[205]</a></span> +than here, and visible perpetually in the daytime +as well as at night.</p> + +<p>If one were to go to the opposite side of the +moon, he would lose sight of the earth until he +came back to the side which is always turned toward +the earth. Even then the earth would never +rise and set at any given place, as the moon does +to us, but would remain all the time at about the +same height above the lunar horizon. The earth +would go through all the phases that the moon +shows to us here, full earth occurring there when +it is new moon here. Our globe would appear to +be nearly four times broader than the moon seems +to us. Its white polar caps of ice and snow, its +dark oceans, and the vast cloud areas would be +very conspicuous. Faint stars, the zodiacal light, +and the filmy solar corona would be visible, probably +even close up to the sun's edge; but although +his rays might shine upon the lunar rocks without +intermission for a fortnight, probably they would +still be too cold to touch with safety. On the side +of the moon turned away from the sun, the temperature +of the moon's surface would fall to that +of space, or many hundred degrees below zero.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p206" id="Page_p206">[206]</a></span></p> + +<h2><a name="CHAPTER_XXIX" id="CHAPTER_XXIX"></a>CHAPTER XXIX<br /> +<br /> +ECLIPSES OF THE MOON</h2> + +<p>Of all the weird happenings of the nighttime sky, +eclipses of the moon are the most impressive. +Rarely is there a year without one. What is the +cause? Simply the earth getting in between sun and +moon, and thereby shutting off the sunlight which at +all other times enables us to see the moon. As the +earth is a dark body it must cast a black shadow on +the side away from the sun, and it is the moon's passing +into this shadow or some part of it that causes a +lunar eclipse.</p> + +<p>Sun and earth being so different in size, the earth's +shadow must stretch away from it into space, growing +smaller and smaller, until at length it comes to +an end—the apex of a cone 857,000 miles long. +If we cut off this shadow at the moon's distance from +the earth, we find it about 6,000 miles in diameter +at that point; and this accounts for the fact that the +curvature on the side of the moon, when the eclipse +is coming on and where it is dropping into the +shadow, is always much less rapid than the curvature +of the moon's own disk is.</p> + +<p>When an eclipse is approaching, the eastern limb +will be duskily darkened for half an hour or more, +because the moon must first pass through the outer +penumbra, or half-shadow which everywhere surrounds +the true shadow itself. If the moon hits +only the upper or lower part of the shadow, the +<span class="pagenum"><a name="Page_p207" id="Page_p207">[207]</a></span> +eclipse will be only partial, and during the progress +of the eclipse it will seem as if the uneclipsed part +had swung or twisted around in the sky, from the +western limb of the moon to the eastern. But when +the moon passes through the middle regions of the +shadow, the eclipse is always total, and direct sunlight +is wholly cut off from every part of the moon's +face, for a greater or less length of time, according +to the part of the shadow through which it passes. +When passing centrally through the shadow, the +total eclipse will last about two hours, as the moon's +diameter is about one-third of the breadth of the +shadow; and the eclipse will be partial about two +hours longer, an hour at beginning and an hour at +the end, because the moon moves over her own +breadth in about an hour.</p> + +<p>While the moon is wholly immersed in the shadow, +her body is nevertheless visible, as a dull tarnished +copper disk; and this is caused by the reddish sunlight +which grazes the earth all around and is refracted +or bent by our atmosphere into the shadow +itself. If this belt or ring of terrestrial atmosphere +happens to be everywhere filled with dense clouds, +as was the case in 1886, even the familiar copper +moon of a total lunar eclipse disappears completely +in the black sky.</p> + +<p>Quite different from a solar eclipse, all the phases +of a lunar eclipse are visible at the same time on the +earth wherever the moon is above the horizon. +Eclipses of the moon are therefore seen with great +frequency at any given place as compared with solar +eclipses, which are restricted to relatively narrow +areas of the earth's surface. Nor are lunar eclipses +of very much significance to the astronomer, mainly +because of the slowness and indefiniteness of the +<span class="pagenum"><a name="Page_p208" id="Page_p208">[208]</a></span> +phenomena. It is a good time to observe occultations +of faint stars at the moon's edge or limb, +and several such programs have been carried out +by cooperation of observatories in widely separate +regions of the world: the object being improvement +in our knowledge of the distance of the moon, and +in the accuracy of the mathematical tables of her +motion. Search by photography for a possible satellite, +or moon of the moon, has been made on several +occasions, though without success.</p> + +<p>A lunar eclipse was first observed and photographed +from an aeroplane, May 2, 1920. At the +request of the writer, two aviators of the United +States navy ascended to a height of 15,000 feet above +Rockaway, and secured many advantages accruing +from great elevation in viewing a celestial phenomenon +of this character.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p209" id="Page_p209">[209]</a></span></p> + +<h2><a name="CHAPTER_XXX" id="CHAPTER_XXX"></a>CHAPTER XXX<br /> +<br /> +TOTAL ECLIPSES OF THE SUN</h2> + +<p>Primitive peoples indulged in every variety of +explanation of mysterious happenings in the sky. +To the Chinese and all through India, a total eclipse +of the sun is caused by "a certain dragon with very +black claws," who, except for their frightening him +away by every conceivable sort of hideous noise, +would most certainly "eat up the sun." The eclipse +always goes off, the sun has never been eaten yet. +Can you convince a Chinaman that Rahu, the +Dragon, wouldn't have eaten up the sun, if his unearthly +din hadn't frightened him away?</p> + +<p>In Japan the eclipse drops poison from the sky +into wells, so the Japanese cover them up. Fontenelle +relates that in the middle of the seventeenth +century a multitude of people shut themselves up in +cellars in Paris during a total eclipse.</p> + +<p>In the Shu-king, an ancient Chinese work, occurs +the earliest record of a total eclipse of the sun, in +the year <span class="smcap2">B. C.</span> 2158. The Nineveh eclipse of <span class="smcap2">B. C.</span> 763 +is perhaps the first of the ancient eclipses of +which we possess a really clear description on the +Assyrian eponym tablets in the British Museum. +It is the eclipse possibly referred to in the Book +of Amos, viii.</p> + +<p>But of all the ancient eclipses none perhaps exceeds +in interest the famous eclipse of Thales, <span class="smcap2">B. C.</span> +585, May 28. It is the first eclipse to have been +predicted, probably by means of the saros, or 18-year +<span class="pagenum"><a name="Page_p210" id="Page_p210">[210]</a></span> +period of eclipses, which is useful as an +approximate method even at the present day. But +the accident of a war between the Lydians and the +Medes has added greatly to the historic interest, +because the combatants were so terrified by the +sudden turning of day into night that they at once +concluded a peace cemented by two marriages.</p> + +<p>Very many of the ancient eclipses have been of great +use to the historian in verifying dates, and mathematical +astronomers have employed them in correcting +the lunar tables, or intricate mathematical data +by which the motion of the moon is predicted.</p> + +<p>Coming down to the middle of the sixth century, +we find the first eclipse recorded in England, +in the "Saxon Chronicle," <span class="smcap2">A. D.</span> 538. During the +epoch of the Arabian Nights several eclipses were +witnessed at Bagdad, <span class="smcap2">A. D.</span> 829 to 928, and many a +century later by Ibu-Jounis, court astronomer of +Hakem, the Caliph of Egypt. Nothing is more interesting +than to search the quaint records of these +ancient eclipses. One occurring in 1560, when +Tycho Brahe was but fourteen, had much to do +with turning his permanent interest toward mathematics +and astronomy. The eclipse of 1612 was the +first "seen through a tube," the telescope having +been invented only a few years before. "Paradise +Lost" was completed about 1665, and the censorship +was still in existence; and it is matter of +record that the oft-quoted passage,</p> + +<div class="poem"> +<div class="i4"> "As when the Sun, new risen,</div> +<div class="i4">Looks through the horizontal misty air,</div> +<div class="i4">Shorn of his beams; or from behind the Moon,</div> +<div class="i4">In dim eclipse, disastrous twilight sheds</div> +<div class="i4">On half the nations, and with fear of change</div> +<div class="i4">Perplexes monarchs."</div> +</div> + +<div style="margin-left: 60%"> +<i>P. L.</i>, i. 594 +</div> + +<p><span class="pagenum"><a name="Page_p211" id="Page_p211">[211]</a></span></p> + +<div class="justify">was strongly urged as sufficient reason for suppressing +the entire epic.</div> + +<p>London was favored with the outflashing corona, +May 3, 1715, and a pamphlet was issued in prediction, +entitled "The Black Day, or a Prospect of +Doomsday."</p> + +<p>The first American eclipse expedition was on +occasion of the totality of Oct. 27, 1780, sent out +by Harvard College and the American Academy of +Arts and Sciences under Professor Samuel Williams +to Penobscot. There was a fine total eclipse +from Albany to Boston on June 16, 1806, and many +important observations of it were made in this +country.</p> + +<p>But it was not till the European eclipse of 1842 +that research got fully under way, because the germ +of the new astronomy, particularly as applied to +the sun, had begun its development; and the significance +of the corona was obvious, if it could be +proved a true appendage of the sun. Photography +had not long been discovered, and the corona of +1851 was the first to be automatically registered +on a daguerreotype. In 1860 it was proved that +prominences and corona both belong to the sun and +not to the moon.</p> + +<p>The great Indian eclipse of 1868 brought the important +discovery that the prominences can be +observed at any time without an eclipse by means +of the spectroscope. In 1869 bright lines were +found in the spectrum of the corona, one line in +the green indicating the presence of an element not +then known on the earth and hence called coronium. +In 1870 the reversing layer or stratum of +the sun was discovered. In 1878 a vast ecliptic +extension of the streams of the corona many millions +<span class="pagenum"><a name="Page_p212" id="Page_p212">[212]</a></span> +of miles both east and west of the sun was +first seen. This is now known to be the type of +corona characteristic of minimum spots on the sun. +In 1882 the spectrum of the corona was first photographed +and in 1889 excellent detail photographs +of the corona were taken. In 1893 it was shown +that the corona quite certainly rotates bodily with +the sun. In 1896 actual spectrum photographs of +the reversing layer established its existence beyond +doubt—"flash spectrum" it is often called. In 1898 +the long ecliptic streamers of the corona were successfully +photographed for the first time. In 1900 +the depth of the reversing layer was found to average +500 miles, the heat of the corona was first +measured by the bolometer, and many observations +showed that the coronal streamers, in part at least, +partake of the nature of electric discharges.</p> + +<p>All subsequent total eclipses have been carefully +observed, in whatever part of the world they may +happen, and each has added new results of significance +to our theories of the corona and its relation +to the radiant energy of the sun. In very +recent eclipses the cinematograph has been brought +into action as an efficient adjunct of observation; +in 1914 the first successful "movie" of the eclipse +was secured in Sweden, and in 1918 Frost of the +Yerkes Observatory first applied the cinematograph +to registry of the "flash spectrum," and Stebbins +tested out his photo-electric cell on the corona, making +the brightness 0.5 that of the full moon. In +1914 (Russia) and again in 1919 (on the Atlantic) +the obvious advantages of the aeroplane in ecliptic +observation and photography were sought by the +writer, though unsuccessfully. The photographic +tests, however, conducted in preparation for these +<span class="pagenum"><a name="Page_p213" id="Page_p213">[213]</a></span> +expeditions proved the entire practicability of +securing eclipse results of much value, independently +of clouds below.</p> + +<p>Eclipses in the near future will be total in Australia +about six minutes on September 21, 1922; in +California and Mexico about four minutes on September +10, 1923; and along a line from Toronto to +Nantucket about two minutes on the morning of +January 24, 1925.</p> + +<p>To all spectators, savage or civilized, scientist +or layman, a total eclipse is wonderful and impressive. +Langley said: "The spectacle is one of +which, though the man of science may prosaically +state the facts, perhaps only the poet could render +the impression." Very gradually the moon steals +its way across the face of the sun, the lessened +light is hardly noticed. If one is near a tree through +whose foliage the sunlight filters, an extraordinary +sight is seen; the ground all about is covered with +luminous crescents, instead of the overlapping +disks which were there before the eclipse came on; +in both cases they are images of the disk of the +sun at the time, and the narrowing crescents will +be watched with interest as totality approaches. +Then the shadow bands may be seen flitting across +the landscape, like "visible wind." They are probably +related to our atmosphere and the very slender +crescent from which true sunlight still comes.</p> + +<p>Then for a few seconds the moon's actual +shadow may be caught in its approach, very suddenly +the darkness steals over the landscape and—totality +is on. How lucky if there are no clouds! +Every eye is riveted on "the incomparable corona, +a silvery, soft, unearthly light, with radiant +streamers, stretching at times millions of uncomprehended +<span class="pagenum"><a name="Page_p214" id="Page_p214">[214]</a></span> +miles into space, while the rosy flaming +protuberances skirt the black rim of the moon +in ethereal splendor."</p> + +<p>Then it is now or never with observer and photographer. +Months of diligent preparations at home followed +by weeks of tedious journey abroad, with days +of strenuous preparation and rehearsals at the station—all +go for naught unless the whole is tuned up +to perfect operation the instant totality begins. It +may last but a minute, or even less; in 1937, however, +total eclipse will last 7 minutes 20 seconds, the +longest ever observed, and within half a minute of +the longest possible. All is over as suddenly as it +came on. The first thing is to complete records, develop +plates, and see if everything worked perfectly.</p> + +<p>There is great utility back of all eclipse research, +on account of its wide bearing on meteorology and +terrestrial physics, and possibly the direct use of +solar energy for industrial purposes. With this +purpose in view the astronomer devotes himself unsparingly +to the acquisition of every possible fact +about the sun and his corona.</p> + +<p>Considering the earth as a whole, the number of +total eclipses will average nearly seventy to the +century. But at any given place, one may count himself +very fortunate if he sees a single total eclipse, +although he may see several partial ones without +going from home. Then, too, there are annular or +ring eclipses, averaging seven in eight years. But +had one been born in Boston or New York in the +latter part of the eighteenth century, he might have +lived through the entire nineteenth century and a +long way into the twentieth without seeing more +than one total eclipse of the sun. In London in 1715 +no total eclipse had been visible for six centuries. +<span class="pagenum"><a name="Page_p215" id="Page_p215">[215]</a></span> +However, taking general averages, and recalling the +comparatively narrow belt of total eclipse, every +part of the earth is likely to come within range +of the moon's shadow once in about three and a +half centuries.</p> + +<p>The longest total eclipses always occur near the +equator; this is because an observer on the equator +is carried eastward by the earth's rotation at a +velocity of about 1,000 miles per hour, so that he +remains longer in the moon's shadow which is passing +over him in the same direction with a velocity +about twice as great.</p> + +<p>The general circumstances of total eclipses are +readily foretold by means of the ancient Chaldean +period of eclipses known as the saros. It is 18 +years and 10 or 11 days in length (according to +the number of leap years intervening). In one +complete saros, forty-one solar eclipses will generally +happen, but only about one-fourth of them +will be total. The saros is a period at the end of +which the centers of sun and moon return very +nearly to their relative positions at the beginning +of the cycle. So, in general, the eclipse of any +year will be a repetition of one which took place 18 +years before, and another very similar in circumstances +will happen 18 years in the future. Three +periods of the saros, or 54 years and 1 month, will +usually bring about a return of any given eclipse +to any particular part of the earth, so far as longitude +is concerned, though the returning track will +lie about 600 miles to the north or south of the one +54 years earlier.</p> + +<p>Paths of total eclipses frequently intersect, if +large areas like an entire country are considered; +Spain, for instance, where total eclipses have occurred +<span class="pagenum"><a name="Page_p216" id="Page_p216">[216]</a></span> +in 1842, 1860, 1870, 1900 and 1905. Besides +crossing Spain, the tracks of totality on May +28, 1900, and August 30, 1905, were unique in intersecting +exactly over a large city—Tripoli in Barbary, +on both of which occasions the writer's expeditions +to that city were rewarded with perfect +observing conditions in that now Italian province +on the edge of the great desert.</p> + +<p>Kepler was the first astronomer to calculate +eclipses with some approach to scientific form, as +exemplified in his Rudolphine Tables. His method +was of course geometrical. But La Grange, who +applied the methods of more refined analysis to the +problem, was the first to develop a method by which +an eclipse and all its circumstances could be accurately +predicted for any part of the earth. To many +minds, the prediction of an eclipse affords the best +illustration of the superior knowledge of the astronomer: +it seems little short of the marvelous. But +recalling that the motion of the moon follows the +law of gravitation, and that its position in the sky +is predictable for years in advance with a high degree +of precision, it will readily be seen how the +arrival of the moon's shadow, and hence the total +eclipses of the sun, can be foretold for any place +over which the shadow passes.</p> + +<p>All these data derived by the mathematician are +known as the elements of the eclipse, and they +are prepared many years in advance and published +in the nautical almanacs and astronomical ephemerides +issued by the leading nations. Buchanan's +"Treatise on Eclipses" will supply all the technical +information regarding the prediction of eclipses +that anyone desirous of inquiring into this phase +of the problem may desire.</p> + +<p><span class="pagenum"><a name="Page_p217" id="Page_p217">[217]</a></span> +So important are total eclipses in the scheme of +modern solar research, and so necessary are clear +skies in order that expeditions may be favored with +success, that every effort is now made to ascertain +the weather chances at particular stations along +the line of eclipse many years in advance. This +method of securing preliminary cloud observations +for a series of years has proved especially useful +for the eclipses of 1893, 1896, 1900, and 1918; and +had it been employed in Russia for totality of +1914, many well-equipped expeditions might have +been spared disaster. The California and Mexico +totality of 1923 does not require this forethought, +as the regions visited are quite likely to be free +from cloud; but observations are now in process of +accumulation for the total eclipse of 1925. The out-look +for clear skies on that occasion, the total +eclipse nearest New York for more than a century, +is not very promising. The path of totality passes +over Marquette, Michigan, Rochester and Poughkeepsie, +New York, Newport, Rhode Island, and +Nantucket about nine in the morning.</p> + +<p>Everyone who saw it will remember the last +total eclipse in this part of the world—on June 8, +1918, visible from Oregon to Florida. Many will +recall the last total eclipse that was visible before +that in the eastern part of the United States, on +May 28, 1900, visible in a narrow path from New +Orleans to Norfolk. One's father or grandfather +will perhaps remember the total eclipse of July 29, +1878, which passed over the United States from +Pike's Peak to Texas (it was the writer's maiden +eclipse), and another on August 7, 1869, which +passed southeasterly over Iowa and Kentucky. On +all these occasions the paths of total eclipse were +<span class="pagenum"><a name="Page_p218" id="Page_p218">[218]</a></span> +dotted with numerous observing parties, many of +them equipped with elaborate apparatus for studying +and photographing the solar corona and prominences, +together with a multitude of other phenomena +which are seen only when total eclipses +take place.</p> + +<p>Looking forward rather than backward, a striking +series, or family, of eclipses happens in the +future: it is the series of May, 1901 and 1919, recurring +again on June 8, 1937 (over the Pacific +Ocean), June 20, 1955 (through India, Siam, and +Luzon), and June 30, 1973 (visible in Sahara, +Abyssinia, and Somali). Already in 1919 this +totality was 6 minutes 50 seconds in duration; in +1937, as already mentioned, it will be 7 minutes 20 +seconds, and at the subsequent returns even longer +yet, approaching the estimated maximum of 7 +minutes 58 seconds which has never been observed. +This remarkable series of total eclipses is longer in +duration than any others during a thousand years. +Its next subsequent return is in 1991, occurring with +the eclipsed sun practically at noon in the zenith of +Mount Popocatepetl in Mexico.</p> + +<p>Whatever may be the progress of solar research +during the intervening years, it is impossible to +imagine the alert astronomer of that remote day +without incentive for further investigation of the +sun's corona, in which are concealed no doubt many +secrets of the sun's evolution from nebula to star.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p219" id="Page_p219">[219]</a></span></p> + +<h2><a name="CHAPTER_XXXI" id="CHAPTER_XXXI"></a>CHAPTER XXXI<br /> +<br /> +THE SOLAR CORONA</h2> + +<p>"And what is the sun's corona?" mildly asked +a college professor of a student who might +better have answered "Not prepared."</p> + +<p>"I did know, Professor, but I have forgotten," +was his reply.</p> + +<p>"What an incalculable loss to science," returned +the professor with a twinkle. "The only man who +ever knew what the sun's corona is, and he has +forgotten!"</p> + +<p>Only in part has the mystery of the corona been +cleared by the research of the present day. Our +knowledge proceeds but slowly, because the corona +has never been seen except during total eclipses of +the sun; and astronomers, as a matter of fact, have +never had a fair chance at it. Two total eclipses +happen on the average of every three years; their +average duration is only two or three minutes; +totality can be seen only in a narrow path about +a hundred miles wide, though it may be several +thousand miles long; there is usually about equal +chance of cloud with clear skies; and fully three-fourths +of the totality areas of the globe are unavailable +because covered by water. So that even +if we imagine the tracks of eclipses quite thickly +populated with astronomers and telescopes, at least +one every hundred miles, how much solid watching +of the corona would this permit? Only a little more +than one week's time in a whole century.</p> + +<p><span class="pagenum"><a name="Page_p220" id="Page_p220">[220]</a></span> +The true corona is at least a triple phenomenon +and a very complex one. The photographs reveal +it much as the eye sees it, with all its complexity +of interlacing streamers projected into a flat, or +plane, surrounding the disk of the dark moon which +hides the true sun completely. But we must keep +in mind the fact that the sun is a globe, not a +disk, and that the streamers of the corona radiate +more or less from all parts of the surface of the +solar sphere, much as quills from a porcupine.</p> + +<p>From the sun's magnetic poles branch out the +polar rays, nearly straight throughout their visible +extent. Gradually as the coronal rays originate at +points around the solar disk farther and farther +removed from the poles, they are more and more +curved. Very probably they extend into the equatorial +regions, but it is not easy to trace them there +because they are projected upon and confused with +the filaments having their origin remote from the +poles. Then there is the inner equatorial corona, +apparently connected intimately with truly solar +phenomena, quite as the polar rays are. The third +element in the composite is the outer ecliptic corona, +for the most part made up of long streamers. +This is most fully developed at the time of the fewest +spots on the sun. It is traceable much farther +against the black sky with the naked eye than by +photography. Without any doubt it is a solar appendage +and possibly it may merge into the zodiacal +light.</p> + +<p>Naturally this superb spectacle must have been +an amazing sight to the beholders of antiquity who +were fortunate enough to see it. Historical references +are rare: perhaps the earliest was by Plutarch +about <span class="smcap2">A. D.</span> 100, who wrote of it, "A radiance +<span class="pagenum"><a name="Page_p221" id="Page_p221">[221]</a></span> +shone round the rim, and would not suffer darkness +to become deep and intense." Philostratus a century +later mentions the death of the emperor Domitian at +Ephesus as "announced" by a total eclipse.</p> + +<p>Kepler thought the corona was evidence of a +lunar atmosphere; indeed, it was not until the +middle of the 19th century that its lack of relation +to the moon was finally demonstrated. Later observers, +Wyberd in 1652 and Ulloa, got the impression +that the corona turned round the disk +catherine-wheel fashion, "like an ignited wheel +in fireworks, turning on its center." But no later +observer has reported anything of the sort. Quite +the contrary, there it stands against the black sky +in motionless magnificence a colorless pearly mass +of wisps and streamers for the most part nebulous +and ill-defined, fading out very irregularly into the +black sky beyond, but with a complex interlacing of +filaments, sometimes very sharply defined near the +solar poles. It defies the skill of artist and draughtsman +to sketch it before it is gone.</p> + +<p>Photograph it? Yes, but there are troubles. Of +course the camera work is superior to sketches by +hand. As Langley used to say, "The camera has +no nerves, and what it sets down we may rely on." +Foremost among the photographic difficulties is the +wide variation in intensity of the coronal light in +different regions of the corona. If a plate is exposed +long enough to get the outer corona, the +exceeding brightness of the inner corona overexposes +and burns out that part of the plate or film. +If the exposure is short, we get certain regions of +the inner corona excellently, but the outer regions +are a blank because they can be caught only by a +long exposure.</p> + +<p><span class="pagenum"><a name="Page_p222" id="Page_p222">[222]</a></span> +So the only way is to take a series of pictures +with a wide range of exposures, and then by careful +and artistic handwork, combine them all into +a single drawing. Wesley of London has succeeded +eminently in work of this character, and his drawings +of the sun's corona, visible at total eclipses +from 1871 onward, in possession of the Royal +Astronomical Society, are the finest in existence. +They give a vastly better idea of the corona, as +the eye sees it, than any single photograph possibly +can.</p> + +<p>The early observers apparently never thought of +the corona as being connected with the sun. It was +a halo merely, and so drawn. Its real structure was +neither known, depicted, or investigated. Sketches +were structureless, as any aureola formed by stray +sunlight grazing the moon might naturally be. That +the rays are curved and far from radial round the +sun was shown for the first time in the sketches +of 1842, and in 1860 Sir Francis Galton observed +that the long arms or streamers "do not radiate +strictly from the center."</p> + +<p>The inner corona had first been recorded photographically +on a daguerreotype plate during the +eclipse of 1851, but the lens belonged to a heliometer, +and was of course uncorrected for the photographic +rays. The wet collodion plates of the +eclipse of 1860, by De la Rue, proved that not only +the prominences but the corona were truly solar, +because his series of technically perfect pictures +revealed the steady and unchanged character of +these phenomena while the moon's disk was passing +over them as totality progressed. And at the +eclipse of 1869, Young put the solar theory of the +corona beyond the shadow of any further doubt +<span class="pagenum"><a name="Page_p223" id="Page_p223">[223]</a></span> +by examination of its light with the spectroscope +and discovering a green line in the spectrum due +to incandescent vapor of a substance not then +identified with anything terrestrial, and therefore +called coronium.</p> + +<p>The total brilliance of the corona was very differently +estimated by the earlier observers, though +pretty carefully measured at later eclipses. The +standard full moon is used for reference, and at +one eclipse the corona falls short of, while at another +it will exceed the full moon in brightness. +Variations in brilliancy are quite marked: at one +eclipse it was nearly four times as bright as the +full moon. Much evidence has already accumulated +on this question; but whether the observed variations +are real, or due mainly to the varying relative +sizes of sun and moon at different eclipses, is +not yet known. The coronal light is largely bluish +in tint, and this is the region of the spectrum most +powerfully absorbed by our atmosphere. Eclipses +are observed by different expeditions located at +stations where the eclipsed sun stands at very different +altitudes above the horizon; besides this the +localities of observation are at varied elevations +above sea level; so that the varying amount of +absorption of the coronal light renders the problem +one of much difficulty.</p> + +<p>The long ecliptic streamers of the corona were +first seen by Newcomb and Langley during the +totality of 1878. On one side of the sun there was +a stupendous extension of at least twelve solar +diameters, or nearly 11 millions of miles. Langley +observed from the summit of Pike's Peak, over +14,000 feet high, and was sure that he was witnessing +a "real phenomenon heretofore undescribed." +<span class="pagenum"><a name="Page_p224" id="Page_p224">[224]</a></span> +The vast advantage of elevation was apparent +also from the fact that he held the corona +for more than four minutes after true totality had +ended. These streamers are characteristic of the +epoch of minimum spots on the sun, as Ranyard +first suggested. It was found that this type of +corona had been recorded also in 1867; and it has +reappeared in 1889, 1900 and 1911, and will doubtless +be visible again in 1922.</p> + +<p>How rapidly the streamers of the corona vary +is not known. Occasionally an observer reports +having seen the filaments vibrate rapidly as in the +aurora borealis, but this is not verified by others +who saw the same corona perfectly unmoving. Comparisons +of photographs taken at widely separate +stations during the same eclipse have shown that at +least the corona remained stationary for hours at +a time. Whether it may be unchanged at the end +of a day, or a week, or a month, is not known; because +no two total eclipses can ever happen nearer +each other than within an interval of 173 days, or +one-half of the eclipse year. And usually the interval +between total eclipses is twice or three times +this period.</p> + +<p>Theories of what the solar corona may be are +very numerous. The extreme inner corona is perhaps +in part a sort of gaseous atmosphere of the +sun, due to matter ejected from the sun, and kept +in motion by forces of ejection, gravity, and repulsion +of some sort. Meteoric matter is likely concerned +in it, and Huggins suggested the débris of +disintegrating comets. Schuster was in agreement +with Huggins that the brighter filaments of the +corona might be due to electric discharges, but it +seems very unlikely that any single hypothesis can +completely account for the intricate tracery of so +complex a phenomenon.</p> + +<p><span class="pagenum"><a name="Page_p224p1" id="Page_p224p1">[224i]</a></span></p> + +<div class="fig_center" style="width: 636px;"> +<img src="images/p224_1.png" width="636" height="587" alt="" /> +<div class="fig_caption"><span class="smcap">Solar Corona and Prominences.</span> Photographed during a total eclipse of the +sun, June 8, 1918. (<i>Courtesy, American Museum of Natural History.</i>)</div> +</div> + +<p><span class="pagenum"><a name="Page_p224p2" id="Page_p224p2">[225i]</a></span></p> + +<div class="fig_center" style="width: 650px;"> +<img src="images/p224_2top.png" width="650" height="517" alt="" /> +<div class="fig_caption"><span class="smcap">Venus, Showing Crescent Phase of the Planet.</span> Venus is the earth's +nearest neighbor on the side toward the sun. (<i>Photo, Yerkes Observatory.</i>)</div> +</div> + +<div class="fig_center" style="width: 643px;"> +<img src="images/p224_2bot.png" width="643" height="543" alt="" /> +<div class="fig_caption"><span class="smcap">Mars, the Planet Next Beyond the Earth.</span> The photograph shows +one of the white polar caps. The caps are thought to be snow or ice and +may indicate the existence of atmosphere. (<i>Photo, Yerkes Observatory.</i>)</div> +</div> + +<p><span class="pagenum"><a name="Page_p225" id="Page_p225">[225]</a></span> +Elaborate spectroscopic programs have been +carried out at recent eclipses, affording evidence +that certain regions are due to incandescent matter +of lower temperature than the sun's surface. A +small part of the light of the corona is sunlight reflected +from dark particles possibly meteoric, but +more likely dust particles or fog of some sort. This +accounts for the weakened solar spectrum with +Fraunhofer absorption lines, and this part of the +light is polarized.</p> + +<p>Many have been the attempts to see, or photograph, +the corona without an eclipse. None of +them has, however, succeeded as yet. Huggins got +very promising results nearly forty years ago, and +success was thought to have been reached; but subsequent +experiments on the Riffelberg in 1884 and +later convinced him that his results related only to +a spurious corona. In 1887 the writer made an unsuccessful +attempt to visualize the corona from the +summit of Fujiyama, and Hale tried both optical +and photographic methods on Pike's Peak in 1893 +without success. He devised later a promising +method by which the heat of the corona in different +regions can be measured by the bolometer, and an +outline corona afterward sketched from these +results.</p> + +<p>Still another method of attacking the problem +occurred to the writer in 1919, which has not yet +been carried out. It would take advantage of recent +advances in aeronautics, and contemplates an +artificial eclipse in the upper air by means of a +black spherical balloon. This would be sent up to +an altitude of perhaps 40,000 feet, where it would +<span class="pagenum"><a name="Page_p226" id="Page_p226">[226]</a></span> +partake of the motion of the air current in which +it came to equilibrium. Then a snapshot camera +would be mounted on an aeroplane, in which the +aviator would ascend to such a height that the balloon +just covered the sun, as the moon does in a +total eclipse. With the center of the balloon in line +with the sun's center, he would photograph the +regions of the sky immediately surrounding the sun, +against which the corona is projected. As the entire +apparatus would be above more than an entire +half of the earth's atmosphere, the experiment +would be well worth the attempt, as pretty much +everything else has been tried and found wanting. +Needless to say, the importance of seeing the corona +at regular intervals whenever desired, without +waiting for eclipses of the sun, remains as insistent +as ever.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p227" id="Page_p227">[227]</a></span></p> + +<h2><a name="CHAPTER_XXXII" id="CHAPTER_XXXII"></a>CHAPTER XXXII<br /> +<br /> +THE RUDDY PLANET</h2> + +<p>Mars is a planet next in order beyond the +earth, and its distance from the sun averages +141½ million miles. It has a relatively rapid motion +among the stars, its color is reddish, and, when +nearest to us, it is perhaps the most conspicuous +object in the sky.</p> + +<p>Mars appeared to the ancients just as it does to us +to-day. Aristotle recorded an observation of Mars, +356 <span class="smcap2">B. C.</span>, when the moon passed over the planet, or +occulted it, as our expression is. Galileo made the +first observations of Mars with a telescope in 1610, +and his little instrument was powerful enough to +enable him to discover that the planet had phases, +though it did not pass through all the phases that +Mercury and Venus do. This was obvious from the +fact that Mars is always at a greater distance from +the sun than we are, and the phase can only be +gibbous, or about like the moon when midway between +full and quarter.</p> + +<p>Many observers in the seventeenth century followed +up the planet with such feeble optical power +as the telescopes of that epoch provided: Fontana +(who made the first sketch), Riccioli and Bianchini +in Italy, Cassini in France, Huygens in Holland, and +later Sir William Herschel in England.</p> + +<p>It was Cassini who first made out the whitish +spots or polar caps of Mars in 1666, but not until +<span class="pagenum"><a name="Page_p228" id="Page_p228">[228]</a></span> +after Huygens had noted the fact that Mars turned +round on an axis in a period but little longer than +the earth's. Cassini followed it up later with a +more accurate value; and observations in our own +day, when combined with these early ones, enable +us to say that the Martian day is equal to 24 +hours 37 minutes 22.67 seconds, accurate probably +to the hundredth part of a second.</p> + +<p>When we know that a planet turns round on an +axis, we know that it has a day. When we know +the direction of the axis in space or in relation to the +plane of its path round the sun, we know that it +has seasons: we can tell their length and when they +begin and end. It did not take many years of observation +to prove that the axis round which Mars +turns is tilted to the plane of its path round the +sun by an angle practically the same as that at +which the earth's axis is tilted. So there is the immediate +inference that on Mars the order and perhaps +the character of the seasons is much the same +as here on the earth.</p> + +<p>At least two things, however, tend to modify +them. First, the year of Mars is not 365 days like +ours, but 687 days. Each of the four seasons on +Mars, therefore, is proportionally longer than our +seasons are. Then comes the question of atmosphere—how +much of an atmosphere does Mars +really possess in proportion to ours, and how would +its lesser amount modify the blending of the +seasons into one another?</p> + +<p>All discussion of Mars and the problems of existence +of life upon that planet hinge upon the character +and extent of Martian atmosphere. The +planet seems never to be covered, as the earth +usually is, with extensive areas of cloud which to +<span class="pagenum"><a name="Page_p229" id="Page_p229">[229]</a></span> +an observer in space would completely mask its +oceans and continents. Nearly all the time Mars +in his equatorial and temperate zones is quite clear +of clouds. A few whitish spots are occasionally +seen to change their form and position in both +northern and southern latitudes, and they vary +with the progress of the day on Mars, as clouds +naturally would. But Schiaparelli, perhaps the best +of all observers, thought them to be not low-lying +clouds of the nimbus type that would produce rains, +but rather a veil of fog, or perhaps a temporary +condensation of vapor, as dew or hoar frost. But +the strongest argument for an atmosphere is based +on the temporary darkening or obscuration of well +known and permanent markings on the surface of +Mars. These are more or less frequently observed +and clouds afford the best explanation of their +occurrence.</p> + +<p>So much for evidence supplied by the telescope +alone. When, however, we employ the spectroscope +in conjunction with the telescope, another sort of +evidence is at hand. Several astronomers have +reached the conclusion that watery vapor exists in +the atmosphere of Mars, while other astronomers +equipped with equal or superior apparatus, and +under equally favorable or even better conditions, +have reached the remarkable conclusion that the +spectra of Mars and the moon are identical in every +particular. From this we should be led to infer +that Mars has perhaps no more atmosphere than +the moon has, that is to say, none whatever that +present instruments and methods of investigation +have enabled us to detect.</p> + +<p>What then, shall we conclude? Simply that the +atmosphere of Mars is neither very dense nor extensive. +<span class="pagenum"><a name="Page_p230" id="Page_p230">[230]</a></span> +Probably its lower strata close to the planet's +surface are about as dense as the earth's atmosphere +is at the summits of our highest mountains.</p> + +<p>This conclusion is not unwelcome, if we keep a +few fundamental facts in clear and constant view. +Mars is a planet of intermediate size between the +earth and the moon: twice the moon's diameter +(2,160 miles) very nearly equals the diameter of +Mars (4,200 miles), and twice the diameter of +Mars does not greatly exceed the earth's diameter +(7,920 miles). As to the weights or masses of +these bodies, Mars is about one-ninth, and the moon +one-eightieth of the earth. The atmospheric envelope +of the earth is abundant, the moon has none +as far as we can ascertain; so it seems safe to +infer that Mars has an atmosphere of slight density: +not dense enough to be detected by spectroscopic +methods, but yet dense enough to enable us +to explain the varying telescopic phenomena of the +planet's disk which we should not know how to account +for, if there were no atmosphere whatever. +One astronomer has, indeed, gone so far as to calculate +that in comparison with our planet Mars is +entitled to one-twentieth as much atmosphere as +we have, and that the mercurial barometer at "sea +level" would run about five and a half inches, as +against thirty inches on the earth.</p> + +<p>In general, then, the climate of Mars is probably +very much like that of a clear season on a very +high terrestrial table land or mountain—a climate +of wide extremes, with great changes of temperature +from day to night. The inequality of Martian +seasons is such that in his northern hemisphere the +winter lasts 381 days and the summer only 306 +days.</p> + +<p><span class="pagenum"><a name="Page_p231" id="Page_p231">[231]</a></span> +Now, the polar caps of Mars, which are reasonably +assumed to be due to snow or hoar frost, attain +their maximum three or four months after the +winter solstice, and their minimum about the same +length of time after the summer solstice. This +lagging should be interpreted as an argument for a +Martian atmosphere with heat-storing qualities, +similar to that possessed by the earth.</p> + +<p>Upon this characteristic, indeed, depends the +climate at the surface of Mars: whether it is at all +similar to our own, and whether fluid water is a +possibility on Mars or not. While the cosmic relations +of the planet in its orbit are quite the same as +ours, nevertheless the greater distance of Mars +diminishes his supply of direct solar heat to about +half what we receive. On the other hand, his distance +from the sun during his year of motion +around it varies much more widely than ours, so +that he receives when nearest the sun about one-half +more of solar heat than he does when farthest away.</p> + +<p>Southern summers on Mars, therefore, must be +much hotter, and southern winters colder than the +corresponding seasons of his northern hemisphere. +Indeed, the length of the southern summer, nearly +twice that of the terrestrial season, sometimes +amply suffices to melt all the polar ice and snow, +as in October, 1894, when the southern polar cap +of Mars dwindled rapidly and finally vanished +completely.</p> + +<p>Very interesting in this connection are the researches +of Stoney on the general conditions affecting +planetary atmospheres and their composition. +According to the kinetic theory, if the molecules of +gases which are continually in motion travel outward +from the center of a planet, as they frequently +<span class="pagenum"><a name="Page_p232" id="Page_p232">[232]</a></span> +must, and with velocities surpassing the +limit that a planet's gravity is capable of controlling, +these molecules will effect a permanent +escape from the planet, and travel through space in +orbits of their own.</p> + +<p>So the moon is wholly without atmosphere +because the moon's gravity is not powerful enough +to retain the molecules of its component gases. So +also the earth's atmosphere contains no helium or +free hydrogen. So, too, Mars is possessed of insufficient +force of gravity to retain water vapor, and +the Martian atmosphere may therefore consist +mainly of nitrogen, argon, and carbon dioxide.</p> + +<p>As everyone knows, the axis of the earth if extended +to the northern heavens would pass very +near the north polar star, which on that account is +known as Polaris. In a similar manner the axis +of Mars pierces the northern heavens about midway +between the two bright stars Alpha Cephei +and Alpha Cygni (Deneb). The direction of this +axis is pretty accurately known, because the measurement +of the polar caps of the planet as they turn +round from night to night, year in and year out, has +enabled astronomers to assign the inclination of the +axis with great precision.</p> + +<p>These caps are a brilliant white, and they are +generally supposed to be snow and ice. They +wax and wane alternately with the seasons on +Mars, being largest at the end of the Martian winter +and smallest near the end of summer. The +existence of the polar caps together with their +seasonal fluctuations afford a most convincing +argument for the reality of a Martian atmosphere, +sufficiently dense to be capable of diffusing and +transporting vapor.</p> + +<p><span class="pagenum"><a name="Page_p233" id="Page_p233">[233]</a></span> +The northern cap is centered on the pole almost +with geometric exactness, and as far as the 85th +parallel of latitude. On the other hand, the south +polar cap is centered about 200 miles from the true +pole, and this distance has been observed to vary +from one season to another. No suggestion has been +made to account for this singular variation. On one +occasion it stretched down to Martian latitude 70 +degrees and was over 1,200 miles in diameter.</p> + +<p>Pickering watched the changing conditions of +shrinking of the south polar cap in 1892 with a +large telescope located in the Andes of Peru. Mars +was faithfully followed on every night but one from +July 13 to September 9, and the apparent alterations +in this cap were very marked, even from night +to night. As the snows began to decrease, a long +dark line made its appearance near the middle of +the cap, and gradually grew until it cut the cap in +two. This white polar area (and probably also the +northern one in similar fashion) becomes notched +on the edge with the progress of its summer season; +dark interior spots and fissures form, isolated +patches separate from the principal mass, and later +seem to dissolve and disappear. Possibly if one +were located on Mars and viewing our earth with a +big telescope, the seasonal variation of our north +and south polar caps might present somewhat similar +phenomena. All the recent oppositions of Mars +have been critically observed by Pickering from an +excellent station in Jamaica.</p> + +<p>Quite obviously the fluctuations of the polar caps +are the key to the physiographic situation on Mars, +and they are made the subject of the closest +scrutiny at every recurring opposition of the planet. +Several observers, Lowell in particular, record a +<span class="pagenum"><a name="Page_p234" id="Page_p234">[234]</a></span> +bluish line or a sort of retreating polar sea, following +up the diminishing polar cap as it shrinks with +the advance of summer. It is said that no such line +is visible during the formation of the polar cap with +the approach of winter. All such results of critical +observation, just on the limit of visibility, have to +be repeated over and over again before they become +part of the body of accepted scientific fact. And in +many instances the only sure way is to fall back on +the photographic record, which all astronomers, +whether prejudiced or not, may have the opportunity +to examine and draw their individual conclusions.</p> + +<p>Already the approaching opposition of 1924, the +most favorable since the invention of the telescope, +is beginning to attract attention, and preparations +are in progress, of new and more powerful instruments, +with new and more sensitive photographic +processes, by means of which many of the present +riddles of Mars may be solved.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p235" id="Page_p235">[235]</a></span></p> + +<h2><a name="CHAPTER_XXXIII" id="CHAPTER_XXXIII"></a>CHAPTER XXXIII<br /> +<br /> +THE CANALS OF MARS</h2> + +<p>Then there are the so-called canals of Mars, +about which so much is written and relatively +little known. Faint markings which resemble them +in character were first drawn in 1840 and later in +1864, but Schiaparelli, the famous Italian astronomer, +is probably their original discoverer, when +Mars was at its least distance from the earth in +1877. He made the first accurate detailed map of +Mars at this time, and most of the important or +more conspicuous canals (<i>canali</i>, he called them in +Italian, that is, channels merely, without any reference +whatever to their being watercourses) were +accurately charted by him.</p> + +<p>At all the subsequent close approaches of Mars, +the canals have been critically studied by a wide +range of astronomical observers, and their conclusions +as to the nature and visibility of the canals +have been equally wide and varied. The most +favorable oppositions have occurred in 1892 and +1894, also in 1907 and 1909. On these occasions +a close minimum distance of Mars was reached, that +is, about 35 millions of miles; but in 1924 the planet +makes the closest approach in a period of nearly a +thousand years. Its distance will not much exceed +34 millions of miles.</p> + +<p>But although this is a minimum distance for Mars, +it must not be forgotten that it is a really vast +<span class="pagenum"><a name="Page_p236" id="Page_p236">[236]</a></span> +distance, absolutely speaking; it is something like +150 times greater than the distance of the moon. +With no telescopic power at our command could we +possibly see anything on the moon of the size of +the largest buildings or other works of human intelligence; +so that we seem forever barred from +detecting anything of the sort on Mars.</p> + +<p>Nevertheless, the closest scrutiny of the ruddy +planet by observers of great enthusiasm and intelligence, +coupled with imagination and persistence, +have built up a system of canals on Mars, covering +the surface of the planet like spider webs over +a printed page, crossing each other at intersecting +spots known as "lakes," and embodying a +wealth of detail which challenges criticism and +explanation.</p> + +<p>To see the canals at all requires a favorable +presentation of Mars, a steady atmosphere and a +perfect telescope, with a trained eye behind it. Not +even then are they sure to be visible. The training +of the eye has no doubt much to do with it. So +photography has been called in, and very excellent +pictures of Mars have already been taken, some +nearly half as large as a dime, showing plainly the +lights and shades of the grander divisions of the +Martian surface, but only in a few instances revealing +the actual canals more unmistakably than they +are seen at the eyepiece.</p> + +<p>The appearance and degree of visibility of the +canals are variable: possibly clouds temporarily +obscure them. But there is a certain capriciousness +about their visibility that is little understood. In +consequence of the changing physical aspects, as to +season, on Mars and his orbital position with reference +to the earth, some of the canals remain for +<span class="pagenum"><a name="Page_p237" id="Page_p237">[237]</a></span> +a long time invisible, adding to the intricacy of the +puzzle.</p> + +<p>For the most part the canals are straight in their +course and do not swerve much from a great circle +on the planet. But their lengths are very different, +some as short as 250 miles, some as long as 4,000 +miles; and they often join one another like spokes +in the hub of a wheel, though at various angles. As +depicted by Lowell and his corps of observers at +Flagstaff, Arizona, the canal system is a truly marvelous +network of fine darkish stripes. Their color +is represented as a bluish green.</p> + +<p>Each marking maintains its own breadth throughout +its entire length, but the breadth of all the canals +is by no means the same: the narrowest are perhaps +fifteen to twenty miles wide, and the broadest probably +ten times that. At least that must be the +breadth of the Nilosyrtis, which is generally regarded +as the most conspicuous of all the canals. The +Lowell Observatory has outstripped all others in the +number of canals seen and charted, now about 500.</p> + +<p>What may be the true significance of this remarkable +system of markings it is impossible to conclude +at present. Schiaparelli from his long and critical +study of them, their changes of width and color, was +led to think that they may be a veritable hydrographic +system for distributing the liquid from the +melting polar snows. In this case it would be difficult +to escape the conviction that the canals have, at +least in part, been designed and executed with a +definite end in view.</p> + +<p>Lowell went even farther and built upon their +behavior an elaborate theory of life on the planet, +with intelligent beings constructing and opening +new canals on Mars at the present epoch. Pickering +<span class="pagenum"><a name="Page_p238" id="Page_p238">[238]</a></span> +propounded the theory that the canals are not water-bearing +channels at all, but that they are due to +vegetation, starting in the spring when first seen +and vitalized by the progress of the season poleward, +the intensity of color of the vegetation coinciding +with the progress of the season as we observe it.</p> + +<p>Extensive irrigation schemes for conducting +agricultural operations on a large scale seem a very +plausible explanation of the canals, especially if we +regard Mars as a world farther advanced in its life +history than our own. Erosion may have worn the +continents down to their minimum elevation, rendering +artificial waterways not difficult to build; +while with the vanishing Martian atmosphere and +absence of rains, the necessity of water for the support +of animal and vegetal life could only be met +by conducting it in artificial channels from one region +of the planet to another.</p> + +<p>Interesting as this speculative interpretation is, +however, we cannot pass by the fact that many competent +astronomers with excellent instruments finely +located have been unable to see the canals, and therefore +think the astronomers who do see them are +deceived in some way. Also many other astronomers, +perhaps on insufficient grounds, deny their +existence <i>in toto</i>.</p> + +<p>Many patient years of labor would be required to +consult all the literature of investigation of the +planet Mars, but much of the detail has been +critically embodied in maps at different epochs, by +Kayser, Proctor, Green, and Dreyer. And Flammarion +in two classic volumes on Mars has presented +all the observations from the earliest time, +together with his own interpretation of them. Areography +is a term sometimes applied to a description +<span class="pagenum"><a name="Page_p239" id="Page_p239">[239]</a></span> +of the surface of Mars, and it is scarcely an exaggeration +to say that areography is now better +known than the geography of immense tracts of the +earth.</p> + +<p>For some reason well recognized, though not at all +well understood, Mars although the nearest of all +the planets, Venus alone excepted, is an object by +no means easy to observe with the telescope. Possibly +its unusual tint has something to do with this. +With an ordinary opera glass examine the moon +very closely, and try to settle precise markings, +colors, and the nature of objects on her surface; +Mars under the best conditions, scrutinized with +our largest and best telescope, presents a problem +of about the same order of difficulty. There are +delicate and changing local colors that add much +uncertainty. Nevertheless, the planet's leading +features are well made out, and their stability since +the time of the earliest observers leaves no room to +doubt their reality as parts of a permanent planetary +crust.</p> + +<p>The border of the Martian disk is brighter than +the interior, but this brightness is far from uniform. +Variations in the color of the markings often depend +on the planet's turning round on its axis, and the +relation of the surface to our angle of vision. If +we keep in mind these obstacles to perfect vision in +our own day, it is easy to see why the early users of +very imperfect telescopes failed to see very much, +and were misled by much that they thought they +saw. Then, too, they had to contend, as we do, with +unsteadiness of atmosphere, which is least troublesome +near the zenith.</p> + +<p>As their telescopes were all located in the northern +hemisphere, the northern hemisphere of Mars +<span class="pagenum"><a name="Page_p240" id="Page_p240">[240]</a></span> +is the one best circumstanced for their investigation; +because at the remote oppositions of Mars, +which always happen in our northern winter with +the planet in high north declination, it is always the +north pole of Mars which is presented to our +view. Whereas the close oppositions of the planet +always come in our northern midsummer, with +Mars in south declination and therefore passing +through the zenith of places in corresponding south +latitude.</p> + +<p>With Mars near opposition, high up from the +horizon, a fairly steady atmosphere, and a magnifying +power of at least 200 diameters, even the most +casual observer could not fail to notice the striking +difference in brightness of the two hemispheres: +the northern chiefly bright and the southern +markedly dark. Formerly this was thought to indicate +that the southern hemisphere of Mars was +chiefly water and the northern land, much as is the +case on the earth: with this difference, however, +that water and land on the earth are proportioned +about as eleven to four.</p> + +<p>But Mars in its general topography presents no +analogy with the present relation of land and water +on the earth. There seems no reason to doubt that +the northern regions with their prevailing orange +tint, in some places a dark red and in others fading +to yellow and white, are really continental in character. +Other vast regions of the Martian surface are +possibly marshy, the varying depth of water causing +the diversity of color. If we could ever catch a +reflection of sunlight from any part of the surface +of Mars, we might conclude that deep water exists +on the planet; but the farther research progresses, +the more complete becomes the evidence that permanent +<span class="pagenum"><a name="Page_p241" id="Page_p241">[241]</a></span> +water areas on Mars, if they exist at all, +are extremely limited.</p> + +<p>Since 1877 Mars has been known to possess two +satellites, which were discovered in August of that +year by Hall at Washington. Moons of this planet +had long been suspected to exist and on one or two +previous occasions critically looked for, though +without success. In the writings of Dean Swift +there is a fanciful allusion to the two moons of +Mars; and if astronomers had chanced to give +serious attention to this, Phobos and Deimos, as +Hall named them, might have been discovered long +before.</p> + +<p>They are very small bodies, not only faint in the +telescope, but actually of only ten or twenty miles +diameter; and from the strange relation that +Phobos, the inner moon, moves round Mars three +times while the planet itself is turning round only +once on its axis, some astronomers incline to the +hypothesis that this moon at least was never part of +Mars itself, but that it was originally an inner or +very eccentric member of the asteroid group, which +ventured within the sphere of gravitation of Mars, +was captured by that planet, and has ever since +been tributary to it as a secondary body or satellite.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p242" id="Page_p242">[242]</a></span></p> + +<h2><a name="CHAPTER_XXXIV" id="CHAPTER_XXXIV"></a>CHAPTER XXXIV<br /> +<br /> +LIFE IN OTHER WORLDS</h2> + +<p>Popular interest in astronomy is exceedingly +wide, but it is very largely confined to the idea of +resemblances and differences between our earth +and the bodies of the sky. The question most +frequently asked the astronomer is, "Have any of +the stars got people on them?" Or more specifically, +"Is Mars inhabited?" The average questioner will +not readily be turned off with yes or no for an +answer. He may or may not know that it is quite +impossible for astronomers to ascertain anything +definite in this matter, most interesting as it is. +What he wants to find out is the view of the individual +astronomer on this absorbing and ever recurring +inquiry.</p> + +<p>We ought first to understand what is meant by +the manifestation here on the earth called life, and +agree concerning the conditions that render it +possible. Apparently they are very simple. We +may or may not agree that a counterpart of life, or +life of a wholly different type from ours, may exist +on other planets under conditions wholly diverse +from those recognized as essential to its existence +here. The problem of the origin of life is, in the +present state of knowledge, highly speculative and +hardly within the domain of science. Here on +earth, life is intimately associated with certain +chemical compounds, in which carbon is the common +<span class="pagenum"><a name="Page_p243" id="Page_p243">[243]</a></span> +element without which life would not exist. Also +hydrogen, oxygen, and nitrogen are present, with +iron, sulphur, phosphorus, magnesium and a few +less important elements besides. But carbon is the +only substance absolutely essential. Protoplasm +cannot be built without it, and protoplasm makes up +the most of the living cell. Closely related to carbon +is silica also, as a substitution in certain organic +compounds. Protoplasm is able to stand very low +temperatures, but its properties as a living cell cease +when the temperature reaches 150 Fahrenheit.</p> + +<p>Animal life as it exists on the earth to-day appears +to have been here many million years. The +palæontologists agree that all life originated in the +waters of the earth. It has passed through evolutionary +stages from the lowest to the highest. +Throughout this vast period the astronomer is able +to say that the conditions of the earth which +appear to be essential to the maintenance of life +have been pretty constantly what they are to-day. +The higher the type of life, the narrower the range +of conditions under which it thrives. Man can +exist at the frigid poles even if the temperature is +75 degrees below Fahrenheit zero; and in the +deserts and the tropics, he swelters under temperatures +of 115 degrees, but he still lives. At these +extremes, however, he can scarcely be said to thrive.</p> + +<p>We have, then, a relatively narrow range of temperatures +which seems to be essential to his comfortable +existence and development: we may call it +150 degrees in extent. Had not the surface temperature +of the earth been maintained within this range +for indefinite ages, in the regions where the human +race has developed, quite certainly man would not be +here. How this equability of temperature has been +<span class="pagenum"><a name="Page_p244" id="Page_p244">[244]</a></span> +maintained does not now matter. Clearly the earth +must have existed through indefinite ages in the +process of cooling down from temperatures of at +least 6,000 degrees.</p> + +<p>During this stage the temperature of the surface +was earth-controlled. Then this period merged +very gradually into the stage where life became +possible, and the temperature of the surface became, +as it now is, sun-controlled. How many years are +embraced in this span of periods, or ages, we have +no means of knowing. But of the sequence of +periods and the secular diminution of temperature, +we may be certain.</p> + +<p>Then there is the equally important consideration +of water necessary for the origination, support, +and development of life. We cannot conceive of +life existing without it. On the earth water is +superabundant, and has been for indefinite ages in +the past. There is little evidence that the oceans +are drying up; although the commonly accepted +view is that the waters of the earth will very gradually +disappear. Water can exist in the fluid state, +which is essential to life, at all temperatures between +32 degrees and 680 degrees F.</p> + +<p>Air to breathe is essential to life also. The atmosphere +which envelops the earth is at least 100 miles +in depth, and its own weight compresses it to a +tension of nearly 15 pounds to the square inch at +sea level. This atmosphere and its physical properties +have had everything to do with the development +of animal life on the planet. Without it and +its remarkable property of selective absorption, +which imprisons and diffuses the solar heat, it is inconceivable +that the necessary equability of surface +temperature could be maintained. This appears to +<span class="pagenum"><a name="Page_p245" id="Page_p245">[245]</a></span> +be quite independent of the chemical constituents of +the atmosphere, and is perhaps the most important +single consideration affecting the existence of life +on a planet. If the surface of a planet is partly +covered with water, it will possess also an atmosphere +containing aqueous vapor.</p> + +<p>Heat, water, and air: these three essentials determine +whether there is life on a planet or not. Of +course there must be nutrition suitable to the organism; +mineral for the vegetal, and vegetal for +the animal. But the narrow range of variation +appears to be the striking thing: relatively but a +few degrees of temperature, and a narrow margin of +atmospheric pressure. If this pressure is doubled +or trebled, as in submarine caissons, life becomes insupportable. +If, on the other hand, it is reduced +even one-third, as on mountains even 13,000 feet +high, the human mechanism fails to function, partly +from lack of oxygen necessary in vitalizing the +blood, but mainly because of simple reduction of +mechanical pressure.</p> + +<p>If, then, we conceive of life in other worlds and +it is agreed that life there must manifest itself much +as it does here, our answer to the question of habitability +of the planets must follow upon an investigation +of what we know, or can reasonably surmise, +about the surface temperatures of these bodies, +whether they have water, and what are the probable +physical characteristics of their atmospheres.</p> + +<p>We may inquire about each planet, then, concerning +each of these details.</p> + +<p>The case of Mercury is not difficult. At an +average distance of only 36 million miles from the +sun, and with a large eccentricity of orbit which +brings it a fifth part nearer, conditions of temperature +<span class="pagenum"><a name="Page_p246" id="Page_p246">[246]</a></span> +alone must be such as to forbid the existence of +life. The solar heat received is seven times greater +than at the earth, and this is perhaps sufficient +reason for a minimum of atmosphere, as indicated +by observation. If no air, then quite certainly no +water, as evaporation would supply a slight atmosphere. +But according to the kinetic theory of gases, +the mass of Mercury, only a very small fraction of +that of the sun, is inadequate to retain an atmospheric +envelope. If, however, the planet's day and +year are equal, so that it turns a constant face to +the sun, surface conditions would be greatly complicated, +so that we cannot regard the planet as +absolutely uninhabitable on the hemisphere that is +always turned away from the sun.</p> + +<p>Venus at 67 millions of miles from the sun +presents conditions that are quite different. She +receives double the solar heat that we do, but possessing +an atmosphere perhaps threefold denser +than ours, as reliably indicated by observations of +transits of Venus, the intensity of the heat and its +diffusion may be greatly modified. What the selective +absorption of the atmosphere of Venus may be, +we do not know. Nor is the rotation time of the +planet definitely ascertained: if equal to her year, +as many observations show and as indicated by the +theory of tidal evolution, there may well be certain +regions on the hemisphere perpetually turned away +from the sun where temperature conditions are +identical with those on the tropical earth, and where +every condition for the origin and development of +life is more fully met than anywhere else in the +solar system. Whether Venus has water distributed +as on the earth we do not know, as her surface is +never seen, owing to dense clouds under which she +<span class="pagenum"><a name="Page_p247" id="Page_p247">[247]</a></span> +is always enshrouded. Her cloudy condition possibly +indicates an overplus of water.</p> + +<p>Is the moon inhabited? Quite certainly not: no +appreciable air, no water, and a surface temperature +unmodified by atmosphere—rising perhaps to +100 degrees F. during the day, which is a fortnight +in length, and falling at night to 300 degrees below +zero, if not lower.</p> + +<p>Is Mars inhabited? The probable surface temperature +is much lower than the earth's, because +Mars receives only half as much solar heat as we do; +and more important still, the atmosphere of Mars is +neither so dense nor so extensive as our own. +Seasons on Mars are established, much the same as +here, except that they are nearly twice as long as +ours; and alternate shrinking and enlarging of the +polar caps keeps even pace with the seasons, thereby +indicating a certainty of atmosphere whose +equatorial and polar circulation transports the +moisture poleward to form the snow and ice of +which the polar caps no doubt consist.</p> + +<p>There is a variety of evidence pointing to an atmosphere +on Mars of one-third to one-half the density +of our own: an atmosphere in which free +hydrogen could not exist, although other gases +might. The spectroscopic evidence of water vapor +in the Martian atmosphere is not very strong. It +is very doubtful whether water exists on Mars in +large bodies: quite certainly not as oceans, though +the evidence of many small "lakes" is pretty well +made out. With very little water, a thin atmosphere +and a zero temperature, is Mars likely to be +inhabited at the present time? The chances are +rather against it. If, however, the past development +of the planet has progressed in the way usually +<span class="pagenum"><a name="Page_p248" id="Page_p248">[248]</a></span> +considered as probable, we may be practically certain +that Mars has been inhabited in the past, when +water was more abundant, and the atmosphere more +dense so as to retain and diffuse the solar heat.</p> + +<p>Biologists tell me that they hardly know enough +regarding the extreme adaptability of organisms +to environment to enable them to say whether life +on such a planet as Mars would or would not keep +on functioning with secular changes of moisture and +temperature. The survival of a race might be insured +against extremely low temperatures by dwelling +in sub-Martian caves, and sufficient water might +be preserved by conceivable engineering and mechanical +schemes; but the secular reduction of the +quantity and pressure of atmosphere—it is not easy +to see how a race even more advanced than ourselves +could maintain itself alive under serious lack of an +element so vital to existence. Both Wallace, the +great biologist, and Arrhenius, the eminent chemist +(but biologist, astronomer, and physicist as well), +both reject the habitation theory of Mars, regarding +the so-called canals as quite like the luminous +streaks on the moon; that is, cracks in the volcanic +crust caused by internal strains due to the heated +interior. Wallace, indeed, argues that the planet is +absolutely uninhabitable.</p> + +<p>The asteroids, or minor planets? We may dismiss +them with the simple consideration that their +individual masses are so insignificant and their +gravity so slight that no atmosphere can possibly +surround them. Their temperatures must be exceedingly +low, and water, if present at all, can only +exist in the form of ice.</p> + +<p>Jupiter, the giant planet, presents the opposite +extreme. His mass is nearly a thousandth part of +<span class="pagenum"><a name="Page_p249" id="Page_p249">[249]</a></span> +the sun's, and is sufficient to retain a very high +temperature, probably approximating to the condition +we call red-hot. This precludes the possibility +of life at the outset, although the indications of a +very dense atmosphere many thousand miles in +depth are unmistakable.</p> + +<p>Of Saturn, one thirty-five hundredth the mass of +the sun, practically the same may be said. Proctor +thought it quite likely that Saturn might be habitable +for living creatures of some sort, but he regarded +the planet as on many accounts unsuitable +as a habitation for beings constituted like ourselves. +Mere consideration of surface temperature precludes +the possibility of life in the present stage of +Saturn's development; but the consensus of opinion +is to the effect that life may make its appearance on +these great planets at some inconceivably remote +epoch in the future when the surface temperature is +sufficiently reduced for life processes to begin. Discoveries +of algæ flourishing in hot springs approaching +200 degrees Fahrenheit make it possible that +these beginnings may take place earlier and at +much higher temperatures than have hitherto been +thought possible.</p> + +<p>A century ago, when the ring of Saturn was +believed to be a continuous plane, this was a favorite +corner of the solar system for speculation as to +habitability; but now that we know the true constitution +of the rings, no one would for a moment +consider any such possibility. Conditions may, +however, be quite different with Saturn's huge +satellite Titan, the giant moon of the solar system. +Its diameter makes it approximately the size of the +planet Mars; and although it is much farther removed +from the sun, its relative nearness to the +<span class="pagenum"><a name="Page_p250" id="Page_p250">[250]</a></span> +highly heated globe of Saturn may provide that +equability of temperature which is essential to life +processes.</p> + +<p>Also the three inner Galilean moons of Jupiter, +especially III which is about the size of Titan, are +excellently placed for life possibilities, as far as +probable temperature is concerned, but we have of +course no basis for surmising what their conditions +may be as to air and water, except that their small +mass would indicate a probable deficiency of those +elements.</p> + +<p>Uranus and Neptune are planets so remote, and +their apparent disks are so small, that very little is +known about their physical condition. They are +each about one-third the diameter of Jupiter, and +the spectrum of Uranus shows broad diffused bands, +indicating strong absorption by a dense atmosphere +very different from that of the earth. Indications +are that Neptune has a similar atmosphere.</p> + +<p>It is possible that the denser atmospheres of these +remote planets may be so conditioned as to selective +absorption that the relatively slender supply of solar +heat may be conserved, and thus insure a relatively +high surface temperature when the sun comes into +control. If our theories of origin of the planets are +to be trusted, we may rather suppose that Uranus +and Neptune are still in a highly heated condition; +that life has not yet made its appearance on them, +but that it will begin its development ages before +Saturn and Jupiter have cooled to the requisite temperature.</p> + +<p>Comets? In his <i>Lettres Cosmologiques</i> (1765) +Lambert considers the question of habitability of +the comets, naturally enough in his day, because he +thought them solid bodies surrounded by atmosphere, +<span class="pagenum"><a name="Page_p251" id="Page_p251">[251]</a></span> +and related to the planets. The extremes of +temperature at perihelia and aphelia to which comets +are subjected did not bother him particularly.</p> + +<p>After calculating that the comet of 1680, "being +160 times nearer to the sun than we are ourselves, +must have been subjected to a degree of heat +25,600 times as great as we are," Lambert goes on +to say: "Whether this comet was of a more compact +substance than our globe, or was protected in some +other way, it made its perihelion passage in safety, +and we may suppose all its inhabitants also passed +safely. No doubt they would have to be of a more +vigorous temperament and of a constitution very +different from our own. But why should all living +beings necessarily be constituted like ourselves? Is +it not infinitely more probable that amongst the +different globes of the universe a variety of organizations +exist, adapted to the wants of the people +who inhabit them, and fitting them for the places +in which they dwell, and the temperatures to which +they will be subjected? Is man the only inhabitant +of the earth itself? And if we had never seen +either bird or fish, should we not believe that the +air and water were uninhabitable? Are we sure +that fire has not its invisible inhabitants, whose +bodies, made of asbestos, are impenetrable to flame? +Let us admit that the nature of the beings who +inhabit comets is unknown to us; but let us not +deny their existence, and still less the possibility +of it."</p> + +<p>Little enough is really known about the physical +nature of comets even now, but what we do know indicates +incessant transformation and instability of +conditions that would render life of any type exceedingly +difficult of maintenance.</p> + +<p><span class="pagenum"><a name="Page_p252" id="Page_p252">[252]</a></span> +A word about Sir William Herschel's theory of +the sun and its habitability. He thought the core of +the sun a dark, solid body, quite cold, and surrounded +by a double layer, the inner one of which +he conceived to act as a sort of fire screen to shield +the sun proper against the intense heat of the outer +layer, or photosphere by which we see it. Viewed +in this light, the sun, he says, "appears to be nothing +else than a very eminent, large and lucid planet, +evidently the first, or, in strictness of speaking, the +only primary one of our system…. It is most probably +also inhabited, like the rest of the planets, by +beings whose organs are adapted to the peculiar +circumstances of that vast globe." But physics and +biology were undeveloped sciences in Herschel's days.</p> + +<p>Herschel knew, however, that the stars are all +suns, so that he must have conceived that they are +inhabited also, quite independently of the question +whether they possess retinues of planets, after the +manner of our solar system.</p> + +<p>This again is a question to which the astronomer +of the present day can give no certain answer. So +immensely distant are even the nearest of these +multitudinous bodies that no telescope can ever be +built large enough or powerful enough to reveal a +dark planet as large as Jupiter, alongside even the +nearest fixed star. Whatever may be the process +of stellar evolution, there doubtless is an era of +many hundreds of millions of years in the life of a +star when it is passing through a planet-maintaining +stage. This would likely depend upon spectral +type, or to be indicated by it; and as about half of +the stars are of the solar type, it would be a reasonable +inference that at least half of the stars may +have planets tributary to them.</p> + +<p><span class="pagenum"><a name="Page_p253" id="Page_p253">[253]</a></span> +In such a case, the chances must be overwhelmingly +in favor of vast numbers of the planets of +other stellar systems being favorably circumstanced +as to heat and moisture for the maintenance of life +at the present time. That is, they are habitable, +and if habitable, then thousands of them are no +doubt inhabited now. But astronomers know absolutely +nothing about this question, nor are they able +to conceive at present any way that may lead them +to any definite knowledge of it. There is, indeed, +one piece of quasi-evidence which might reasonably +be interpreted as implying that it is more likely +that the stars are not attended by families of planets +than that they are.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p254" id="Page_p254">[254]</a></span></p> + +<h2><a name="CHAPTER_XXXV" id="CHAPTER_XXXV"></a>CHAPTER XXXV<br /> +<br /> +THE LITTLE PLANETS</h2> + +<p>Along toward the end of the eighteenth century +and the beginning of the nineteenth, astronomers +were leading a quiet unexcited life. Sir William +Herschel had been knighted by King George for +his discovery of the outer planet Uranus, and practically +everything seemed to be known and discovered +in the solar system with a single exception. Between +Mars and Jupiter there existed an obvious +gap in the planetary brotherhood.</p> + +<p>Could it be possible that some time in the remote +cosmic past a planet had actually existed there, and +that some celestial cataclysm had blown it to fragments? +If so, would they still be traveling round +the sun as individual small planets? And might it +not be possible to discover some of them among the +faint stars that make up the belt of the zodiac in +which all the other planets travel?</p> + +<p>So interesting was this question that the first international +association of astronomers banded themselves +together to carry on a systematic search +round the entire zodiacal heavens in the faint hope +of detecting possible fragments of the original +planet of mere hypothesis.</p> + +<p>The astronomers of that day placed much reliance +on what is known as Bode's law—not a law at all, +but a mere arithmetical succession of numbers +which represented very well the relative distances +<span class="pagenum"><a name="Page_p255" id="Page_p255">[255]</a></span> +of all the planets from the sun. And the distance +of the newly found Uranus fitted in so well with +this law that the utter absence of a planet in the gap +between Mars and Jupiter became very strongly +marked.</p> + +<p>Quite by accident a discovery of one of the +guessed-at small planetary bodies was made, on +January 1, 1801, in Palermo, Sicily, by Piazzi, who +was regularly occupied in making an extensive catalogue +of the stars. His observations soon showed +that the new object he had seen could not be a fixed +star, because it moved from night to night among +the stars. He concluded that it was a planet, and +named it Ceres (1), for the tutelary goddess of +Sicily.</p> + +<p>Other astronomers kept up the search, and another +companion planet, Pallas (2) was found in +the following year. Juno (3) was found in 1804, +and Vesta (4), the largest and brightest of all the +minor planets, in 1807. Vesta is sometimes bright +enough when nearest the earth to be seen with the +naked eye; but it was the last of the brighter ones, +and no more discoveries of the kind were made till +the fifth was found in 1845. Since then discoveries +have been made in great abundance, more and more +with every year till the number of little planets at +present known is very near 1,000.</p> + +<p>The early asteroid hunters found the search +rather tedious, and the labor increased as it became +necessary to examine the increasing thousands of +fainter and fainter stars that must be observed in +order to detect the undiscovered planets, which +naturally grow fainter and fainter as the chase is +prolonged. First a chart of the ecliptic sky had to +be prepared containing all the stars that the telescope +<span class="pagenum"><a name="Page_p256" id="Page_p256">[256]</a></span> +employed in the search would show. Some of +the most detailed charts of the sky in existence were +prepared in connection with this work, particularly +by the late Dr. Peters of Hamilton College. Once +such charts are complete, they are compared with +the sky, night after night when the moon is absent. +Thousands upon thousands of tedious hours are +spent in this comparison, with no result whatever +except that chart and sky are found to correspond +exactly.</p> + +<p>But now and then the planet hunter is rewarded +by finding a new object in the sky that does not +appear on his chart. Almost certainly this is a +small planet, and only a few night's observation will +be necessary to enable the discoverer to find out +approximately the orbit it is traveling in, and +whether it is out-and-out a new planet or only one +that had been previously recognized, and then lost +track of.</p> + +<p>Nearly all the minor planets so far found have +had names assigned to them principally legendary +and mythological, and a nearly complete catalogue +of them, containing the elements of their orbits +(that is, all the mathematical data that tell us about +their distance from the sun and the circumstances +of their motion around him) is published each year in +the "Annuaire du Bureau des Longitudes" at Paris. +But these little planets require a great deal of care +and attention, for some astronomers must accurately +observe them every few years, and other astronomers +must conduct intricate mathematical computations +based on these observations; otherwise they +get lost and have to be discovered all over again. +Professor Watson, of the University of Michigan +and later of the University of Wisconsin, endowed +the 22 asteroids of his own discovery, leaving to the +National Academy of Sciences a fund for prosecuting +this work perpetually, and Leuschner is now +ably conducting it.</p> + +<p><span class="pagenum"><a name="Page_p256p1" id="Page_p256p1">[256i]</a></span></p> + +<div class="fig_center" style="width: 644px;"> +<img src="images/p256_1top.png" width="644" height="542" alt="" /> +<div class="fig_caption"><span class="smcap">Jupiter, Largest of the Planets.</span> The irregular belts change their +mutual relation and shapes because they do not represent land, but +are part of the atmosphere. (<i>Photo, Yerkes Observatory.</i>)</div> +</div> + +<div class="fig_center" style="width: 645px;"> +<img src="images/p256_1bot.png" width="645" height="509" alt="" /> +<div class="fig_caption"><span class="smcap">The Planet Neptune and its Satellite.</span> The photograph required +an exposure of the plate for one hour. (<i>Photo, Yerkes Observatory.</i>)</div> +</div> + +<p><span class="pagenum"><a name="Page_p256p2" id="Page_p256p2">[257i]</a></span></p> + +<div class="fig_center" style="width: 646px;"> +<img src="images/p256_2top.png" width="646" height="347" alt="" /> +<div class="fig_caption"><span class="smcap">Saturn, as Seen Through the 40-inch Refractor</span>, at the time +when only the edge of the rings is visible, showing condensations. +(<i>Photo, Yerkes Observatory.</i>)</div> +</div> + +<div class="fig_center" style="width: 644px;"> +<img src="images/p256_2bot.png" width="644" height="538" alt="" /> +<div class="fig_caption"><span class="smcap">Saturn, Photographed Through the 40-inch Refractor.</span> The +rings appear opened to the fullest extent they can be seen from the +earth. The picture was made July 7, 1898. (<i>Photo, Yerkes Observatory.</i>)</div> +</div> + +<p><span class="pagenum"><a name="Page_p257" id="Page_p257">[257]</a></span> +While the number of the asteroids is gratifyingly +large, their individual size is so small and their total +mass so slight that, even if there are a hundred +thousand of them (as is wholly possible), they would +not be comparable in magnitude with any one of the +great planets. Vesta, the largest, is perhaps 400 +miles in diameter, and if composed of substances +similar to those which make up the earth, its mass +may be perhaps one twenty-thousandth of the earth's +mass. If we calculate the surface gravity on such a +body, we find it about one-thirtieth of what it is +here; so that a rifle ball, if fired on Vesta with a +muzzle velocity of only 2,000 feet a second, might +overmaster the gravity of the little planet entirely +and be projected in space never to return.</p> + +<p>If, as is likely, some of the smallest asteroids are +not more than ten miles in diameter, their gravity +must be so feeble a force that it might be overcome +by a stone thrown from the hand. There is no reliable +evidence that any of the asteroids are surrounded +by atmospheric gases of any sort. Probably +they are for the most part spherical in form, +although there is very reliable evidence that a few +of the asteroids, being variable in the amount of +sunlight that they reflect, are irregular in form, +mere angular masses perhaps.</p> + +<p>The network of orbits of the asteroids is inconceivable +complicated. Nevertheless, there is a wide +variation in their average distance from the sun, +and their periods of traveling round him vary in a +similar manner, the shortest being only about three +<span class="pagenum"><a name="Page_p258" id="Page_p258">[258]</a></span> +years. While the longest is nearly nine years in +duration, the average of all their periods is a little +over four years. The gap in the zone of asteroids, +at a distance from the sun equal to about five-eighths +that of Jupiter, is due to the excessive disturbing +action of Jupiter, whose periodic time is just twice +as long as that of a theoretical planet at this distance.</p> + +<p>The average inclination of their orbits to the +plane of the ecliptic is not far from 8 degrees. But +the orbit of Pallas, for example, is inclined 35 degrees, +and the eccentricities of the asteroid orbits +are equally erratic and excessive. Both eccentricity +and inclination of orbit at times suggest a possible +relation to cometary orbits, but nothing has ever +been definitely made out connecting asteroids and +comets in a related origin.</p> + +<p>No comprehensive theory of the origin of the asteroid +group has yet been propounded that has met +with universal acceptance. According to the nebular +hypothesis the original gaseous material, which +should have been so concentrated as to form a planet +of ordinary type, has in the case of the asteroids collected +into a multitude of small masses instead of +simply one. That there is a sound physical reason +for this can hardly be denied. According to the +Laplacian hypothesis, the nearness of the huge +planetary mass of Jupiter just beyond their orbits +produced violent perturbations which caused the +original ring of gaseous material to collect into +fragmentary masses instead of one considerable +planet. The theory of a century ago that an original +great planet was shattered by internal explosive +forces is no longer regarded as tenable.</p> + +<p>To astronomers engaged upon investigation of +distances in the solar system, the asteroid group has +<span class="pagenum"><a name="Page_p259" id="Page_p259">[259]</a></span> +proved very useful. The late Sir David Gill employed +a number of them in a geometrical research +for finding the sun's distance, and more recently the +discovery of Eros (433) has made it possible to +apply a similar method for a like purpose when it +approaches nearest to the earth in 1924 and 1931. +Then the distance of Eros will be less than half that +of Mars or even Venus at their nearest.</p> + +<p>When the total number of asteroids discovered +has reached 1,000, with accurate determination of +all their orbits, we shall have sufficient material for +a statistical investigation of the group which ought +to elucidate the question of its origin, and bear on +other problems of the cosmogony yet unsolved. +Present methods of discovery of the asteroids by +photography replace entirely the old method by +visual observation alone, with the result that discoveries +are made with relatively great ease and +rapidity.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p260" id="Page_p260">[260]</a></span></p> + +<h2><a name="CHAPTER_XXXVI" id="CHAPTER_XXXVI"></a>CHAPTER XXXVI<br /> +<br /> +THE GIANT PLANET</h2> + +<p>I can never forget as a young boy my first +glimpse of the planet Jupiter and his moons; it +was through a bit of a telescope that I had put +together with my own hands; a tube of pasteboard, +and a pair of old spectacle lenses that chanced to +be lying about the house.</p> + +<p>In the field of view I saw five objects; four of +them looking quite alike, and as if they were stars +merely (they were Jupiter's moons), while the +fifth was vastly larger and brighter. It was circular +in shape, and I thought I could see a faint +darkish line across the middle of it.</p> + +<p>This experience encouraged me immensely, and +I availed myself eagerly of the first chance to see +Jupiter through a bigger and better glass. Then +I saw at once that I had observed nothing wrongly, +but that I had seen only the merest fraction of +what there was to see.</p> + +<p>In the first place, the planet's disk was not perfectly +circular, but slightly oval. Inquiring into +the cause of this, we must remember that Jupiter +is actually not a flat disk but a huge ball or globe, +more than ten times the diameter of the earth, +which turns swiftly round on its axis once every +ten hours as against the earth's turning round in +twenty-four hours. Then it is easy to see how the +centrifugal force bulges outward the equatorial +<span class="pagenum"><a name="Page_p261" id="Page_p261">[261]</a></span> +regions of Jupiter, so that the polar regions are +correspondingly drawn inward, thereby making the +polar diameter shorter than the equatorial one, +which is in line with the moons or satellites. The +difference between the two diameters is very +marked, as much as one part in fifteen. All the +planets are slightly flattened in this way, but Jupiter +is the most so of all except Saturn.</p> + +<p>The little darkish line across the planet's middle +region or equator was found to be replaced by several +such lines or irregular belts and spots, often +seen highly colored, especially with reflecting telescopes; +and they are perpetually changing their +mutual relation and shapes, because they are not +solid territory or land on Jupiter, but merely the +outer shapes of atmospheric strata, blown and torn +and twisted by atmospheric circulation on this +planet, quite the same as clouds in the atmosphere +on the earth are.</p> + +<p>Besides this the axial turning of Jupiter brings +an entirely different part of the planet into view +every two or three hours; so that in making a map +or chart of the planet, an arbitrary meridian must +be selected. Even then the process is not an easy +one, and it is found that spots on Jupiter's equator +turn round in 9 hours 50 minutes, while other +regions take a few minutes longer, the nearer the +poles are approached. The Great Red Spot, about +30,000 miles long and a quarter as much in breadth +has been visible for about half a century. Bolton, +an English observer, has made interesting studies of +it very recently.</p> + +<p>The four moons, or satellites, which a small +telescope reveals, are exceedingly interesting on +many accounts. They were the first heavenly +<span class="pagenum"><a name="Page_p262" id="Page_p262">[262]</a></span> +bodies seen by the aid of the telescope, Galileo having +discovered them in 1610. They travel round +Jupiter much the same as the moon does round the +earth, but faster, the innermost moon about four +times per week, the second moon about twice a +week, the third or largest moon (larger than the +planet Mercury) once a week, and the outermost +in about sixteen days. The innermost is about +260,000 miles from Jupiter, and the outermost more +than a million miles. From their nearness to the +huge and excessively hot globe of Jupiter, some +astronomers, Proctor especially, have inclined to the +view that these little bodies may be inhabited.</p> + +<p>Jupiter has other moons; a very small one, close +to the planet, which goes round in less than twelve +hours, discovered by Barnard in 1892. Four others +are known, very small and faint and remote from +the planet, which travel slowly round it in orbits of +great magnitude. The ninth, or outermost, is at a +distance of fifteen and one-half million miles from +Jupiter, and requires nearly three years in going +round the planet. It was discovered by Nicholson +at the Lick Observatory in 1914. The eighth was +discovered by Melotte at Greenwich in 1908, and +is peculiar in the great angle of 28 degrees, at +which its orbit is inclined to the equator of Jupiter. +The sixth and seventh satellites revolve round Jupiter +inside the eighth satellite, but outside the +orbit of IV; and they were discovered by photography +at the Lick Observatory in 1905 by Perrine, +now director of the Argentine National Observatory +at Cordoba.</p> + +<p>The ever-changing positions of the Medicean +moons, as Galileo called the four satellites that he +discovered—their passing into the shadow in +<span class="pagenum"><a name="Page_p263" id="Page_p263">[263]</a></span> +eclipse, their transit in front of the disk, and their +occultation behind it—form a succession of phenomena +which the telescopist always views with +delight. The times when all these events take +place are predicted in the "Nautical Almanac," many +thousand of them each year, and the predictions +cover two or three years in advance.</p> + +<p>Jupiter, as the naked eye sees him high up in +the midnight sky, is the brightest of all the planets +except Venus; indeed, he is five times brighter than +Sirius, the brightest of all the fixed stars. His +stately motion among the stars will usually be +visible by close observation from day to day, and +his distance from the earth, at times when he is +best seen, is usually about 400 million miles. +Jupiter travels all the way round the sun in twelve +years; his motion in orbit is about eight miles +a second.</p> + +<p>The eclipses of Jupiter's moons, caused by passing +into the shadow of the planet, would take place +at almost perfectly regular intervals, if our distance +from Jupiter were invariable. But it was +early found out that while the earth is approaching +Jupiter the eclipses take place earlier and +earlier, but later and later when the earth is moving +away. The acceleration of the earliest eclipse +added to the retardation of the latest makes 1,000 +seconds, which is the time that light takes in crossing +a diameter of the earth's orbit round the sun. +Now the velocity of light is well known to be 186,300 +miles per second, so we calculate at once and +very simply that the sun's distance from the earth, +which is half the diameter of the orbit, equals 500 +times 186,300, or 93,000,000 miles.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p264" id="Page_p264">[264]</a></span></p> + +<h2><a name="CHAPTER_XXXVII" id="CHAPTER_XXXVII"></a>CHAPTER XXXVII<br /> +<br /> +THE RINGED PLANET</h2> + +<p>Saturn is the most remote of all the planets +that the ancient peoples knew anything about. +These anciently known planets are sometimes +called the lucid or naked-eye planets—five in number: +Mercury, Venus, Mars, Jupiter, and Saturn. +Saturn shines as a first-magnitude star, with a +steady straw-colored light, and is at a distance of +about 800 million miles from the earth when best +seen. Saturn travels completely round the sun in +a little short of thirty years, and the telescope, +when turned to Saturn, reveals a unique and +astonishing object; a vast globe somewhat similar +to Jupiter, but surrounded by a system of rings +wholly unlike anything else in the universe, as far +as at present known; the whole encircled by a +family of ten moons or satellites. The Saturnian +system, therefore, is regarded by many as the most +wonderful and most interesting of all the objects +that the telescope reveals.</p> + +<p>At first the flattening of the disk of Saturn is +not easily made out, but every fifteen years (as +1921 and 1936) the earth comes into a position +where we look directly at the thin edge of the +rings, causing them to completely disappear. Then +the remarkable flattening of the poles of Saturn is +strikingly visible, amounting to as much as one-tenth +of the entire diameter. The atmospheric belt +system is also best seen at these times.</p> + +<p><span class="pagenum"><a name="Page_p265" id="Page_p265">[265]</a></span> +But the rings of Saturn are easily the most fascinating +features of the system. They can never +be seen as if we were directly above or beneath +the planet so they never appear circular, as they +really are in space, but always oval or elliptical in +shape. The minor axis or greatest breadth is +about one-half the major axis or length. The latter +is the outer ring's actual diameter, and it +amounts to 170,000 miles, or two and one-half times +the diameter of Saturn's globe.</p> + +<p>There are in fact no less than four rings; an +outer ring, sometimes seen to be divided near its +middle; an inner, broader and brighter ring; and +an innermost dusky, or crape ring, as it is often +called. This comes within about 10,000 miles of +the planet itself. After the form and size of the +rings were well made out, their thickness, or rather +lack of thickness, was a great puzzle.</p> + +<p>If a model about a foot in diameter were cut out +of tissue paper, the relative proportion of size and +thickness would be about right. In space the thickness +is very nearly 100 miles, so that, when we look +at the ring system edge-on, it becomes all but invisible +except in very large telescopes. Clearly a +ring so thin cannot be a continuous solid object +and recent observations have proved beyond a +doubt that Saturn's rings are made up of millions +of separate particles moving round the planet, each +as if it were an individual satellite.</p> + +<p>Ever since 1857 the true theory of the constitution +of the Saturnian ring has been recognized on +theoretic grounds, because Clerke-Maxwell founded +the dynamical demonstration that the rings could +be neither fluid nor solid, so that they must be +made up of a vast multitude of particles traveling +<span class="pagenum"><a name="Page_p266" id="Page_p266">[266]</a></span> +round the planet independently. But the physical +demonstration that absolutely verified this conclusion +did not come until 1895, when, as we have +said in a preceding chapter, Keeler, by radial velocity +measures on different regions of the ring by +means of the spectroscope, proved that the inner +parts of the ring travel more swiftly round the +planet than the outer regions do. And he further +showed that the rates of revolution in different +parts of the ring exactly correspond to the periods +of revolution which satellites of Saturn would have, +if at the same distance from the center of the +planet. The innermost particles of the dusky ring, +for example, travel round Saturn in about five +hours, while the outermost particles of the outer +bright ring take 137 hours to make their revolution. +For many years it was thought that the Saturnian +ring system was a new satellite in process +of formation, but this view is no longer entertained; +and the system is regarded as a permanent +feature of the planet, although astronomers are +not in entire agreement as to the evolutionary +process by which it came into existence—whether +by some cosmic cataclysm, or by gradual development +throughout indefinite aeons, as the rest of the +solar system is thought to have come to its present +state of existence. Possibly the planetesimal hypothesis +of Chamberlin and Moulton affords the true +explanation, as the result of a rupture due to excessive +tidal strain.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p267" id="Page_p267">[267]</a></span></p> + +<h2><a name="CHAPTER_XXXVIII" id="CHAPTER_XXXVIII"></a>CHAPTER XXXVIII<br /> +<br /> +THE FARTHEST PLANETS</h2> + +<p>On the 13th of March, 1781, between 10 and 11 +P. M., as Sir William Herschel was sweeping +the constellation Gemini with one of his great reflecting +telescopes, one star among all that passed +through the field of view attracted his attention. +Removing the eyepiece and applying another with +a higher magnifying power, he found that, unlike +all the other stars, this one had a small disk and was +not a mere point of light, as all the fixed stars seem +to be.</p> + +<p>A few nights' observation showed that the +stranger was moving among the stars, so he thought +it must be a comet; but a week's observation following +showed that he had discovered a new member +of the planetary system, far out beyond Saturn, +which from time immemorial had been assumed to +be the outermost planet of all. This, then, was the +first real discovery of a planet, as the finding of the +satellites of Jupiter had been the first of all astronomical +discoveries. Herschel's discovery occasioned +great excitement, and he named the new planet +Georgium Sidus or the Georgian, after his King. +The King created him a knight and gave him a +pension, besides providing the means for building a +huge telescope, 40 feet long, with which he subsequently +made many other astronomical discoveries. +The planet that Herschel discovered is now called +Uranus.</p> + +<p><span class="pagenum"><a name="Page_p268" id="Page_p268">[268]</a></span> +Uranus is an object not wholly impossible to see +with the naked eye, if the sky background is clear +and black, and one knows exactly where to look for +it. Its brightness is about that of a sixth magnitude +star or a little fainter. Its average distance from +the sun is about 1,800 million miles and it takes +eighty-four years to complete its journey round the +sun, traveling only a little more than four miles a +second. When we examine Uranus closely with a +large telescope, we find a small disk slightly greenish +in tint, very slightly flattened, and at times faint +bands or belts are apparently seen. Uranus is about +30,000 miles in diameter, and is probably surrounded +by a dense atmosphere. Its rotation time is 10 h. +50 m.</p> + +<p>Uranus is attended by four moons or satellites, +named Ariel, Umbriel, Titania, and Oberon, the last +being the most remote from the planet. This system +of satellites has a remarkable peculiarity: the plane +of the orbits in which they travel round Uranus is +inclined about 80 degrees to the plane of the ecliptic, +so that the satellites travel backward, or in a retrograde +direction; or we might regard their motion +as forward, or direct, if we considered the planes +of their orbits inclined at 100 degrees.</p> + +<p>For many years after the discovery of Uranus +it was thought that all the great bodies of the solar +system had surely been found. Least of all was any +planet suspected beyond Uranus until the mathematical +tables of the motion of Uranus, although +built up and revised with the greatest care and thoroughness, +began to show that some outside influence +was disturbing it in accordance with Newton's +law of gravitation. The attraction of a still more +distant planet would account for the disturbance, +<span class="pagenum"><a name="Page_p269" id="Page_p269">[269]</a></span> +and since no such planet was visible anywhere a +mathematical search for it was begun.</p> + + +<h3>NEPTUNE</h3> + +<p>Wholly independently of each other, two young +astronomers, Adams of England and Le Verrier of +France, undertook to solve the unique problem of +finding out the position in the sky where a planet +might be found that would exactly account for the +irregular motion of Uranus. Both reached practically +identical results. Adams was first in point of +time, and his announcement led to the earliest observation, +without recognition of the new planet +(July 30, 1846), although it was Le Verrier's work +that led directly to the new planet's being first seen +and recognized as such (September 23, 1846). Figuring +backward, it was found that the planet had +been accidentally observed in Paris in 1795, but its +planetary character had been overlooked.</p> + +<p>Neptune is the name finally assigned to this historical +planet. It is thirty times farther from the +sun than the earth, or 2,800 million miles; its +velocity in orbit is a little over three miles per +second, and it consumes 164 years in going once +completely round the sun. So faint is it that a +telescope of large size is necessary to show it plainly. +The brightness equals that of a star of the eighth +magnitude, and with a telescope of sufficient magnifying +power, the tiny disk can be seen and measured. +The planet is about 30,000 miles in diameter, +and is not known to possess more than one moon or +satellite. If there are others, they are probably too +faint to be seen by any telescope at present in +existence.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p270" id="Page_p270">[270]</a></span></p> + +<h2><a name="CHAPTER_XXXIX" id="CHAPTER_XXXIX"></a>CHAPTER XXXIX<br /> +<br /> +THE TRANS-NEPTUNIAN PLANET</h2> + +<p>Investigation of the question of a possible +trans-Neptunian planet was undertaken by the +writer in 1877. As Neptune requires 164 years to +travel completely round the sun, and the period +during which it has been carefully observed embraces +only half that interval, clearly its orbit cannot +be regarded as very well known. Any possible +deviations from the mathematical orbit could not +therefore be traced to the action of a possible unknown +planet outside. But the case was different +with Uranus, which showed very slight disturbances, +and these were assumed to be due to a possible +planet exterior to both Uranus and Neptune. +As a position for this body in the heavens was indicated +by the writer's investigation, that region of +the sky was searched by him with great care in +1877-1878 with the twenty-six-inch telescope at +Washington; and photographs of the same region +were afterward taken by others, though only with +negative results.</p> + +<p>In 1880, Forbes of Edinburgh published his investigation +of the problem from an entirely independent +angle. Families of comets have long been +recognized whose aphelion distances correspond so +nearly with the distances of the planets that these +comet families are now recognized as having been +created by the several planets, which have reduced +<span class="pagenum"><a name="Page_p271" id="Page_p271">[271]</a></span> +the high original velocities possessed by the comets +on first entering the solar system.</p> + +<p>Their orbits have ever since been ellipses with +their aphelia in groups corresponding to the distances +of the planets concerned. Jupiter has a large +group of such comets, also Saturn. Uranus and +Neptune likewise have their families of comets, and +Forbes found two groups with average distances +far outside of Neptune; from which he drew the inference +that there are two trans-Neptunian planets. +The position he assigned to the inner one agreed +fairly well with the writer's planet as indicated by +unexplained deviations of Uranus.</p> + +<p>The theoretical problem of a trans-Neptunian +planet has since been taken up by Gaillot and Lau +of Paris, the late Percival Lowell, and W. H. Pickering +of Harvard. The photographic method of search +will, it is expected, ultimately lead to its discovery. +On account of the probable faintness of the planet, +at least the twelfth or thirteenth magnitude, Metcalf's +method of search is well adapted to this practical +problem. When near its opposition the motion +of Neptune retrograding among the stars amounts +to five seconds of arc in an hour; while the trans-Neptunian +planet would move but three seconds. +By shifting the plate this amount hourly during exposure, +the suspected object would readily be detected +on the photographic plate as a minute and +nearly circular disk, all the adjacent stars being +represented by short trails.</p> + +<p>Interest in a possible planet or planets outside the +orbit of Neptune is likely to increase rather than +diminish. To the ancients seven was the perfect +number, there were seven heavenly bodies already +known, so there could be no use whatever in looking +<span class="pagenum"><a name="Page_p272" id="Page_p272">[272]</a></span> +for an eighth. The discovery of Uranus in 1781 +proved the futility of such logic, and Neptune followed +in 1846 with further demonstration, if need +be. The cosmogony of the present day sets no outer +limit to the solar system, and some astronomers advocate +the existence of many trans-Neptunian +planets.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p273" id="Page_p273">[273]</a></span></p> + +<h2><a name="CHAPTER_XL" id="CHAPTER_XL"></a>CHAPTER XL<br /> +<br /> +COMETS—THE HAIRY STARS</h2> + +<p>Comets—hairy stars, as the origin of the name +would indicate—are the freaks of the heavens. +Of great variety in shape, some with heads and some +without, some with tails and some without, moving +very slowly at one time and with exceedingly high +velocity at another, in orbits at all possible angles +of inclination to the general plane of the planetary +paths round the sun, their antics and irregularities +were the wonder and terror of the ancient world, +and they are keenly dreaded by superstitious people +even to the present day.</p> + +<p>Down through the Middle Ages the advent of a +comet was regarded as:</p> + +<div class="poem"> +<div class="i4">Threatening the world with famine, plague and war;</div> +<div class="i4">To princes, death; to kingdoms, many curses;</div> +<div class="i4">To all estates, inevitable losses;</div> +<div class="i4">To herdsmen, rot; to plowmen, hapless seasons;</div> +<div class="i4">To sailors, storms; to cities, civil treasons.</div> +</div> + +<p>Comets appeared to be marvelous objects, as well +as sinister, chiefly because they bid apparent defiance +to all law. Kepler had shown that the moon +and the planets travel in regular paths—slightly +elliptical to be sure, but nevertheless unvarying. +None of the comets were known to follow regular +paths till the time of Halley late in the seventeenth +century, when, as we have before told, a fine comet +<span class="pagenum"><a name="Page_p274" id="Page_p274">[274]</a></span> +made its appearance, and Halley calculated its orbit +with much precision. Comparing this with the +orbits of comets that had previously been seen, he +found its path about the sun practically identical +with that of at least two comets previously observed +in 1531 and 1607.</p> + +<p>So Halley ventured to think all these comets were +one and the same body, and that it traveled round the +sun in a long ellipse in a period of about seventy-five +or seventy-six years. We have seen how his prediction +of its return in 1758 was verified in every +particular. On the comet's return in 1910, Crowell +and Crommelin of Greenwich made a thorough +mathematical investigation of the orbit, indicating +that the year 1986 will witness its next return to the +sun.</p> + +<p>There is a class of astronomers known as comet-hunters, +and they pass hours upon hours of clear, +sparkling, moonless nights in search for comets. +They are equipped with a peculiar sort of telescope +called a comet-seeker, which has an object glass +usually about four or five inches in diameter, and a +relatively short length of focus, so that a larger +field of view may be included. Regions near the +poles of the heavens are perhaps the most fruitful +fields for search, and thence toward the sun till its +light renders the sky too bright for the finding of +such a faint object as a new comet usually is at the +time of discovery. Generally when first seen it +resembles a small circular patch of faint luminous +cloud.</p> + +<p>When a suspect is found, the first thing to do is +to observe its position accurately with relation to the +surrounding stars. Then, if on the next occasion +when it is seen the object has moved, the chances +<span class="pagenum"><a name="Page_p275" id="Page_p275">[275]</a></span> +are that it is a comet; and a few days' observation +will provide material from which the path of the +comet in space can be calculated. By comparing this +with the complete lists of comets, now about 700 in +number, it is possible to tell whether the comet is +a new one, or an old one returning. The total number +of comets in the heavens must be very great, and +thousands are doubtless passing continually undetected, +because their light is wholly overpowered +by that of the sun. Of those that are known, perhaps +one in twelve develops into a naked-eye comet, +and in some years six or seven will be discovered. +With sufficiently powerful telescopes, there are as +a rule not many weeks in the year when no comet +is visible. Brilliant naked-eye comets are, however, +infrequent.</p> + +<p>Comets, except Halley's, generally bear the name +of their discoverer, as Donati (1858), and Pons-Brooks +(1893). Pons was a very active discoverer of +comets in France early in the nineteenth century: he +was a doorkeeper at the observatory of Marseilles, +and his name is now more famous in astronomy than +that of Thulis, then the director of the Observatory, +who taught and encouraged him. Messier was +another very successful discoverer of comets in +France, and in America we have had many: Swift, +Brooks, and Barnard the most successful.</p> + +<p>How bright a comet will be and how long it will +be visible depends upon many conditions. So the +comets vary much in these respects. The first comet +of 1811 was under observation for nearly a year and +a half, the longest on record till Halley's in 1910. +In case a comet eludes discovery and observation +until it has passed its perihelion, or nearest point to +the sun, its period of visibility may be reduced to a +<span class="pagenum"><a name="Page_p276" id="Page_p276">[276]</a></span> +few weeks only. The brightest comets on record +were visible in 1843 and 1882: so brilliant were they +that even the effulgence of full daylight did not overpower +them. In particular the comet of 1843 was +not only excessively bright, but at its nearest approach +to the earth its tail swept all the way across +the sky from one horizon to the other. It must have +looked very much like the straight beam of an +enormous searchlight, though very much brighter.</p> + +<p>The tails of comets are to the naked eye the most +compelling thing about them, and to the ancient +peoples they were naturally most terrifying. Their +tails are not only curved, but sometimes curved with +varying degrees of curvature, and this circumstance +adds to their weirdness of appearance. If we examine +the tail of a comet with a telescope, it vanishes +as if there were nothing to it: as indeed one may +almost say there is not. Ordinarily, only the head +of the comet is of interest in the telescope. When +first seen there is usually nothing but the head +visible, and that is made up of portions which +develop more or less rapidly, presenting a succession +of phenomena quite different in different +comets.</p> + +<p>When first discovered a comet is usually at a great +distance from the sun, about the distance of Jupiter; +and we see it, not as we do the planets, by sunlight +reflected from them, but by the comet's own light. +This is at that time very faint, and nearly all comets +at such a distance look alike: small roundish hazy +patches of faint, cloudlike light, with very often a +concentration toward the center called the nucleus, +on the average about 4,000 miles in diameter. Approach +toward the sun brightens up the comet more +and more, and the nucleus usually becomes very +<span class="pagenum"><a name="Page_p277" id="Page_p277">[277]</a></span> +much brighter and more starlike. Then on the +sunward side of the nucleus, jetlike streamers or +envelopes appear to be thrown off, often as if in +parallel curved strata, or concentrically. As they +expand and move outward from the nucleus, these +envelopes grow fainter and are finally merged in +the general nebulosity known as the comet's head, +which is anywhere from 30,000 to 100,000 miles in +diameter. As a rule, this is an orderly development +which can be watched in the telescope from hour to +hour and from night to night; but occasionally a +cometary visitor is quite a law to itself in development, +presenting a fascinating succession of unpredictable +surprises.</p> + +<p>Then follows the development of the comet's tail, +perhaps more striking than anything that has preceded +it. Here a genuine repulsion from the sun +appears to come into play. It may be an electrical +repulsion. Much of the material projected from the +comet's nucleus, seems to be driven backward or repelled +by the sun, and it is this that goes to form +the tail. The particles which form the tail then +travel in modified paths which nevertheless can be +calculated. The tail is made up of these luminous +particles and it expands in space much in the form +of a hollow, horn-shaped cone, the nucleus being +near the tip of the horn.</p> + +<p>Some comets possess multiple tails with different +degrees of curvature, Donati's for example. Usually +there is a nearly straight central dark space, marking +the axis of the comet, and following the nucleus. +But occasionally this is replaced by a thin light +streak very much less in breadth than the diameter +of the head. Cometary tails are sometimes 100 million +miles in length.</p> + +<p><span class="pagenum"><a name="Page_p278" id="Page_p278">[278]</a></span> +Three different types of cometary tails are recognized. +First, the long straight ones, apparently made +up of matter repelled by the sun twelve to fifteen +times more powerfully than gravitation attracts it. +Such particles must be brushed away from the +comet's head with a velocity of perhaps five miles +a second, and their speed is continually increasing. +Probably these straight tails are due to hydrogen. +The second type tails are somewhat curved, or plume-like, +and they form the most common type of cometary +tail. In them the sun's repulsion is perhaps +twice its gravitational attraction, and hydrocarbons +in some form appear to be responsible for tails of +this character. Then there is a third type, much less +often seen, short and quickly curving, probably due +to heavier vapors, as of chlorine, or iron, or sodium, +in which the repulsive force is only a small fraction +of that of gravitation.</p> + +<p>Many features of this theory of cometary tails +are borne out by examination of their light with the +spectroscope, although the investigation is as yet +fragmentary. It is evident that the tail of a comet +is formed at the expense of the substance of the +nucleus and head; so that the matter repelled is +forever dissipated through the regions of space +which the comet has traveled. Comets must lose +much of their original substance every time they +return to perihelion. Comets actually age, therefore, +and grow less and less in magnitude of material +as well as brightness, until they are at last opaque, +nonluminous bodies which it becomes impossible to +follow with the telescope.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p279" id="Page_p279">[279]</a></span></p> + +<h2><a name="CHAPTER_XLI" id="CHAPTER_XLI"></a>CHAPTER XLI<br /> +<br /> +WHERE DO COMETS COME FROM?</h2> + +<p>Where do comets come from? The answer to +this question is not yet fully made out. Most +likely they have not all had a similar origin, and +theories are abundant. Apparently they come into +the solar system from outer space, from any direction +whatsoever. The depths of interstellar space +seem to be responsible for most, if not all, of the new +ones. Whether they have come from other stars or +stellar systems we cannot say.</p> + +<p>While comets are tremendous in size or volume, +their mass or the amount of real substance in them +is relatively very slight. We know this by the effect +they produce on planets that they pass near, or +rather by the effect that they fail to produce. +The earth's atmosphere weighs about one two hundred +and fifty thousandth as much as the earth itself, +but a comet's entire mass must be vastly less +than this. Even if a comet were to collide with the +earth head on, there is little reason to believe that +dire catastrophe would ensue. At least twice the +earth is known to have passed through the tail of +a comet, and the only effect noticed was upon the +comet itself; its orbit had been modified somewhat +by the attraction of the earth. If the comet were +a small one, collision with any of the planets would +result in absorption and dissipation of the comet +into vapor.</p> + +<p><span class="pagenum"><a name="Page_p280" id="Page_p280">[280]</a></span> +The whole of a large comet has perhaps as much +mass or weight as a sphere of iron a hundred +miles in diameter. Even this could not wreck the +earth, but the effect would depend upon what part +of the earth was hit. A comet is very thin and +tenuous, because its relatively small mass is distributed +through a volume so enormous. So it is +probable that the earth's atmosphere could scatter +and burn up the invading comet, and we should +have only a shower of meteors on an unprecedented +scale. Diffusion of noxious gases through the atmosphere +might vitiate it to some extent, though +probably not enough to cause the extinction of +animal life.</p> + +<p>Every comet has an interesting history of its +own, almost indeed unique. One of the smallest +comets and the briefest in its period round the sun +is known as Encke's comet. It is a telescopic comet +with a very short tail, its time of revolution is +about three and a half years, and it exhibits a +remarkable contraction of volume on approach to +the sun.</p> + +<p>Biela's comet has a period about twice as long. +At one time it passed within about 15 million miles +of the earth, and somewhere about the year 1840 +this comet divided into two distinct comets, which +traveled for months side by side, but later separated +and both have since completely disappeared. +Perhaps the most beautiful of all comets is that discovered +by Donati of Florence in 1858. Its coma +presented the development of jets and envelopes in +remarkable perfection, and its tail was of the secondary +or hydrocarbon type, but accompanied by +two faint streamer tails, nearly tangential to the +main tail and of the hydrogen type. Donati's +<span class="pagenum"><a name="Page_p281" id="Page_p281">[281]</a></span> +comet moves in an ellipse of extraordinary length, +and it will not return to the sun for nearly 2,000 +years.</p> + +<p>The most brilliant comet of the last half century +is known as the great comet of 1882. In a clear +sky it could readily be seen at midday. On September +17 it passed across the disk of the sun and was +practically as bright as the surface of the sun itself. +The comet had a multiple nucleus and a hydrocarbon +tail of the second type, nearly a hundred +million miles in length. Doubtless this great comet +is a member of what is known as a cometary group, +which consists of comets having the same orbit +and traveling tandem round the sun. The comets +of 1668, 1843, 1880, 1882 and 1887 belong to this +particular group, and they all pass within 300,000 +miles of the sun's surface, at a maximum velocity +exceeding 300 miles a second. They must therefore +invade the regions of the solar corona, the +inference being that the corona as well as the comet +is composed of exceedingly rare matter.</p> + +<p>Photography of comets has developed remarkably +within recent years, especially under the deft +manipulation of Barnard, whose plates, in particular +during his residence at the Lick Observatory +on Mount Hamilton, California, show the +features of cometary heads and tails in excellent +definition. Halley's comet, at the 1910 apparition, +was particularly well photographed at many +observatories.</p> + +<p>The question is often asked, When will the next +comet come? If a large bright comet is meant, +astronomers cannot tell. At almost any time one +may blaze into prominence within only a few days. +During the latter half of the last century, bright +<span class="pagenum"><a name="Page_p282" id="Page_p282">[282]</a></span> +comets appeared at perihelion at intervals of eight +years on the average. Several of the lesser and +fainter periodic comets return nearly every year, +but they are mostly telescopic, and are rarely seen +except by astronomers who are particularly interested +in observing them.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p283" id="Page_p283">[283]</a></span></p> + +<h2><a name="CHAPTER_XLII" id="CHAPTER_XLII"></a>CHAPTER XLII<br /> +<br /> +METEORS AND SHOOTING STARS</h2> + +<p>"Falling stars," or "shooting stars," have +been familiar sights in all ages of the world, but +the ancient philosophers thought them scarcely +worthy of notice. According to Aristotle they were +mere nothings of the upper atmosphere, of no more +account than the general happenings of the weather. +But about the end of the eighteenth century and +the beginning of the nineteenth the insufficiency of +this view began to be fully recognized, and interplanetary +space was conceived as tenanted by shoals +of moving bodies exceedingly small in mass and +dimension as compared with the planets.</p> + +<p>Millions of these bodies are all the time in collision +with the outlying regions of our atmosphere; +and by their impact upon it and their friction in +passing swiftly through it, they become heated to +incandescence, thus creating the luminous appearances +commonly known as shooting stars. For the +most part they are consumed or dissipated in vapor +before reaching the solid surface of the earth; but +occasionally a luminous cloud or streak is left glowing +in the wake of a large meteor, which sometimes +remains visible for half an hour after the passage +of the meteor itself. These mistlike clouds projected +upon the dark sky have been especially studied +by Trowbridge of Columbia University.</p> + +<p>Many more meteors are seen during the morning +hours, say from four to six, than at any other nightly +<span class="pagenum"><a name="Page_p284" id="Page_p284">[284]</a></span> +period of equal length, because the visible sky is at +that time nearly centered around the general direction +toward which the earth is moving in its +orbit round the sun; so that the number of meteors +that would fall upon the earth if at rest is increased +by those which the earth overtakes by its own +motion. Also from January to July while the earth +is traveling from perihelion to aphelion, fewer +meteors are seen than in the last half of the year; +but this is chiefly because of the rich showers encountered +in August and November.</p> + +<p>Although the descent of meteoric bodies from the +sky was pretty generally discredited until early in +the nineteenth century, such falls had nevertheless +been recorded from very early times. They were +usually regarded as prodigies or miracles, and such +stones were commonly objects of worship among +ancient peoples. For example, the Phrygian Stone, +known as the "Diana of the Ephesians which fell +down from Jupiter," was a famous stone built into +the Kaaba at Mecca, and even to-day it is revered by +Mohammedans as a holy relic. Perhaps the earliest +known meteoric fall is that historically recorded in +the Parian Chronicle as having occurred in the +island of Crete, <span class="smcap2">B. C.</span> 1478. Also in the imperial +museum of Petrograd is the Pallas or Krasnoiarsk +iron, perhaps three-quarters of a ton in weight, +found in 1772 by Pallas, the famous traveler, at +Krasnoiarsk, Siberia.</p> + +<p>But a fall of meteoric stones that chanced upon +the department of Orne, France, in 1805, led to a +critical investigation by Biot, the distinguished +physicist and academician. According to his report +a violent explosion in the neighborhood of L'Aigle +had been heard for a distance of seventy-five miles +<span class="pagenum"><a name="Page_p285" id="Page_p285">[285]</a></span> +around, and lasting five or six minutes, about 1 P. M. +on Tuesday, April 26. From several adjoining +towns a rapidly moving fireball had been seen in a +sky generally clear, and there was absolutely no +room for doubt that on the same day many stones +fell in the neighborhood of L'Aigle. Biot estimated +their number between two and three thousand, and +they were scattered over an elliptical area more +than six miles long, and two and a half miles broad. +Thenceforward the descent of meteoric matter from +outer space upon the earth has been recognized as +an unquestioned fact.</p> + +<p>The origin of these bodies being cosmic, meteors +may be expected to fall upon the earth without reference +to latitude, or season, or day and night, or +weather. On entering our upper atmosphere their +temperature must be that of space, many hundred +degrees below zero; and their velocities range from +ten miles per second upward. But atmospheric resistance +to their flight is so great that their velocity +is quickly reduced: at ground impact it does not exceed +a few hundred feet per second. On January 1, +1869, several meteoric stones fell on ice only a few +inches thick in Sweden, rebounding without either +breaking through the ice or being themselves +fractured.</p> + +<p>Naturally the flight of a meteor through the atmosphere +will be only a few seconds in duration, +and owing to the sudden reduction of velocity, it will +continue to be luminous throughout only the upper +part of its course. Visibility generally begins at an +elevation of about seventy miles, and ends at +perhaps half that altitude.</p> + +<p>What is the origin of meteors? Theories there +are in great abundance: that they come from the +<span class="pagenum"><a name="Page_p286" id="Page_p286">[286]</a></span> +sun, that they come from the moon, that they come +from the earth in past ages as a result of volcanic +action, and so on. But there are many difficulties in +the way of acceptance of these and several other +theories. That all meteors were originally parts of +cometary masses is however a theory that may be +accepted without much hesitation.</p> + +<p>Comets have been known to disintegrate. Biela's +comet even disappeared entirely, so that during a +shower of Biela meteors in November, 1885, an +actual fragment of the lost comet fell upon the earth, +at Mazapil, Mexico. And as the Bielid meteors encounter +the earth with the relatively low velocity of +ten miles a second, we may expect to capture other +fragments in the future. Numerous observers saw +the weird disintegration of the nucleus of the great +comet of 1882, well recognized as a member of the +family of the comet of 1843. As these comets are +fellow voyagers through space along the same orbit, +probably all five members of the family, with perhaps +others, were originally a single comet of unparalleled +magnitude.</p> + +<p>The Brooks comet of 1890 affords another instance +of fragmentary nucleus. The oft-repeated action of +solar forces tending to disrupt the mass of a comet +more and more, and scatter its material throughout +space, the secular dismemberment of all comets becomes +an obvious conclusion. During the hundreds +of millions of years that these forces are known to +have been operant, the original comets have been +broken up in great numbers, so that elliptical rings +of opaque meteoric bodies now travel round the sun +in place of the comets.</p> + +<p>These bodies in vast numbers are everywhere +through space, each too small to reflect an appreciable +<span class="pagenum"><a name="Page_p287" id="Page_p287">[287]</a></span> +amount of sunlight, and becoming visible only +when they come into collision with our outer atmosphere. +The practical identity of several such +meteor streams and cometary orbits has already +been established, and there is every reason for assigning +a similar origin to all meteoric bodies. +Meteors, then, were originally parts of comets, which +have trailed themselves out to such extent that +particles of the primal masses are liable to be picked +up anywhere along the original cometary paths. The +historic records of all countries contain trustworthy +accounts of meteoric showers. Making due allowances +for the flowery imagery of the oriental, it +is evident that all have at one time or another seen +much the same thing. In <span class="smcap2">A. D.</span> 472, for instance, the +Constantinople sky was reported alive with flying +stars. In October, 1202, "stars appeared like waves +upon the sky; and they flew about like grasshoppers." +During the reign of King William II occurred +a very remarkable shower in which "stars seemed +to fall like rain from heaven."</p> + +<p>But the showers of November, 1799 and 1833, are +easily the most striking of all. The sky was filled +with innumerable fiery trails and there was not a +space in the heavens a few times the size of the moon +that was not ablaze with celestial fireworks. Frequently +huge meteors blended their dazzling brilliancy +with the long and seemingly phosphorescent +trails of the shooting stars.</p> + +<p>The interval of thirty-four years between 1799 +and 1833 appeared to indicate the possibility of a +return of the shower in November of 1866 or 1867, +and all the people of that day were aroused on this +subject and made every preparation to witness the +spectacle. Extemporized observatories were established, +<span class="pagenum"><a name="Page_p288" id="Page_p288">[288]</a></span> +watchmen were everywhere on the lookout, +and bells were to be rung the minute the shower +began. The newspapers of the day did little to allay +the fears of the multitude, but the critical days of +November, 1866, passed with disappointment in +America. In Europe, however, a fine shower was +seen, though it was not equal to that of 1833. The +astronomers at Greenwich counted many thousand +meteors. In November of 1867, however, American +astronomers were gratified by a grand display, +which, although failing to match the general expectation, +nevertheless was a most striking spectacle, +and the careful preparation for observing it afforded +data of observation which were of the greatest scientific +value. The actual orbits of these bodies in space +became known with great exactitude, and it was +found that their general path was identical with that +of the first comet of 1866, which travels outward +somewhat beyond the planet Uranus. When the +visible paths of these meteors are traced backward, +all appear as if they originated from the constellation +Leo. So they are known as Leonids, and a return +of the shower was confidently predicted for +November, 1900-1901, which for unknown reasons +failed to appear.</p> + +<p><span class="pagenum"><a name="Page_p288p1" id="Page_p288p1">[288i]</a></span></p> + +<div class="fig_center" style="width: 637px;"> +<img src="images/p288_1top.png" width="637" height="497" alt="" /> +<div class="fig_caption"><span class="smcap">Two Views of Halley's Comet.</span> Taken with the same camera from +the same position, one on May 12, and the other on May 15, 1910. +(<i>Photo, Mt. Wilson Solar Observatory.</i>)</div> +</div> + +<div class="fig_center" style="width: 645px;"> +<img src="images/p288_1bot.png" width="645" height="508" alt="" /> +<div class="fig_caption"><span class="smcap">Swift's Comet of 1892.</span> This comet showed extraordinary and rapid +transformations, one day having a dozen streamers in its tail, another +only two. (<i>Photo by Prof. E. E. Barnard.</i>)</div> +</div> + +<p><span class="pagenum"><a name="Page_p288p2" id="Page_p288p2">[289i]</a></span></p> + +<div class="fig_center" style="width: 648px;"> +<img src="images/p288_2.png" width="648" height="361" alt="" /> +<div class="fig_caption">A LARGE METEOR TRAIL IN THE FIELD WITH FINE NEBULÆ. +(<i>Photo, Yerkes Observatory.</i>)</div> +</div> + +<p><span class="pagenum"><a name="Page_p289" id="Page_p289">[289]</a></span> +During the last half century meteors have been +pretty systematically observed, especially by the astronomers +of Italy and Denning of England, so that +several hundred distinct showers are now known, +their radiant points fall in every part of the heavens, +and there is scarcely a clear moonless night when +careful watching for meteors will be unrewarded. +Besides November, the months of August (Perseids), +April (Lyrids), and December (Geminids) +are favorable. Following in tabular form is a fairly +comprehensive list of the meteoric showers of the +year, with the positions of the radiant points and +the epochs of the showers according to Denning:</p> + +<h2>RADIANT POINT</h2> + +<table style="width: 80%" summary="Meteor Shower Radiant Point"> +<tr> + <td class="brdtp2 brdbt">Name of Shower</td> + <td class="brdtp2 brdbt brdlf">R. A.</td> + <td class="brdtp2 brdbt brdlf">Decl.</td> + <td class="brdtp2 brdbt brdlf">Date of Shower</td> +</tr> +<tr> + <td align="left">Quadrantids</td> + <td class="brdlf text_rt">230°</td> + <td class="brdlf">+53°</td> + <td class="brdlf" align="left">Jan. 2-4</td> +</tr> +<tr> + <td align="left">Zeta Cepheids</td> + <td class="brdlf text_rt">331°</td> + <td class="brdlf">+56°</td> + <td class="brdlf" align="left">Jan. 25</td> +</tr> +<tr> + <td align="left">Alpha Leonids</td> + <td class="brdlf text_rt">155°</td> + <td class="brdlf">+14°</td> + <td class="brdlf" align="left">Feb. 19-March 1</td> +</tr> +<tr> + <td align="left">Tau Leonids</td> + <td class="brdlf text_rt">166°</td> + <td class="brdlf"> +4°</td> + <td class="brdlf" align="left">March 1-4</td> +</tr> +<tr> + <td align="left">Beta Ursids</td> + <td class="brdlf text_rt">161°</td> + <td class="brdlf">+58°</td> + <td class="brdlf" align="left">March 13-24</td> +</tr> +<tr> + <td align="left">Lyrids</td> + <td class="brdlf text_rt">271°</td> + <td class="brdlf">+33°</td> + <td class="brdlf" align="left">April 20-22</td> +</tr> +<tr> + <td align="left">Gamma Aquarids</td> + <td class="brdlf text_rt">338°</td> + <td class="brdlf"> -2°</td> + <td class="brdlf" align="left">May 1-6</td> +</tr> +<tr> + <td align="left">Zeta Herculids</td> + <td class="brdlf text_rt">246°</td> + <td class="brdlf">+29°</td> + <td class="brdlf" align="left">May 18-26</td> +</tr> +<tr> + <td align="left">Eta Pegasids</td> + <td class="brdlf text_rt">330°</td> + <td class="brdlf">+28°</td> + <td class="brdlf" align="left">May 30-June 4</td> +</tr> +<tr> + <td align="left">Theta Boötids</td> + <td class="brdlf text_rt">213°</td> + <td class="brdlf">+53°</td> + <td class="brdlf" align="left">June 27-28</td> +</tr> +<tr> + <td align="left">Alpha Capricornids</td> + <td class="brdlf text_rt">304°</td> + <td class="brdlf">-12°</td> + <td class="brdlf" align="left">July 15-28</td> +</tr> +<tr> + <td align="left">Delta Aquarids</td> + <td class="brdlf text_rt">339°</td> + <td class="brdlf">-11°</td> + <td class="brdlf" align="left">July 25-30</td> +</tr> +<tr> + <td align="left">Perseids</td> + <td class="brdlf text_rt">45°</td> + <td class="brdlf">+57°</td> + <td class="brdlf" align="left">Aug. 10-12</td> +</tr> +<tr> + <td align="left">Omicron Draconids</td> + <td class="brdlf text_rt">291°</td> + <td class="brdlf">+60°</td> + <td class="brdlf" align="left">Aug. 15-25</td> +</tr> +<tr> + <td align="left">Zeta Draconids</td> + <td class="brdlf text_rt">262°</td> + <td class="brdlf">+63°</td> + <td class="brdlf" align="left">Aug. 21-Sept. 2</td> +</tr> +<tr> + <td align="left">Piscids</td> + <td class="brdlf text_rt">348°</td> + <td class="brdlf"> +2°</td> + <td class="brdlf" align="left">Sept. 4-14</td> +</tr> +<tr> + <td align="left">Alpha Andromedids</td> + <td class="brdlf text_rt">4°</td> + <td class="brdlf">+28°</td> + <td class="brdlf" align="left">Sept. 27</td> +</tr> +<tr> + <td align="left">Epsilon Arietids</td> + <td class="brdlf text_rt">40°</td> + <td class="brdlf">+20°</td> + <td class="brdlf" align="left">Oct. 11-24</td> +</tr> +<tr> + <td align="left">Orionids</td> + <td class="brdlf text_rt">92°</td> + <td class="brdlf">+15°</td> + <td class="brdlf" align="left">Oct. 17-24</td> +</tr> +<tr> + <td align="left">Epsilon Perseids</td> + <td class="brdlf text_rt">61°</td> + <td class="brdlf">+35°</td> + <td class="brdlf" align="left">Nov. 5</td> +</tr> +<tr> + <td align="left">Leonids</td> + <td class="brdlf text_rt">150°</td> + <td class="brdlf">+23°</td> + <td class="brdlf" align="left">Nov. 13-15</td> +</tr> +<tr> + <td align="left">Epsilon Taurids</td> + <td class="brdlf text_rt">64°</td> + <td class="brdlf">+22°</td> + <td class="brdlf" align="left">Nov. 14-25</td> +</tr> +<tr> + <td align="left">Andromedids</td> + <td class="brdlf text_rt">25°</td> + <td class="brdlf">+43°</td> + <td class="brdlf" align="left">Nov. 17-23</td> +</tr> +<tr> + <td align="left">Beta Geminids</td> + <td class="brdlf text_rt">119°</td> + <td class="brdlf">+31°</td> + <td class="brdlf" align="left">Dec. 1-12</td> +</tr> +<tr> + <td align="left">Geminids</td> + <td class="brdlf text_rt">108°</td> + <td class="brdlf">+33°</td> + <td class="brdlf" align="left">Dec. 1-14</td> +</tr> +<tr> + <td align="left">Alpha Ursæ Majorids</td> + <td class="brdlf text_rt">161°</td> + <td class="brdlf">+58°</td> + <td class="brdlf" align="left">Dec. 18-21</td> +</tr> +<tr> + <td class="brdbt" align="left">Kappa Draconids</td> + <td class="brdbt brdlf text_rt">194°</td> + <td class="brdbt brdlf">+68°</td> + <td class="brdbt brdlf" align="left">Dec. 18-28</td> +</tr> +</table> + +<p>The year 1916 was exceptional in providing an +abundant and previously unknown shower on +June 28, and its stream has nearly the same orbit +as that of the Pons-Winnecke periodic comet. Useful +observations of meteors are not difficult to make, +and they are of service to professional astronomers +investigating the orbits of these bodies, among whom +are Mitchell and Olivier of the University of Virginia.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p290" id="Page_p290">[290]</a></span></p> + +<h2><a name="CHAPTER_XLIII" id="CHAPTER_XLIII"></a>CHAPTER XLIII<br /> +<br /> +METEORITES</h2> + +<p>Meteorites, the name for meteors which have +actually gone all the way through our atmosphere, +are never regular in form or spherical. As a +rule the iron meteorites are covered with pittings or +thumb marks, due probably to the resistance and impact +of the little columns of air which impede its +progress, together with the unequal condition and +fusibility of their surface material. The work done +by the atmosphere in suddenly checking the meteor's +velocity appears in considerable part as heat, fusing +the exterior to incandescence. This thin liquid shell +is quickly brushed off, making oftentimes a luminous +train.</p> + +<p>But notwithstanding the exceedingly high temperature +of the exterior, enforced upon it for the brief +time of transit through the atmosphere, it is probable +that all large meteorites, if they could be reached +at once on striking the earth, would be found to +be cold, because the smooth, black, varnishlike +crust which always incases them as a result of +intense heat is never thick. On one occasion a +meteor which was seen to fall in India was dug out +of the ground as quickly as possible, and found to +be, not hot as was expected, but coated thickly over +with ice frozen on it from the moisture in the surrounding +soil.</p> + +<p><span class="pagenum"><a name="Page_p291" id="Page_p291">[291]</a></span> +As to the composition of shooting stars, and their +probable mass, and its effect upon the earth, our +data are quite insufficient. The lines of sodium and +magnesium have been hurriedly caught in the spectroscope, +and, estimating on the basis of the light +emitted by them, the largest meteors must weigh +ounces rather than pounds. Nevertheless, it is interesting +to inquire what addition the continual fall +of many millions daily upon the earth makes to its +weight: somewhere between thirty and fifty thousand +tons annually is perhaps a conservative estimate, +but even this would not accumulate a layer +one inch in thickness over the entire surface of the +earth in less than a thousand million years.</p> + +<p>Many hundreds of the meteors actually seen to +fall, together with those picked up accidentally, are +recovered and prized as specimens of great value in +our collections, the richest of which are now in New +York, Paris, and London. The detailed investigation +of them is rather the province of the chemist, +the crystallographer and the mineralogist than of +the astronomer whose interest is more keen in their +life history before they reach the earth. To distinguish +a stony meteorite from terrestrial rock substances +is not always easy, but there is usually little +difficulty in pronouncing upon an iron meteorite. +These are most frequently found in deserts, because +the dryness of the climate renders their oxidation +and gradual disappearance very slow.</p> + +<p>The surface of a suspected iron meteorite is +polished to a high luster and nitric acid is poured +upon it. If it quickly becomes etched with a characteristic +series of lines, or a sort of cross-hatching, +it is almost certain to be a meteorite. Occasionally +carbon has been found in meteorites, and the existence +<span class="pagenum"><a name="Page_p292" id="Page_p292">[292]</a></span> +of diamond has been suspected. The minerals +composing meteorites are not unlike terrestrial +materials of volcanic origin, though many of them +are peculiar to meteorites only. More than one-third +of all the known chemical elements have been +found by analysis in meteorites, but not any new +ones.</p> + +<p>Meteoric iron is a rich alloy containing about ten +per cent of nickel, also cobalt, tin, and copper in much +smaller amount. Calcium, chlorine, sodium, and +sulphur likewise are found in meteoric irons. At +very high temperatures iron will absorb gases and +retain them until again heated to red heat. Carbonic +oxide, helium, hydrogen, and nitrogen are +thus imprisoned, or occluded, in meteoric irons in +very small quantities; and in 1867, during a London +lecture by Graham, a room in the Royal Institution +was for a brief space illuminated by gas brought to +earth in a meteorite from interplanetary space. +Meteorites, too, have been most critically investigated +by the biologist, but no trace of germs of +organic life of any type has so far been found. +Farrington of Chicago has published a full +descriptive catalogue of all the North American +meteorites.</p> + +<p>Recent investigations of the radioactivity of meteorites +show that the average stone meteorite is +much less radioactive than the average rock, and +probably less than one-fourth as radioactive as +in average granite. The metallic meteorites examined +were found about wholly free from radioactivity.</p> + +<p>From shooting stars, perhaps the chips of the +celestial workshop, or more possibly related to the +planetesimals which the processes of growth of the +<span class="pagenum"><a name="Page_p293" id="Page_p293">[293]</a></span> +universe have swept up into the vastly greater +bodies of the universe, transition is natural to the +stars themselves, the most numerous of the heavenly +bodies, all shining by their own light, and all inconceivably +remote from the solar system, which +nevertheless appears to be not far removed from +the center of the stellar universe.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p294" id="Page_p294">[294]</a></span></p> + +<h2><a name="CHAPTER_XLIV" id="CHAPTER_XLIV"></a>CHAPTER XLIV<br /> +<br /> +THE UNIVERSE OF STARS</h2> + +<p>Our consideration of the solar system hitherto +has kept us quite at home in the universe. The +outer known planets, Uranus and Neptune, are indeed +far removed from the sun, and a few of the +comets that belong to our family travel to even +greater distances before they begin to retrace their +steps sunward. When we come to consider the vast +majority of the glistening points on the celestial +sphere—all in fact except the five great planets, +Mercury, Venus, Mars, Jupiter, and Saturn—we are +dealing with bodies that are self-luminous like the +sun, but that vary in size quite as the bodies of the +solar system do, some stars being smaller than the +sun and others many hundred fold larger than +he is; some being "giants," and others "dwarfs." +But the overwhelming remoteness of all these +bodies arrests our attention and even taxes our +credulity regarding the methods that astronomers +have depended on to ascertain their distances +from us.</p> + +<p>Their seeming countlessness, too, is as bewildering +as are the distances; though, if we make actual +counts of those visible to the naked eye within a +certain area, in the body of the "Great Bear," for +example, the great surprise will be that there are so +few. And if the entire dome of the sky is counted, +at any one time, a clear, moonless sky would reveal +<span class="pagenum"><a name="Page_p295" id="Page_p295">[295]</a></span> +perhaps 2,500, so that in the entire sky, northern +and southern, we might expect to find 5,000 to 6,000 +lucid stars, or stars visible to the naked eye.</p> + +<p>But when the telescope is applied, every accession +of power increases the myriads of fainter and +fainter stars, until the number within optical reach +of present instruments is somewhere between 400 +and 500 millions. But if we were to push the 100-inch +reflector on Mount Wilson to its limit by photography +with plates of the highest sensitiveness, +millions upon millions of excessively faint stars +would be plainly visible on the plates which the +human eye can never hope to see directly with any +telescope present or future, and which would doubtless +swell the total number of stars to a thousand +millions. Recent counts of stars by Chapman and +Melotte of Greenwich tend to substantiate this +estimate.</p> + +<p>What have astronomers done to classify or catalogue +this vast array of bodies in the sky? Even +before making any attempt to estimate their number, +there is a system of classification simply by the +amount of light they send us, or by their apparent +stellar magnitudes—not their actual magnitudes, for +of those we know as yet very little. We speak of +stars of the "first magnitude," of which there are +about 20, Sirius being the brightest and Regulus the +faintest. Then there are about 65 of the second, or +next fainter, magnitude, stars like Polaris, for example, +which give an amount of light two and a half +times less than the average first magnitude star. +Stars of the third magnitude are fainter than those +of the second in the same ratio, but their number increases +to 200; fourth magnitude, 500; fifth magnitude, +1,400; sixth magnitude, 5,000, and these are so +<span class="pagenum"><a name="Page_p296" id="Page_p296">[296]</a></span> +faint that they are just visible on the best nights +without telescopic aid.</p> + +<p>Decimals express all intermediate graduations of +magnitude. Astronomers carry the telescopic magnitudes +much farther, till a magnitude beyond the +twentieth is reached, preserving in every case the +ratio of two and one-half for each magnitude in relation +to that numerically next to it. Even Jupiter +and Venus, and the sun and moon, are sometimes +calculated on this scale of stellar magnitude, numerically +negative, of course, Venus sometimes being as +bright as magnitude -4.3, and the sun -26.7.</p> + +<p>Knowing thus the relation of sun, moon, and +stars, and the number of the stars of different magnitudes, +it is possible to estimate the total light from +the stars. This interesting relation comes out this +way: that the stars we cannot see with the naked +eye give a greater total of light than those we can +because of their vastly greater numbers. And if +we calculate the total light of all the brighter stars +down to magnitude nine and one-half, we find it +equal to 1/80th of the light of the average full moon.</p> + +<p>Many stars show marked differences in color, and +strictly speaking the stars are now classified by +their colors. The atmosphere affects star colors +very considerably, low altitudes, or greater thickness +of air, absorbing the bluish rays more strongly +and making the stars appear redder than they really +are. Aldebaran, Betelgeuse and Antares are well-known +red stars, Capella and Alpha Ceti yellowish, +Vega and Sirius blue, and Procyon and Polaris +white. Among the telescopic stars are many of a +deep blood-red tint, variable stars being numerous +among them. Double stars, too, are often complementary +in color. There is evidence indicating +<span class="pagenum"><a name="Page_p297" id="Page_p297">[297]</a></span> +change of color of a very few stars in long periods +of time; Sirius, for example, two thousand years ago +was a red star, now it is blue or bluish white. But the +meaning of color, or change of color in a star is as +yet only incompletely ascertained. It may be connected +with the radiative intensity of the star, or +its age, or both.</p> + +<p>The late Professor Edward C. Pickering was +famous for his life-long study and determination of +the magnitudes of the stars. Standards of comparison +have been many, and have led to much unnecessary +work. Pickering chose Polaris as a +standard and devised the meridian photometer, an +ingenious instrument of high accuracy, in which the +light of a star is compared directly with that of the +pole star by reflection. All the bright stars of both +the northern and the southern skies are worked +into a standard system of magnitudes known as +HP, or the Harvard Photometry.</p> + +<p>Astronomers make use of several different kinds +of magnitude for the stars: the apparent magnitude, +as the eye sees it, often called the visual magnitude; +the photographic magnitude, as the photographic +plate records it, and these are now determined with +the highest accuracy; the photovisual magnitude, +quite the same as the visual, but determined photographically +on an isochromatic plate with a yellow +screen or filter, so that the intensity is nearly the +same as it appears to the eye. The difference between +the star's visual or photovisual magnitude +and its photographic magnitude is called its color-index, +and is often used as a measure of the star's +color. Light of the shorter wave lengths, as blue +and violet, affects the photographic plate more +rapidly than the reds and yellows of longer wave +<span class="pagenum"><a name="Page_p298" id="Page_p298">[298]</a></span> +length by which the eye mainly sees; so that red +stars will appear much fainter and blue stars much +brighter on the ordinary photographic plate than +the eye sees them.</p> + +<p>So great are the differences of color in the stars +that well-known asterisms, with which the eye is +perfectly familiar, are sometimes quite unrecognizable +on the photographic plate, except by relative +positions of the stars composing them. White stars +affect the eye and the plate about equally, so that +their visual or photovisual and photographic magnitudes +are about equal. The studies of the colors +of the stars, the different methods of determining +them, and the relations of color to constitution have +been made the subject of especial investigation by +Seares of Mount Wilson and many other astronomers.</p> + +<p>Centuries of the work of astronomers have been +faithfully devoted to mapping or charting the stars +and cataloguing them. Just as we have geographical +maps of countries, so the heavens are parceled out +in sections, and the stars set down in their true +relative positions just as cities are on the map. Recent +years have added photographic charts, especially +of detailed regions of the sky; but owing to +spectral differences of the stars, their photographic +magnitudes are often quite different from their visual +magnitudes. From these maps and charts the +positions of the stars can be found with much precision; +but if we want the utmost accuracy, we +must go to the star catalogues—huge volumes oftentimes, +with stellar positions set down therein with +the last degree of precision.</p> + +<p>First there will be the star's name, and in the next +column its magnitude, and in a third the star's +<span class="pagenum"><a name="Page_p299" id="Page_p299">[299]</a></span> +right ascension. This is its angular distance eastward +around the celestial sphere starting from the +vernal equinox, and it corresponds quite closely to +the longitude of a place which we should get from a +gazetteer, if we wished to locate it on the earth. +Then another column of the catalogue will give the +star's declination, north or south of the equator, +just as the gazetteer will locate a city by its north +or south latitude.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p300" id="Page_p300">[300]</a></span></p> + +<h2><a name="CHAPTER_XLV" id="CHAPTER_XLV"></a>CHAPTER XLV<br /> +<br /> +STAR CHARTS AND CATALOGUES</h2> + +<p>Who made the first star chart or catalogue? +There is little doubt that Eudoxus (<span class="smcap2">B. C.</span> 200) +was the first to set down the positions of all the +brighter stars on a celestial globe, and he did this +from observations with a gnomon and an armillary +sphere. Later Hipparchus (<span class="smcap2">B. C.</span> 130) constructed +the first known catalogue of stars, so that astronomers +of a later day might discover what changes +are in progress among the stars, either in their relative +positions or caused by old stars disappearing +or new stars appearing at times in the heavens. +Hipparchus was an accurate observer, and he discovered +an apparent and perpetual shifting of the +vernal equinox westward, by which the right ascensions +of the stars are all the time increasing. +He determined the amount of it pretty accurately, +too. His catalogue contained 1,080 stars, and is +printed in the "Almagest" of Ptolemy.</p> + +<p>Centuries elapsed before a second star catalogue +was made, by Ulugh-Beg, an Arabian astronomer, +<span class="smcap2">A. D.</span> 1420, who was a son of Tamerlane, the Tartar +monarch of Samarcand, where the observations for +the catalogue were made. The stars were mainly +those of Ptolemy, and much the same stars were reobserved +by Tycho Brahe (<span class="smcap2">A. D.</span> 1580) with his +greatly improved instruments, thus forming the +<span class="pagenum"><a name="Page_p301" id="Page_p301">[301]</a></span> +third and last star catalogue of importance before +the invention of the telescope.</p> + +<p>From the end of the seventeenth century onward, +the application of the telescope to all the types of instruments +for making observations of star places +has increased the accuracy many-fold. The entire +heavens has been covered by Argelander in the +northern hemisphere, and Gould in the southern—over +700,000 stars in all. Many government observatories +are still at work cataloguing the stars. The +Carnegie Institution of Washington maintains a +department of astrometry under Boss of Albany, +which has already issued a preliminary catalogue +of more than 6,000 stars, and has a great general +catalogue in progress, together with investigations +of stellar motions and parallaxes. This catalogue +of star positions will include proper motions of +stars to the seventh magnitude.</p> + +<p>In 1887 on proposal of the late Sir David Gill, +an international congress of astronomers met at +Paris and arranged for the construction of a photographic +chart of the entire heavens, allotting the +work to eighteen observatories, equipped with photographic +telescopes essentially alike. The total number +of plates exceeds 25,000. Stars of the fourteenth +magnitude are recorded, but only those including +the eleventh magnitude will be catalogued, perhaps +2,000,000 in all. The expense of this comprehensive +map of the stars has already exceeded $2,000,000, +and the work is now nearly complete. Turner of +Oxford has conducted many special investigations +that have greatly enhanced the progress of this international +enterprise.</p> + +<p>Other great photographic star charts have been +carried through by the Harvard Observatory, with +<span class="pagenum"><a name="Page_p302" id="Page_p302">[302]</a></span> +the annex at Arequipa, Peru, employing the Bruce +photographic telescope, a doublet with 24-inch +lenses; also Kapteyn of Groningen has catalogued +about 300,000 stars on plates taken at Cape Town. +Charting and cataloguing the stars, both visually +and photographically, is a work that will never be +entirely finished. Improvements in processes will +be such that it can be better done in the future than +it is now, and the detection of changes in the fainter +stars and investigation of their motions will necessitate +repetition of the entire work from century +to century.</p> + +<p>The origin of the names of individual stars is a +question of much interest. The constellation figures +form the basis of the method, and the earliest names +were given according to location in the especial +figure; as for instance, Cor Scorpii, the heart of the +Scorpion, later known as Antares or Alpha Scorpii. +The Arabians adopted many star names from the +Greeks, and gave about a hundred special names +to other stars. Some of these are in common use +to-day, by navigators, observers of meteors and of +variable stars. Sirius, Vega, Arcturus, and a few +other first magnitude stars, are instances.</p> + +<p>But this method is quite insufficient for the +fainter stars whose numbers increase so rapidly. +Bayer, a contemporary of Galileo, originated our +present system, which also employs the names of the +constellations, the Latin genitive in each case, prefixed +by the small letters of the Greek alphabet, +from alpha to omega, in order of decreasing brightness; +and followed by the Roman letters when the +Greek alphabet is exhausted.</p> + +<p>If there were still stars left in a constellation unnamed, +numbers were used, first by Flamsteed, +<span class="pagenum"><a name="Page_p303" id="Page_p303">[303]</a></span> +Astronomer Royal; and numbers in the order of +right ascension in various catalogues are used to +designate hundreds of other stars. The vast bulk +of the stars are, however, nameless; but about one +million are identifiable by their positions (right +ascension and declination) on the celestial sphere.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p304" id="Page_p304">[304]</a></span></p> + +<h2><a name="CHAPTER_XLVI" id="CHAPTER_XLVI"></a>CHAPTER XLVI<br /> +<br /> +THE SUN'S MOTION TOWARD LYRA</h2> + +<p>If Hipparchus or Galileo should return to earth +to-night and look at the stars and constellations +as we see them, there would be no change whatever +discernible in either the brightness of the stars or in +their relative positions. So the name fixed stars +would appear to have been well chosen. Halley in the +seventeenth century was the first to detect that slow +relative change of position of a few stars which is +known as proper motion, and all the modern catalogues +give the proper motions in both right ascension +and declination. These are simply the +small annual changes in position athwart the line +of vision; and, as a whole, the proper motions of the +brighter stars exceed the corresponding motions of +the fainter ones because they are nearer to us. The +average proper motion of the brightest stars is +0".25, and of stars of the sixth magnitude only one-sixth +as great.</p> + +<p>A few extreme cases of proper motion have been +detected, one as large as 9", of an orange yellow +star of the eighth magnitude in the southern constellation +Pictor, and Barnard has recently discovered +a star with a proper motion exceeding 10"; +several determinations of its parallax give 0".52, +corresponding to a distance of 6.27 light years. +Nevertheless, two centuries would elapse before +these stars would be displaced as much as the +<span class="pagenum"><a name="Page_p305" id="Page_p305">[305]</a></span> +breadth of the moon among their neighbors in the +sky. The proper motions of stars are along perfectly +straight lines, so far as yet observed. Ultimately +we may find a few moving in curved paths +or orbits, but this is hardly likely.</p> + +<p>As for a central sun hypothesis, that pointing out +Alcyone in particular, there is no reliable evidence +whatever. Analysis of the proper motions of stars +in considerable numbers, first by Sir William Herschel, +showed that they were moving radially from +the constellation Hercules, and in great numbers +also toward the opposite side of the stellar sphere. +Later investigation places this point, called the +sun's goal, or apex of the sun's way, over in the +adjacent constellation Lyra; and the opposite point, +or the sun's quit, is about halfway between Sirius +and Canopus. By means of the radial velocities of +stars in these antipodal regions of the sky, it is +found that the sun's motion toward Lyra, carrying +all his planetary family along with him, is taking +place at the rate of about 12 miles in every second.</p> + +<p>While the right ascensions of the solar apex as +given by the different investigations have been +pretty uniform, the declination of this point has +shown a rather wide variation not yet explained. +For example, there is a difference of nearly ten +degrees between the declination (+34°.3) of the +apex as determined by Boss from the proper motions +of more than 6,000 stars, and the declination +(+25°.3) found by Campbell from the radial velocities +of nearly 1,200 stars. Several investigations +tend to show that the fainter the stars are, the +greater is the declination of the solar apex. More +remarkable is the evidence that this declination +varies with the spectral type of the stars, the later +<span class="pagenum"><a name="Page_p306" id="Page_p306">[306]</a></span> +types, especially G and K, giving much more northerly +values. On the whole the great amount of research +that has been devoted to the solar motion +relative to the system of the stars for the past +hundred years may be said to indicate a point in +right ascension 18h. (270°) and declination 34° N. +as the direction toward which the sun is moving. +This is not very far from the bright star Alpha +Lyræ, and the antipodal point from which the sun +is traveling is quite near to Beta Columbæ.</p> + +<p>So swift is this motion (nearly twenty kilometers +per second) that it has provided a base line of exceptional +length, and very great service in determining +the average distance of stars in groups or classes. +After thousands of years the sun's own motion combined +with the proper motions of the stars will displace +many stars appreciably from their familiar +places. The constellations as we know them will +suffer slight distortions, particularly Orion, Cassiopeia +and Ursa Major. Identity or otherwise of +spectra often indicates what stars are associated +together in groups, and their community of motion +is known as star drift. Recent investigation of vast +numbers of stars by both these methods have led to +the epochal discovery of star streaming, which indicates +that the stars of our system are drifting by, +or rather through, each other, in two stately and +interpenetrating streams. The grand primary +cause underlying this motion is as yet only surmised.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p307" id="Page_p307">[307]</a></span></p> + +<h2><a name="CHAPTER_XLVII" id="CHAPTER_XLVII"></a>CHAPTER XLVII<br /> +<br /> +STARS AND THEIR SPECTRAL TYPE</h2> + +<p>When in 1872 Dr. Henry Draper placed a very +small wet plate in the camera of his spectroscope +and, by careful following, on account of the +necessarily long exposure, secured the first photographic +spectrum of a star ever taken, he could +hardly have anticipated the wealth of the new field +of research which he was opening. His wife, Anna +Palmer Draper, was his enthusiastic assistant in +both laboratory and observatory, and on his death in +1882, she began to devote her resources very considerably +to the amplification of stellar spectrum +photography. At first with the cooperation of +Professor Young of Princeton, and later through +extension of the facilities of Harvard College +Observatory, whose director, the late Professor +Edward C. Pickering, devoted his energies in very +large part to this matter, all the preliminaries of +the great enterprise were worked out, and a comprehensive +program was embarked upon, which culminated +in the "Henry Draper Memorial," a catalogue +and classification of the spectra of all the stars +brighter than the ninth magnitude, in both the +northern and southern hemispheres.</p> + +<p>One very remarkable result from the investigation +of large numbers of stars according to their +type is the close correlation between a star's luminosity +and its spectral type. But even more remarkable +<span class="pagenum"><a name="Page_p308" id="Page_p308">[308]</a></span> +is the connection between spectral type and +speed of motion. As early as 1892 Monck of Dublin, +later Kapteyn, and still later Dyson, directed attention +to the fact that stars of the Secchi type II had +on the average larger proper motions than those of +type I. In 1903 Frost and Adams brought out the +exceptional character of the Orion stars, the radial +velocities of twenty of which averaged only seven +kilometers per second.</p> + +<p>Soon after, with the introduction of the two-stream +hypothesis, a wider generalization was +reached by Campbell and Kapteyn, whose radial velocities +showed that the average linear velocity increases +continually through the entire series B, A, +F, G, K, M, from the earliest types of evolution to +the latest. The younger stars of early type have +velocities of perhaps five or six kilometers per second, +while the older stars of later type have velocities +nearly fourfold greater.</p> + +<p>The great question that occurs at once is: How do +the individual stars get their motions? The farther +back we go in a star's life history, the smaller we +find its velocity to be. When a star reaches the +Orion stage of development, its velocity is only one-third +of what it may be expected to have finally. +Apparently, then, the stars at birth have no motion, +but gradually acquire it in passing through their +several types or stages of development.</p> + +<p>More striking still is the motion of the planetary +nebulæ, in excess of 25 kilometers per second, while +type A stars move 11 kilometers, type G 15 kilometers, +and type M 17 kilometers per second. Can the +law connecting speed of motion and spectral type +be so general that the planetary nebula is to be regarded +as the final evolutionary stage? Stars have +<span class="pagenum"><a name="Page_p309" id="Page_p309">[309]</a></span> +been seen to become nebulæ, and one astronomer at +least is strongly of the opinion that a single such +instance ought to outweigh all speculation to the +contrary, as that stars originate from nebulæ.</p> + +<p>In his discussion of stellar proper motions, Boss +has reached a striking confirmation of the relation +of speed to type, finding for the cross linear motion +of the different types a series of velocities closely +paralleling those of Kapteyn and Campbell.</p> + +<p>Concerning the marked relation of the luminosities +of the stars to their spectral types, there is a +pronounced tendency toward equality of brightness +among stars of a given type; also the brightness +diminishes very markedly with advance in the stage +of evolution. There has been much discussion as to +the order of evolution as related to the type of +spectrum, and Russell of Princeton has put forward +the hypothesis of giant stars and dwarf stars, each +spectral type having these two divisions, though not +closely related. One class embraces intensely luminous +stars, the other stars only feebly luminous. +When a star is in process of contraction from a +diffused gaseous mass, its temperature rises, according +to Lane's law, until that density is reached where +the loss of heat by radiation exceeds the rise in temperature +due to conversion of gravitational energy +into heat. Then the star begins to cool again. So +that if the spectrum of a star depends mainly on the +effective temperature of the body, clearly the classification +of the Draper catalogue would group stars +together which are nearly alike in temperature, +taking no note as to whether their present temperature +is rising or falling.</p> + +<p>Another classification of stars by Lockyer divides +them according to ascending and descending temperatures. +<span class="pagenum"><a name="Page_p310" id="Page_p310">[310]</a></span> +Russell's theory would assign the succession +of evolutionary types in the order, M<sub>1</sub>, K<sub>1</sub>, +G<sub>1</sub>, F<sub>1</sub>, A<sub>1</sub>, B, A<sub>2</sub>, F<sub>2</sub>, G<sub>2</sub>, K<sub>2</sub>, M<sub>2</sub>, the subscript 1 referring +to the "giants," and 2 to the dwarf stars. +In large part the weight of evidence would appear +to favor the order of the Harvard classification, independently +confirmed as it is by studies of stellar +velocities, Galactic distribution, and periods of binary +stars both spectroscopic and visual, where +Campbell and Aiken find a marked increase in +length of period with advance in spectral type. At +the same time, a vast amount of evidence is accumulating +in support of Russell's theory. Investigations +in progress will doubtless reveal the ground on +which both may be harmonized.</p> + +<p>The publication of the new Henry Draper Catalogue +of Stellar Spectra is in progress, a work of +vast magnitude. The great catalogue of thirty +years ago embraced the spectra of more than ten +thousand stars, and was a huge work for that day; +but the new catalogue utterly dwarfs it, with a +classification much more detailed than in the +earlier work, and with the number of stars increased +more than twenty-fold. This work, projected by the +late director of the Harvard Observatory, has been +brought to a conclusion by the energy and enthusiasm +of Miss Annie J. Cannon through six years of +close application, aided by many assistants. The +catalogue ranges over the stars of both hemispheres, +and is a monument to masterly organization and +completed execution which will be of the highest +importance and usefulness in all future researches +on the bodies of the stellar universe.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p311" id="Page_p311">[311]</a></span></p> + +<h2><a name="CHAPTER_XLVIII" id="CHAPTER_XLVIII"></a>CHAPTER XLVIII<br /> +<br /> +STAR DISTANCES</h2> + +<p>So vast are the distances of the stars that all attempts +of the early astronomers to ascertain +them necessarily proved futile. This led many astronomers +after Copernicus to reject his doctrine +of the earth's motion round the sun, so that they +clung rather to the Ptolemaic view that the earth +was without motion and was the center about which +all the celestial motions took place. The geometry +of stellar distances was perfectly understood, and +many were the attempts made to find the parallaxes +and distances of the stars; but the art of instrument +making had not yet advanced to a stage where +astronomers had the mechanisms that were absolutely +necessary to measure very small angles.</p> + +<p>About 1835, Bessel undertook the work of determining +stellar parallax in earnest. His instrument +was the heliometer, originally designed for measuring +the sun's diameter; but as modified for parallax +work it is the most accurate of all angle-measuring +instruments that the astronomers employ. The star +that he selected was 61 Cygni, not a bright star, +of the sixth magnitude only, but its large proper +motion suggested that it might be one of those +nearest to us. He measured with the heliometer, +at opposite seasons of the year, the distance of 61 +Cygni from another and very small star in the +same field of view, and thus determined the relative +<span class="pagenum"><a name="Page_p312" id="Page_p312">[312]</a></span> +parallax of the two stars. The assumption was +made that the very faint star was very much more +distant than the bright one, and this assumption +will usually turn out to be sound. Bessel got 0".35 +for his parallax of 61 Cygni, and Struve by applying +the same method to Alpha Lyræ, about the +same time, got 0".25 for the parallax of that star.</p> + +<p>These classic researches of Bessel and Struve +are the most important in the history of star distances, +because they were the first to prove that +stellar parallax, although minute, could nevertheless +be actually measured. About the same time +success was achieved in another quarter, and Henderson, +the British astronomer at the Cape of Good +Hope, found a parallax of nearly a whole second +for the bright star Alpha Centauri.</p> + +<p>Although the parallaxes of many hundreds of +stars have been measured since, and the parallaxes +of other thousands of stars estimated, the measured +parallax of Alpha Centauri, as later investigated by +Elkin and Sir David Gill, and found to be 0".75, +is the largest known parallax, and therefore Alpha +Centauri is our nearest neighbor among the stars, +so far as we yet know. This star is a binary system +and the light of the two components together is +about the same as that of Capella (Alpha Aurigæ). +But it is never visible from this part of the world, +being in 60 degrees of south declination: one might +just glimpse it near the southern horizon from Key +West.</p> + +<p>How the distances of the stars are found is not +difficult to explain, although the method of doing it +involves a good deal of complication, interesting to +the practical astronomer only. Recall the method of +getting the moon's distance from the earth: it was +<span class="pagenum"><a name="Page_p313" id="Page_p313">[313]</a></span> +done by measuring her displacement among the stars +as seen from two widely separated observatories, +as near the ends of a diameter of the earth as convenient. +This is the base line, and the angle +which a radius of the earth as seen from the +center of the moon fills, or subtends, is the moon's +parallax.</p> + +<p>So near is the moon that this angle is almost +an entire degree, and therefore not at all difficult +to measure. But if we go to the distance of even +Alpha Centauri, the nearest of the stars, our earth +shrinks to invisibility; so that we must seek a +longer base line. Fortunately there is one, but +although its length is 25,000 times the earth's diameter, +it is only just long enough to make the star +distances measurable. We found that the sun's distance +from the earth was 93 million miles; the +diameter of the earth's orbit is therefore double +that amount. Now conceive the diameter of the +earth replaced by the diameter of the earth's +orbit: by our motion round the sun we are transported +from one extremity of this diameter to the +opposite one in six month's time; so we may measure +the displacement of a star from these two extremities, +and half this displacement will be the +star's parallax, often called the annual parallax +because a year is consumed in traversing its period. +And it is this very minute angle which Bessel and +Struve were the first to measure with certainty, +and which Henderson found to be in the case of +Alpha Centauri the largest yet known.</p> + +<p>Evidently the earth by its motion round the sun +makes every star describe, a little parallactic ellipse; +the nearer the star is the larger this ellipse will +be, and the farther the star the smaller: if the star +<span class="pagenum"><a name="Page_p314" id="Page_p314">[314]</a></span> +were at an infinite distance, its ellipse would become +a point, that is, if we imagine ourselves +occupying the position of the star, even the vast +orbit of the earth, 186 million miles across, would +shrink to invisibility or become a mathematical +point.</p> + +<p>Measurement of stellar parallax is one of many +problems of exceeding difficulty that confront the +practical astronomer. But the actual research nowadays +is greatly simplified by photography, which +enables the astronomer to select times when the air +is not only clear, but very steady for making the +exposures. Development and measurement of the +plates can then be done at any time. Pritchard of +Oxford, England, was among the earliest to appreciate +the advantages of photography in parallax +work, and Schlesinger, Mitchell, Miller, Slocum +and Van Maanen, with many others in this country, +have zealously prosecuted it.</p> + +<p>How shall we intelligently express the vast distances +at which the stars are removed from us? +Of course we can use miles, and pile up the millions +upon millions by adding on ciphers, but that fails to +give much notion of the star's distance. Let us try +with Alpha Centauri: its parallax of 0".75 means +that it is 275,000 times farther from the sun than +the earth is. Multiplying this out, we get 25 trillion +miles, that is, 25 millions of million miles—an +inconceivable number, and an unthinkable +distance.</p> + +<p>Suppose the entire solar system to shrink so that +the orbit of Neptune, sixty times 93 million miles +in diameter, would be a circle the size of the dot +over this letter i. On the same scale the sun itself, +although nearly a million miles in diameter, could +<span class="pagenum"><a name="Page_p315" id="Page_p315">[315]</a></span> +not be seen with the most powerful microscope in +existence; and on the same scale also we should +have to have a circle ten feet in diameter, if the +solar system were imagined at its center and Alpha +Centauri in its circumference.</p> + +<p>So astronomers do not often use the mile as a +yardstick of stellar distance, any more than we +state the distance from London to San Francisco +in feet or inches. By convention of astronomers, +the average distance between the centers of sun +and earth, or 93 million miles, is the accepted +unit of measure in the solar system. So the adopted +unit of stellar distance is the distance traveled by +a wave of light in a year's time: and this unit is +technically called the light-year. This unit of distance, +or stellar yardstick, as we may call it, is +nearly 6 millions of million miles in length. Alpha +Centauri, then, is four and one-third light-years +distant, and 61 Cygni seven and one-fifth light-years +away.</p> + +<p>For convenience in their calculations most astronomers +now use a longer unit called the parsec, +first suggested by Turner. Its length is equal to the +distance of a star whose parallax is one second of +arc; that is, one parsec is equal to about three +and a quarter light-years. Or the light-year is +equal to 0.31 parsec. Also the parsec is equal to +206,000 astronomical units, or about 19 millions of +million miles.</p> + +<p>We have, then four distinct methods of stating +the distance of a star: Sirius, for example, has a +parallax of 0".38 or its distance is two and two-thirds +parsecs, or eight and a half light-years, or +50 millions of million miles. It is the angle of +parallax which is always found first by actual measurement +<span class="pagenum"><a name="Page_p316" id="Page_p316">[316]</a></span> +and from this the three other estimates +of distance are calculated.</p> + +<p>So difficult and delicate is the determination of +a stellar distance that only a few hundred parallaxes +have been ascertained in the past century. +The distance of the same star has been many times +measured by different astronomers, with much +seeming duplication of effort. Comprehensive campaigns +for determining star parallaxes in large +numbers have been undertaken in a few instances, +particularly at the suggestion of Kapteyn, the eminent +astronomer of Groningen, Holland. His catalogue +of star parallaxes is the most complete and +accurate yet published, and is the standard in all +statistical investigations of the stars.</p> + +<p>That we find relatively large parallaxes for +some of the fainter stars, and almost no measurable +parallax for some of the very bright stars is +one of the riddles of the stellar universe. We may +instance Arcturus, in the northern hemisphere and +Canopus in the southern; the latter almost as +bright as Sirius. Dr. Elkin and the late Sir David +Gill determined exhaustively the parallax of Canopus, +and found it very minute, only 0".03, making +its distance in excess of a hundred light-years. The +stupendous brilliancy of this star is apparent if we +remember that the intensity of its light must vary +inversely as the square of the distance; so that if +Canopus were to be brought as near us as even +61 Cygni is, it would be a hundredfold brighter +than Sirius, the brightest of all the stars of the +firmament.</p> + +<p>In researches upon the distribution of the more +distant stars, the method of measuring parallaxes +of individual stars fails completely, and the secular +<span class="pagenum"><a name="Page_p317" id="Page_p317">[317]</a></span> +parallax, or parallactic motion of the stars is employed +instead. By parallactic motion is meant the +apparent displacement in consequence of the solar +motion which is now known with great accuracy, +and amounts to 19.5 kilometers per second. Even +in a single year, then, the sun's motion is twice the +diameter of the earth's orbit, so that in a hundred +or more years, a much longer base line is available +than in the usual type of observations for stellar +parallax. If we ascertain the parallactic motion of +a group of stars, then we can find their average +distance. It is found, for example, that the mean +parallax of stars of the sixth magnitude is 0".014. +Also the mean distances of stars thrown into +classes according to their spectral type have been +investigated by Boss, Kapteyn, Campbell and +others. The complete intermingling of the two +great star streams has been proved, too, by using +the magnitude of the proper motions to measure +the average distances of both streams. These come +out essentially the same, so that the streaming cannot +be due to mere chance relation in the line of +sight.</p> + +<p>Most unexpected and highly important is the discovery +that the peculiar behavior of certain lines +in the spectrum leads to a fixed relation between a +star's spectrum and its absolute magnitude, which +provides a new and very effective method of ascertaining +stellar distances. By absolute magnitudes +are meant the magnitudes the stars would appear to +have if they were all at the same standard distance +from the earth.</p> + +<p>Very satisfactory estimates of the distance of +exceedingly remote objects have been made within +recent years by this indirect method, which is especially +<span class="pagenum"><a name="Page_p318" id="Page_p318">[318]</a></span> +applicable to spiral nebulæ and globular clusters. +The absolute magnitude of a star is inferred +from the relative intensities of certain lines in its +spectrum, so that the observed apparent magnitude +at once enables us to calculate the distance of the +star. Adams and Joy have recently determined +the luminosities and parallaxes of 500 stars by this +spectroscopic method. Of these stars 360 have had +their parallaxes previously measured; and the +average difference between the spectroscopic and +the trigonometric values of the parallax is only +the very small angle 0".0037, a highly satisfactory +verification.</p> + +<p>An indirect method, but a very simple one, and +of the greatest value because it provides the key +to stellar distances with the least possible calculation, +and we can ascertain also the distances of +whole classes of stars too remote to be ascertained +in any other way at present known.</p> + +<p>The problem of spectroscopic determinations of +luminosity and parallax has been investigated at +Mount Wilson with great thoroughness from all +sides, the separate investigations checking each +other. A definitive scale for the spectroscopic determination +of absolute magnitudes has now been +established, and the parallaxes and absolute magnitudes +have already been derived for about 1,800 +stars.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p319" id="Page_p319">[319]</a></span></p> + +<h2><a name="CHAPTER_XLIX" id="CHAPTER_XLIX"></a>CHAPTER XLIX<br /> +<br /> +THE NEAREST STARS</h2> + +<p>Of especial interest are the few stars that we +know are the nearest to us, and the following +table includes all those whose parallax is 0".20 or +greater. There are nineteen in all and nearly half +of them are binary systems. The radial motions +given are relative to the sun. The transverse velocities +are formed by using the measured parallaxes +to transform proper motions into linear measures. +They are given by Eddington in his "Stellar Movements":</p> + +<table style="width: 80%" summary="Nearest Stars"> +<tr> + <td class="brdtp2 brdbt">Star's Name</td> + <td class="brdtp2 brdbt brdlf">Magnitude</td> + <td class="brdtp2 brdbt brdlf">Parallax in Seconds of Arc</td> + <td class="brdtp2 brdbt brdlf">Proper Motion in Seconds of Arc</td> + <td class="brdtp2 brdbt brdlf">Linear Velocity Km. per sec.</td> + <td class="brdtp2 brdbt brdlf">Radial Velocity Km. per sec.</td> + <td class="brdtp2 brdbt brdlf">Spectral Type</td> + <td class="brdtp2 brdbt brdlf">Luminosity (Sun=1)</td> + <td class="brdtp2 brdbt brdlf">Star Stream</td> +</tr> +<tr> + <td class="text_lf">Groombridge 34</td> + <td class="brdlf">8.2</td> + <td class="brdlf">0.28</td> + <td class="brdlf">2.85</td> + <td class="brdlf">48</td> + <td class="brdlf">‥</td> + <td class="brdlf">Ma</td> + <td class="brdlf">0.010</td> + <td class="brdlf">I</td> +</tr> +<tr> + <td class="text_lf">Eta Cassiop</td> + <td class="brdlf">3.6</td> + <td class="brdlf">0.20</td> + <td class="brdlf">1.25</td> + <td class="brdlf">30</td> + <td class="brdlf">+10</td> + <td class="brdlf">F8</td> + <td class="brdlf">1.4</td> + <td class="brdlf">I</td> +</tr> +<tr> + <td class="text_lf">Tau Ceti</td> + <td class="brdlf">3.6</td> + <td class="brdlf">0.33</td> + <td class="brdlf">1.93</td> + <td class="brdlf">28</td> + <td class="brdlf">-16</td> + <td class="brdlf">K</td> + <td class="brdlf">0.50</td> + <td class="brdlf">II</td> +</tr> +<tr> + <td class="text_lf">Epsilon Erid</td> + <td class="brdlf">3.3</td> + <td class="brdlf">0.31</td> + <td class="brdlf">1.00</td> + <td class="brdlf">15</td> + <td class="brdlf">+16</td> + <td class="brdlf">K</td> + <td class="brdlf">0.79</td> + <td class="brdlf">II</td> +</tr> +<tr> + <td class="text_lf">CZ 5h 243</td> + <td class="brdlf">8.3</td> + <td class="brdlf">0.32</td> + <td class="brdlf">8.70</td> + <td class="brdlf">129</td> + <td class="brdlf">+242</td> + <td class="brdlf">G-K</td> + <td class="brdlf">0.007</td> + <td class="brdlf">II</td> +</tr> +<tr> + <td class="text_lf">Sirius</td> + <td class="brdlf">-1.6</td> + <td class="brdlf">0.38</td> + <td class="brdlf">1.32</td> + <td class="brdlf">16</td> + <td class="brdlf">-7</td> + <td class="brdlf">A</td> + <td class="brdlf">48.0</td> + <td class="brdlf">II</td> +</tr> +<tr> + <td class="text_lf">Procyon</td> + <td class="brdlf">0.5</td> + <td class="brdlf">0.32</td> + <td class="brdlf">1.25</td> + <td class="brdlf">19</td> + <td class="brdlf">-3</td> + <td class="brdlf">F5</td> + <td class="brdlf">9.7</td> + <td class="brdlf">I ?</td> +</tr> +<tr> + <td class="text_lf">Lal. 21185</td> + <td class="brdlf">7.6</td> + <td class="brdlf">0.40</td> + <td class="brdlf">4.77</td> + <td class="brdlf">57</td> + <td class="brdlf">‥</td> + <td class="brdlf">Ma</td> + <td class="brdlf">0.009</td> + <td class="brdlf">II</td> +</tr> +<tr> + <td class="text_lf">Lal. 21258</td> + <td class="brdlf">8.9</td> + <td class="brdlf">0.20</td> + <td class="brdlf">4.46</td> + <td class="brdlf">106</td> + <td class="brdlf">‥</td> + <td class="brdlf">Ma</td> + <td class="brdlf">0.011</td> + <td class="brdlf">I</td> +</tr> +<tr> + <td class="text_lf">OA (<span class="smcap2">N</span>) 11677</td> + <td class="brdlf">9.2</td> + <td class="brdlf">0.20</td> + <td class="brdlf">3.03</td> + <td class="brdlf">72</td> + <td class="brdlf">‥</td> + <td class="brdlf">‥</td> + <td class="brdlf">0.008</td> + <td class="brdlf">I</td> +</tr> +<tr> + <td class="text_lf">Alpha Centauri</td> + <td class="brdlf">0.3</td> + <td class="brdlf">0.76</td> + <td class="brdlf">3.66</td> + <td class="brdlf">23</td> + <td class="brdlf">-22</td> + <td class="brdlf">G,K5</td> + <td class="brdlf text_lf" style="padding:0px;"> + <table summary="braced"> + <tr> + <td rowspan="2" style="padding:0px; font-size:2em;">{</td> + <td style="padding:0px;">2.0</td> + </tr> + <tr> + <td>0.6</td> + </tr> + </table> + </td> + <td class="brdlf">I</td> +</tr> +<tr> + <td class="text_lf">OA (<span class="smcap2">N</span>) 17415</td> + <td class="brdlf">9.3</td> + <td class="brdlf">0.27</td> + <td class="brdlf">1.31</td> + <td class="brdlf">23</td> + <td class="brdlf">‥</td> + <td class="brdlf">F</td> + <td class="brdlf">0.004</td> + <td class="brdlf">II</td> +</tr> +<tr> + <td class="text_lf">Pos. Med. 2164</td> + <td class="brdlf">8.8</td> + <td class="brdlf">0.29</td> + <td class="brdlf">2.28</td> + <td class="brdlf">37</td> + <td class="brdlf">‥</td> + <td class="brdlf">K</td> + <td class="brdlf">0.006</td> + <td class="brdlf">I</td> +</tr> +<tr> + <td class="text_lf">Sigma Draco</td> + <td class="brdlf">4.8</td> + <td class="brdlf">0.20</td> + <td class="brdlf">1.84</td> + <td class="brdlf">43</td> + <td class="brdlf">+25</td> + <td class="brdlf">K</td> + <td class="brdlf">0.5</td> + <td class="brdlf">II</td> +</tr> +<tr> + <td class="text_lf">Alpha Aquilæ</td> + <td class="brdlf">0.9</td> + <td class="brdlf">0.24</td> + <td class="brdlf">0.65</td> + <td class="brdlf">13</td> + <td class="brdlf">-33</td> + <td class="brdlf">A5</td> + <td class="brdlf">12.3</td> + <td class="brdlf">I</td> +</tr> +<tr> + <td class="text_lf">61 Cygni</td> + <td class="brdlf">5.6</td> + <td class="brdlf">0.31</td> + <td class="brdlf">5.25</td> + <td class="brdlf">80</td> + <td class="brdlf">-39</td> + <td class="brdlf">K5</td> + <td class="brdlf">0.10</td> + <td class="brdlf">I</td> +</tr> +<tr> + <td class="text_lf">Epsilon Indi</td> + <td class="brdlf">4.7</td> + <td class="brdlf">0.28</td> + <td class="brdlf">4.67</td> + <td class="brdlf">79</td> + <td class="brdlf">-62</td> + <td class="brdlf">K5</td> + <td class="brdlf">0.25</td> + <td class="brdlf">I</td> +</tr> +<tr> + <td class="text_lf">Krüger 60</td> + <td class="brdlf">9.2</td> + <td class="brdlf">0.26</td> + <td class="brdlf">0.92</td> + <td class="brdlf">17</td> + <td class="brdlf">‥</td> + <td class="brdlf">‥</td> + <td class="brdlf">0.005</td> + <td class="brdlf">II</td> +</tr> +<tr> + <td class="text_lf">Lacaille 9352</td> + <td class="brdlf">7.4</td> + <td class="brdlf">0.29</td> + <td class="brdlf">7.02</td> + <td class="brdlf">115</td> + <td class="brdlf">+12</td> + <td class="brdlf">Ma</td> + <td class="brdlf">0.019</td> + <td class="brdlf">I</td> +</tr> +<tr> + <td class="brdbt"></td> + <td class="brdbt brdlf"></td> + <td class="brdbt brdlf"></td> + <td class="brdbt brdlf"></td> + <td class="brdbt brdlf"></td> + <td class="brdbt brdlf"></td> + <td class="brdbt brdlf"></td> + <td class="brdbt brdlf"></td> + <td class="brdbt brdlf"></td> +</tr> +</table> + +<p><span class="pagenum"><a name="Page_p320" id="Page_p320">[320]</a></span> +These stars are distant less than five parsecs +(about 16 light-years) from the sun, so they make +up the closest fringe of the stellar universe immediately +surrounding our system. The large number +of binary systems is quite remarkable. Why +some stars are single and others double is not yet +known. By the spectroscopic method the proportion +is not so large; Campbell finding that about +one quarter of 1,600 stars examined are spectroscopic +binaries, and Frost two-fifths to a half. The +exceptional number of large velocities is very remarkable; +the average transverse motion of the +nineteen stars is fifty kilometers per second, whereas +thirty is about what would have been expected.</p> + +<p>As to star streams to which these nearest stars +belong, eleven are in Stream I and eight in Stream +II, in close accord with the ratio 3:2 given by the +6,000 stars of Boss's catalogue. "We are not able," +says Eddington, "to detect any significant difference +between the luminosities, spectra, or speeds of the +stars constituting the two streams. The thorough +interpenetration of the two star streams is well +illustrated, since we find even in this small volume +of space that members of both streams are mingled +together in just about the average proportion."</p> + +<p><span class="pagenum"><a name="Page_p320p1" id="Page_p320p1">[320i]</a></span></p> + +<div class="fig_center" style="width: 436px;"> +<img src="images/p320_1.png" width="436" height="584" alt="" /> +<div class="fig_caption"><span class="smcap">The Ring Nebula in</span> <i>Lyra</i>. This is the best example of the annular and +elliptic nebulæ, which are not very abundant. (<i>Photo, Mt. Wilson Solar +Observatory.</i>)</div> +</div> + +<p><span class="pagenum"><a name="Page_p320p2" id="Page_p320p2">[321i]</a></span></p> + +<div class="fig_center" style="width: 644px;"> +<img src="images/p320_2.png" width="644" height="485" alt="" /> +<div class="fig_caption"><span class="smcap">The Dumb-bell Nebula of</span> <i>Vulpecula</i>. To take the photograph required an exposure of five +hours. (<i>Photo, Mt. Wilson Solar Observatory.</i>)</div> +</div> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p321" id="Page_p321">[321]</a></span></p> + +<h2><a name="CHAPTER_L" id="CHAPTER_L"></a>CHAPTER L<br /> +<br /> +ACTUAL DIMENSIONS OF THE STARS</h2> + +<p>We have seen that the distances of the stars +from the solar system are immense beyond +conception, and millions upon millions of them are +probably forever beyond our power of ascertaining +by direct measurement what their distance really +is. After we had found the sun's distance and +measured the angle filled by his disk, it was easy +to calculate his actual size. This direct method, +however, fails when we try to apply it to the stars, +because their distances are so vast that no star's +disk fills an angle of any appreciable size; and even +if we try to get a disk with the highest magnifying +powers of a great telescope our efforts end only in +failure. There is, indeed, no instrumentally appreciable +angle to measure.</p> + +<p>How then shall we ascertain the actual dimensions +of the vast spheres which we know the stars +actually are, as they exist in the remotest regions +of space? Clearly by indirect methods only, and it +must be said that astronomers have as yet no general +method that yields very satisfactory results for +stellar dimensions. The actual magnitude of the +variable system of Algol, Beta Persei, is among the +best known of all the stars, because the spectroscope +measures the rate of approach and recession +of Algol when its invisible satellite is in opposite +parts of the orbit; the law of gravitation gives the +<span class="pagenum"><a name="Page_p322" id="Page_p322">[322]</a></span> +mass of the star and the size of its orbit, and so +the length of the eclipse gives the actual size of +the dark, eclipsing body. This figures out to be +practically the same size as that of our sun, while +Algol's own diameter is rather larger, exceeding a +million miles.</p> + +<p>If we try to estimate sizes of stars by their brightness +merely, we are soon astray. Differences of +brightness are due to difference of dimensions, of +course, or of light-giving area; but differences of +distance also affect the brightness, inversely as the +squares of the distances, while differences of temperature +and constitution affect, in very marked +degree, the intrinsic brilliance of the light-emitting +surface of the star. There are big stars and little +stars, stars relatively near to us and stars exceedingly +remote, and stars highly incandescent as well +as others feebly glowing.</p> + +<p>We have already shown how the angular diameters +subtended by many of the stars have been +estimated, through the relation of surface brightness +and spectral type. Antares and Betelgeuse +appear to be the most inviting for investigation, because +their estimated angular diameters are about +one-twentieth of a second of arc. This is the +way in which their direct measurement is being +attempted.</p> + +<p>As early as 1890, Michelson of Chicago suggested +the application of interference methods to the accurate +measurement of very small angles, such as +the diameters of the minor planets, and the satellites +of Jupiter and Saturn, as well as the arc distance +between the components of double stars. Two +portions of the object glass are used, as far apart +as possible on the same diameter, and the interference +<span class="pagenum"><a name="Page_p323" id="Page_p323">[323]</a></span> +fringes produced at the focus of the objective +are then the subject of observation. These +fringes form a series of equidistant interference +bands, and are most distinct when the light comes +from a source subtending an infinitesimal angle. +If the object presents an appreciable angle, the visibility +is less and may even become zero.</p> + +<p>Michelson tested this method on the satellites of +Jupiter at the Lick Observatory in 1891, and +showed its accuracy and practicability. Nevertheless, +the method has not been taken up by astronomers, +until very recently at the Mount Wilson +Observatory, where Anderson has applied it to the +measurement of close double stars. It is found that, +contrary to general expectation, the method gives +excellent results, even if the "seeing" is not the best—2 +on a scale of 10, for instance.</p> + +<p>To simplify the manipulation of the interferometer, +a small plate with two apertures in it is +placed in the converging beam of light coming from +the telescope objective or mirror. The interference +fringes formed in the focal plane are then viewed +with an eyepiece of very high power, many thousand +diameters. The resolving power of the interferometer +is found to be somewhat more than +double that of a telescope of the same aperture. By +applying the interferometer method to Capella, arc +distances of much less than one-twentieth of a +second of arc were measured. More recently the +method has been applied to the great star Betelgeuse +in Orion, whose angular diameter was found to be +0".46, corresponding to an actual diameter of 260,000,000 +miles, if the star's parallax is as small as it +appears to be.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p324" id="Page_p324">[324]</a></span></p> + +<h2><a name="CHAPTER_LI" id="CHAPTER_LI"></a>CHAPTER LI<br /> +<br /> +THE VARIABLE STARS</h2> + +<p>Spectacular as they are to the layman, novæ, +or temporary stars, are to the astronomers +simply a class among many thousands of stars +which they call variables, or variable stars. There +are a few objects classified as irregular variables, +one of which is very remarkable. We refer to Eta +Argus, an erratic variable in the southern constellation +Argo and surrounded by a well-known +nebula. There is a pretty complete record of this +star. Halley in 1677 when observing at Saint +Helena recorded Eta Argus as of the fourth magnitude. +During the 18th century, it fluctuated between +the fourth magnitude and the second. Early +in the 19th it rapidly waxed in brightness, fluctuating +between the first and second magnitudes +from 1822 to 1836. But two years later its light +tripled, rivaling all the fixed stars except Canopus +and Sirius. In 1843 it was even brighter for a few +months, but since then it has declined fairly steadily, +reaching a minimum at magnitude seven and a +half in 1886, with a slight increase in brightness +more recently. A period of half a century has been +suggested, but it is very doubtful if Eta Argus has +any regular period of variation.</p> + +<p>Another very interesting class of variables is +known as the Omicron Ceti type. Nearly all the +time they are very faint, but quite suddenly they +<span class="pagenum"><a name="Page_p325" id="Page_p325">[325]</a></span> +brighten through several magnitudes, and then fade +away, more or less slowly, to their normal condition +of faintness. But the extraordinary thing is +that most of these variables go through their fluctuations +in regular periods: from six months to two +years in length. The type star, Omicron Ceti, or +Mira, is the oldest known variable, having been +discovered by Fabricius in 1596. Most of the time +it is a relatively faint star of the 12th magnitude; +but once in rather less than a year its brightness +runs up to the fourth, third and sometimes even +the second magnitude, where it remains for a week +or ten days, and afterward it recedes more slowly +to its usual faintness, the entire rise and decline +in brightness usually requiring about 100 days. The +spectrum of Omicron Ceti contains many very +bright lines, and a large proportion of the variable +stars are of this type.</p> + +<p>Another class of variables is designated as the +Beta Lyræ type. Their periods are quite regular, +but there are two or more maxima and minima of +light in each period, as if the variation were caused +by superposed relations in some way. Their spectra +show a complexity of helium and hydrogen bands. +No wholly satisfactory explanation has yet been +offered. Probably they are double stars revolving +in very small orbits compared with their dimensions, +their plane of motion passing nearly through +the earth.</p> + +<p>But the most interesting of all the variables are +those of the Algol type, their light curves being +just the reverse of the Omicron Ceti type; that is, +they are at their maximum brightness most of the +time, and then suffer a partial eclipse for a relatively +brief interval. Algol goes through its variations +<span class="pagenum"><a name="Page_p326" id="Page_p326">[326]</a></span> +so frequently that its period is very accurately +known; it is 2d. 20h. 48m. 55.4s. For most of this +period Algol is an easy second magnitude star; then +in about four and a half hours it loses nearly five-sixths +of its light, receding to the fourth magnitude. +Here at minimum it remains for fifteen or +twenty minutes, and then in the next three and a +half hours it regains its full normal brilliancy of +the second magnitude. During these fluctuations +the star's spectrum undergoes no marked changes. +The spectra of all the Algol variables are of the +first or Sirian type.</p> + +<p>To explain the variation of the Algol type of +variables is easy: a dark, eclipsing body, somewhat +smaller than the primary is supposed to be traveling +round it in an orbit lying nearly edgewise to +our line of sight. The gravitation of this dark companion +displaces Algol itself alternately toward and +from the earth, because the two bodies revolve +round their common center of gravity. With the +spectroscope this alternate motion of Algol, now +advancing and now receding at the rate of 26 miles +per second, has been demonstrated; and the period +of this motion synchronizes exactly with the period +of the star's variability.</p> + +<p>Russell and Shapley have made extended studies +of the eclipsing binaries, and developed the formulæ +by which the investigations of their orbits +are conducted. Heretofore, visual binaries and +spectroscopic binaries afforded the only means of +deriving data regarding double systems, but it is +now possible to obtain from the orbits of eclipsing +variables fully as much information relating to +binary systems in general and their bearing on +stellar evolution. After an orbit has been determined +<span class="pagenum"><a name="Page_p327" id="Page_p327">[327]</a></span> +from the photometric data of the light curve, +the addition of spectroscopic data often permits the +calculation of the masses, dimensions and densities +in terms of the sun. Shapley's original investigation +included the orbits of ninety eclipsing variables, +and with the aid of hypothetical parallaxes, +he computed the approximate position of each +system in space. The relation to the Milky Way is +interesting, the condensation into the Galactic +plane being very marked; only thirteen of the +ninety systems being found at Galactic latitudes +exceeding 30 degrees.</p> + +<p>If we can suppose the variable stars covered with +vast areas of spots, perhaps similar to the spots on +the sun, and then combine the variation of these +spot areas with rotation of the star on its axis, +there is a possibility of explanation of many of +the observed phenomena, especially where the +range of variation is small. But for the Omicron +Ceti type, no better explanation offers than that +afforded by Sir Norman Lockyer's collision theory. +First he assumes that these stars are not condensed +bodies, but still in the condition of meteoric +swarms, and the revolution of lesser swarms +around larger aggregations, in elliptic orbits of +greater or less eccentricity, must produce vast +multitudes of collisions; and these collisions, taking +place at pretty regular periods, produce the variable +maximum light by raising hosts of meteoric +particles to a state of incandescence simultaneously.</p> + +<p>The catalogues of variable stars now contain +many thousands of these objects. They are often +designated by the letters R, S, T, and so on, followed +by the genitive form of the name of the constellation +wherein they are found. Most of the recently +<span class="pagenum"><a name="Page_p328" id="Page_p328">[328]</a></span> +found variables have a range of less than +one magnitude. They are so distributed as to be +most numerous in a zone inclined about 18 degrees +to the celestial equator, and split in two near where +the cleft in the Galaxy is located. Nearly all the +temporary stars are in this duplex region. Bailey +of Harvard a quarter century ago began the investigation +of variables in close star clusters, where +they are very abundant, with marked changes of +magnitude within only a few hours.</p> + +<p>Many amateur astronomers afford very great +assistance to the professional investigator of variable +stars by their cooperation in observing these +interesting bodies, in particular the American Association +of Observers of Variable Stars, organized +and directed by William Tyler Olcott.</p> + +<p>For a high degree of accuracy in determining +stellar magnitudes the photo-electric cell is unsurpassed. +Stebbins of Urbana has been very successful +in its application and he discovered the secondary +minimum of Algol with the selenium cell. His +most recent work was done with a potassium cell +with walls of fused quartz, perfected after many +trial attempts. The stars he has recently investigated +are Lambda Tauri, and Pi Five Orionis. Combining +results with those reached by the spectroscope, +the masses of the two component stars of +the former are 2.5 and 1.0 that of the sun, and +the radii are 4.8 and 3.6 times the sun's.</p> + +<p>Russell of Princeton thinks it probable that similar +causes are at work in all these variables. In the +case of the typical Novæ there is evidence that +when the outburst takes place a shell of incandescent +gas is actually ejected by the star at a very +high velocity. What may be the forces that cause +<span class="pagenum"><a name="Page_p329" id="Page_p329">[329]</a></span> +such an explosion can only be guessed. Repeated +outbursts have not, in the case of T Pyxidis, destroyed +the star, because it has gone through this +process three times in the past thirty years. Russell +inclines to regard it as a standard process +occurring somewhere in the stellar universe probably +as often as once a year.</p> + +<p>Novæ, then, cannot be due to collisions between +two stars, for even if we suppose the stars to be +a thousand millions in number, no two should collide +except at average intervals of many million +years. The idea is gaining ground that the stars +are vast storehouses of energy which they are +gradually transforming into heat and radiating +into space. "Under ordinary circumstances, it is +probable that the rate of generation of heat is +automatically regulated to balance the loss by radiation. +But it is quite conceivable that some sudden +disturbance in the substance of the star, near the +surface, might cause an abrupt liberation of a +great amount of energy, sufficient to heat the surface +excessively, and drive the hot material off +into infinite space, in much the form of a shell of +gas, as seems to have been observed in the case of +Nova Aquilæ…. With the rapid advance of our +knowledge of the properties of the stars on one +hand, and of the very nuclei of atoms on the other, +we may, perhaps before many years have passed, +find ourselves nearer a solution of the problem."</p> + +<p>The Cepheid variables increase very rapidly in +brightness from their least light to their maximum, +and then fade out much more slowly, with certain +irregularities or roughnesses of their light-curves +when declining. Their spectral lines also shift in +period with their variations of light. In the case +<span class="pagenum"><a name="Page_p330" id="Page_p330">[330]</a></span> +of these variables, whose regular fluctuation of +light cannot be due to eclipse, and is as a rule +embraced within a few days, there is a fluctuation +in color also between maximum and minimum, as +if there were a periodic change in the star's physical +condition. Eddington and Shapley advocate the +theory of a mechanical pulsation of the star as most +plausible. Knowledge of the internal conditions of +the stars make it possible to predict the period of +pulsation within narrow limits; and for Delta +Cephei this theoretical period is between four and +ten days. Its observed period is five and one-third +days, and corresponding agreement is found in +all the Cepheids so far tested.</p> + +<p>Shapley of Mount Wilson finds that the Cepheid +variables with periods exceeding a day in length +all lie close to the Galactic lane. So greatly have +the studies of these objects progressed that, as before +remarked, when we know the star's period, +we can get its absolute magnitude, and from this +the star's distance. On all sides of the sun, the distances +of the Cepheids range up to 4,000 parsecs. +So they indicate the existence of a Galactic system +far greater in extent than any previously dealt +with.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p331" id="Page_p331">[331]</a></span></p> + +<h2><a name="CHAPTER_LII" id="CHAPTER_LII"></a>CHAPTER LII<br /> +<br /> +THE NOVÆ, OR NEW STARS</h2> + +<p>New stars, or temporary stars, we have already +mentioned in connection with variables. They +are, next to comets, the most dramatic objects in +the heavens. They may be variable stars which, +in a brief period, increase enormously in brightness, +and then slowly wane and disappear entirely, +or remain of a very faint stellar magnitude.</p> + +<p>In the ancient historical records are found +accounts of several such stars. For instance, in +the Chinese annals there is an allusion to such a +stellar outburst in the constellation of Scorpio, +<span class="smcap2">B. C.</span> 134. This was observed also by Hipparchus +and, no doubt, it was the immediate incentive which +led to his construction of the first known catalogue +of stars, so that similar happenings might be detected +in the future. In November, 1572, Tycho +Brahe observed the most famous of all new stars, +which blazed out in the constellation Cassiopeia. +In something over a year it had completely disappeared.</p> + +<p>In 1604-1605 a new star of equal brightness was +seen in Ophiuchus by Kepler; it also faded out to +invisibility in 1606. Kepler and Tycho printed +very complete records of these remarkable objects. +The eighteenth century passed without any new +stars being seen or recorded. There was one of the +fifth magnitude in 1848, and another of the seventh +<span class="pagenum"><a name="Page_p332" id="Page_p332">[332]</a></span> +magnitude in 1860; and in May, 1866, a star of the +second magnitude suddenly made its appearance +in Corona Borealis; and one of the third magnitude +in Cygnus in November, 1876. The latter +was fully observed by Schmidt of Athens and +became a faint telescopic star within a few weeks. +It is now of the fifteenth magnitude.</p> + +<p>In 1885 astronomers were surprised to find suddenly +a new star of the sixth magnitude very close +to the brightest part of the great nebula in Andromeda; +it ran its course in about six months, +fading with many fluctuations in brightness, and +no star is now visible in its position even with +the telescope. Stars of this class are known to +astronomers as Novæ, usually with the genitive of +the constellation name, as Nova Andromedæ.</p> + +<p>In 1891-1892 Nova Aurigæ made its spectacular +appearance and yielded a distinctly double and +complex spectrum for more than a month. Many +pairs of lines indicated a community of origin as +to substance, and accurate measurement showed +a large displacement with a relative velocity of +more than 500 miles per second. For each bright +hydrogen line displaced toward the red there was +a dark companion line or band about equally displaced +toward the violet much as if the weird light +of Nova Aurigæ originated in a solid globe moving +swiftly away from us and plunging into an irregular +nebulous mass as swiftly approaching us. +Parallax observations of Nova Aurigæ made it immensely +remote, perhaps within the Galaxy, and it +still exists as a faint nebulous star.</p> + +<p>In February, 1901, in the constellation Perseus +appeared the most brilliant nova of recent years. +It was first discovered by Dr. Anderson, an +<span class="pagenum"><a name="Page_p333" id="Page_p333">[333]</a></span> +amateur of Glasgow, and at maximum on February +23 it outshone Capella. There were many unusual +fluctuations in its waning brightness. Its +spectrum closely resembled that of Nova Aurigæ, +with calcium, helium, and hydrogen lines. In +August, 1901, an enveloping nebula was discovered, +and a month later certain wisps of this nebulosity +appeared to have moved bodily, at a speed seventy-fold +greater than ever previously observed in the +stellar universe.</p> + +<p>According to Sir Norman Lockyer's meteoritic +hypothesis, a vast nebulous region was invaded, not +by one but by many meteor swarms, under conditions +such that the effects of collision varied greatly +in intensity. The most violent of these collisions +gave birth to Nova Persei itself, and the least +violent occurred subsequently in other parts of the +disturbed nebula, perhaps immeasurably removed. +This explanation would avoid the necessity of supposing +actual motion of matter through space at +velocities heretofore unobserved and inconceivably +high. A recent photograph of Nova Persei, by +Ritchey, reveals a nebulous ring of regular structure +surrounding the star. The great power of the +60-inch has made it possible to photograph even the +spectra of many of the novæ of years ago which are +now very faint. After the lapse of years the characteristic +lines of the nebular spectrum generally +vanish, as if the star had passed out of the nebula—a +plunge into which is generally thought to be the +cause of the great and sudden outburst of light.</p> + +<p>Many novæ have recently been found in the +spiral nebulæ, especially in the great nebula of +Andromeda.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p334" id="Page_p334">[334]</a></span></p> + + +<h2><a name="CHAPTER_LIII" id="CHAPTER_LIII"></a>CHAPTER LIII<br /> +<br /> +THE DOUBLE STARS</h2> + +<p>Examining individual stars of the heavens more +in detail, thousands of them are found to be +double; not the stars that appear double to the naked +eye, as Theta Tauri, Mizar, Epsilon Lyræ, and others; +but pairs of stars much closer together, and requiring +the power of the telescope to divide or separate +them. Only a very few seconds apart they are or, +in many cases, only the merest fraction of a second +of arc. Some of them, called binaries, are found to +be revolving around a common center, sometimes in +only a few years, sometimes in stately periods of +hundreds of years. Many such binary systems are +now known, and the number is constantly increasing. +Castor is one, Gamma Virginis another, +Sirius also is one of these binaries, and a most +interesting one, having a period of revolution of +about 52 years.</p> + +<p>Aitken, of the Lick Observatory, in his work on +binary stars, directs special attention to the correlation +between the elements of known binary +orbits and the star's spectral type, and presents +a statistical study of the distribution of 54,000 +visual double stars, of which the spectra of 3919 +are known. That the masses of binary systems +average about twice that of the sun's mass has long +been known, and this fact can be employed with +confidence in estimates of the probable parallax of +<span class="pagenum"><a name="Page_p335" id="Page_p335">[335]</a></span> +these systems. Aitken applies the test to fourteen +visual systems for which the necessary data are +available, and deduces for them a mean mass of +1.76 times that of the sun. For the spectroscopic +binaries the masses are much greater.</p> + +<p>Triple, quadruple and multiple stars are less +frequent; but many exceedingly interesting objects +of this class exist. Epsilon Lyræ is one, a double-double, +or four stars as seen with slender telescopic +power, and six or seven stars with larger instruments. +Sigma Orionis and 12 Lyncis, also Theta +Cancri and Mu Bootis are good examples of triple +stars.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p336" id="Page_p336">[336]</a></span></p> + +<h2><a name="CHAPTER_LIV" id="CHAPTER_LIV"></a>CHAPTER LIV<br /> +<br /> +THE STAR CLUSTERS</h2> + +<p>From multiple stars the transition is natural to +star clusters although the gap between these types +of stellar objects is very broad. The familiar group +of the winter sky known as the Pleiades is a loose +cluster, showing relatively very few stars even in +telescopes or on photographic plates. The "Beehive," +or cluster known as Praesepe in Cancer, and +a double group in the sword-handle of Perseus, both +just visible to the naked eye, are excellent examples +of star clusters of the average type. When the moon +is absent, they are easily recognized without a telescope +as little patches of nebulous light; but every +increase of optical power adds to their magnificence.</p> + +<p>Then we come in regular succession to the truly +marvelous globular clusters, that for instance in +Hercules. Messier 13, a recent photograph of +which, taken by Ritchey with the 60-inch reflector +on Mount Wilson, reveals an aggregation of more +than 50,000 stars. But the finest specimens are in +the southern hemisphere. Sir John Herschel spent +much time investigating them nearly a century ago +at the Cape of Good Hope. His description of the +cluster in the constellation of Centaurus is as follows: +"The noble globular cluster Omega Centauri +is beyond all comparison the richest and largest object +of the kind in the heavens. The stars are literally +innumerable, and as their total light when received +<span class="pagenum"><a name="Page_p337" id="Page_p337">[337]</a></span> +by the naked eye affects it hardly more than +a star of the fifth or fourth to fifth magnitude, the +minuteness of each star may be imagined."</p> + +<p>Others of these clusters are so remote that the +separate stars are not distinguishable, especially at +the center, and their distances are entirely beyond +our present powers of direct measurement, although +methods of estimating them are in process of development. +If gravitation is regnant among the +uncounted components of stellar clusters, as doubtless +it is, these stars must be in rapid motion, although +our photographs of measurements have been +made too recently for us to detect even the slightest +motion in any of the component stars of a cluster. +The only variations are changes of apparent magnitude, +of a type first detected in a large number of +stars in Omega Centauri, by Bailey of Harvard, who +by comparison of photographs of the globular clusters +was the first to find variable stars quite numerous +in these objects. Their unexplained variations +of magnitude take place with great rapidity, +often within a few hours.</p> + +<p>There are about a hundred of these globular clusters, +and the radial velocities of ten of them have +been measured by Slipher and found to range from a +recession of 410 to an approach of 225 kilometers +per second. These excessive velocities are comparable +with those found for the spiral nebulæ. Shapley +has estimated the distances of many of these bodies, +which contain a large number of variable stars of +the Cepheid type. By assuming their absolute magnitudes +equal to those of similar Cepheids at known +distances, he finds their distance represented by the +inconceivably minute parallax of 0".00012, corresponding +to 30,000 light-years. This research also +<span class="pagenum"><a name="Page_p338" id="Page_p338">[338]</a></span> +places the globular clusters far outside and independent +of our Galactic system of stars. The distribution +of the globular clusters has also been investigated, +and these interesting objects are found +almost exclusively in but one hemisphere of the sky. +Its center lies in the rich star clouds of Scorpio and +Sagittarius. Success in finding the distances of +these objects has made it possible to form a general +idea of their distribution in three-dimensional space.</p> + +<p>The numerous variable stars in any one cluster +are remarkable for their uniformity. Accepting +variables of this type as a constant standard of +absolute brightness, and assuming that the differences +of average magnitude of the variables in different +clusters are entirely due to differences of +distance, the relative distances of many clusters were +ascertained with considerable accuracy. Then it was +found that the average absolute magnitude of the +twenty-five brightest stars in a cluster is also a uniform +standard, or about 1.3 magnitudes brighter +than the mean magnitude of the variables. This +new standard was employed in ascertaining the distances +of other clusters not containing many variables.</p> + +<p>Shapley further shows that the linear dimensions +of the clusters are nearly uniform, and the proper +relative positions in space are charted for sixty-nine +of these objects. We can determine the scale of the +charts, if we know the absolute brightness of our +primary standard—the variable stars; and this is deduced +from a knowledge of the distances of variables +of the same type in our immediate stellar system.</p> + +<p>The most striking of all the globular clusters, +Omega Centauri, comes out the nearest; nevertheless +it is distant 6.5 kiloparsecs. A kiloparsec is a +<span class="pagenum"><a name="Page_p339" id="Page_p339">[339]</a></span> +thousand parsecs, and is the equivalent of 3,256 light-years. +At the inconceivable distance of sixty-seven +kiloparsecs, or more than 200,000 light-years, is the +most remote of the globular clusters, known to astronomers +as N.G.C. 7006, from its number in the +catalogue which records its position in the sky, the +New General Catalogue of nebulæ by Dreyer of +Armagh.</p> + +<p>The clusters are widely scattered, and their center +of diffusion is about twenty kiloparsecs on the +Galactic plane toward the region of Scorpio-Sagittarius. +Marked symmetry with reference to this +plane makes it evident that the entire system of +globular clusters is associated with the Galaxy itself. +But to conceive of this it is necessary to extend our +ideas of the actual dimensions of the Galactic system. +Almost on the circumference of the great +system of globular clusters our local stellar system +is found, and it contains probably all the naked-eye +stars, with millions of fainter ones. Its size seems +almost diminutive, only about one kiloparsec in +diameter. The relative location of our local stellar +system shows why the globular clusters appear to +be crowded into one hemisphere only.</p> + +<p>Shapley suggests that globular clusters can exist +only in empty space, and that when they enter the +regions of space tenanted by stars, they dissolve into +the well-known loose clusters and the star clouds of +the Milky Way. Strangely the radial velocities of +the clusters already observed show that most of them +are traveling toward this region, and that some will +enter the stellar regions within a period of the order +of a hundred million years.</p> + +<p>The actual dimensions of globular clusters are +not easy to determine, because the outer stars are +<span class="pagenum"><a name="Page_p340" id="Page_p340">[340]</a></span> +much scattered. To a typical cluster, Messier 3, +Shapley assigns a diameter of 150 parsecs, which +makes it comparable with the size of the stellar cluster +to which the sun belongs. Also on certain likely +assumptions, he finds that the diameter of the great +cluster in Hercules, the finest one in our northern +sky, is about 350 parsecs, and its distance no less +than 30,000 parsecs; in other words, the staggering +distance that light would require 9,750,000 years +to travel over. While these distances can never be +verified by direct measurement, it lends great weight +to the three methods of indirect measurement, or +estimation, (1) from the diameter of the image of +the clusters, (2) from the mean magnitude of the +twenty-five brightest stars, and (3) from the mean +magnitude of the short period variables, that they +are in excellent agreement.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p341" id="Page_p341">[341]</a></span></p> + +<h2><a name="CHAPTER_LV" id="CHAPTER_LV"></a>CHAPTER LV<br /> +<br /> +MOVING CLUSTERS</h2> + +<p>Recent researches on the proper motions of stars +have brought to light many groups of stars whose +individual members have equal and parallel velocities. +Eddington calls these moving clusters. The +component stars are not exceptionally near to each +other, and it often happens that other stars not belonging +to the group are actually interspersed among +them. They may be likened to double stars which +are permanent neighbors, with some orbital motion, +though exceedingly slow.</p> + +<p>The connection is rather one of origin; occurring +in the same region of space, perhaps, from a single +nebula. They set out with the same motion, and +have "shared all the accidents of the journey together." +Their equality of motion is intact because +any possible deflections by the gravitative pull of the +stellar system is the same for both. Mutual attraction +may tend to keep the stars together, but +their community of motion persists chiefly because +no forces tend to interfere with it. In this way +physically connected pairs may be separated by very +great distances.</p> + +<p>So with the moving clusters: their component +stars may be widely separate on the celestial sphere, +but equality of their motions affords a clue to their +association in groups. The Hyades, a loose cluster +in Taurus, is a group of thirty-nine stars, within +<span class="pagenum"><a name="Page_p342" id="Page_p342">[342]</a></span> +an area of about 15 degrees square, which has +been pretty fully investigated, especially by the late +Professor Lewis Boss; and no doubt many fainter +stars in the same region will ultimately be found to +belong to the same group.</p> + +<p>If we draw arrows on a chart representing the +amount and direction of the proper motions of these +stars, these arrows must all converge toward a point. +This shows that their motions are parallel in space. +It is a relatively compact group, and the close convergence +shows that their individual velocities must +agree within a small fraction of a kilometer per +second. Radial velocity measures of six of the component +stars are in very satisfactory accord, giving +45.6 kilometers per second for the entire group.</p> + +<p>We can get the transverse velocity, and therefrom +the distances of the stars, which are among the +best known in the heavens, because the proper +motions are very accurately known. The mean +parallax of the group by this indirect method comes +out 0".025, agreeing almost exactly with the direct +determination by photography, 0".023, by Kapteyn, +De Sitter, and others.</p> + +<p>Eddington concludes that this Taurus group is a +globular cluster with a slight central condensation. +Its entire diameter is about ten parsecs, and its +known motion enables us to trace its past and +future history. It was nearest the sun 800,000 +years ago, when it was at about half its present +distance. Boss calculated that in 65 million years, +if the present motion is maintained, this group +will have receded so far as to appear like an ordinary +globular cluster 20' in diameter, its stars +ranging from the ninth to the twelfth apparent magnitude. +We may infer that the motion will likely +<span class="pagenum"><a name="Page_p343" id="Page_p343">[343]</a></span> +continue undisturbed, because there are interspersed +among the group many stars not belonging to it, and +these have neither scattered its members nor sensibly +interfered with the parallelism of their motion.</p> + +<p>Another moving cluster, the similarity of proper +motion of whose component stars was first pointed +out by Proctor, is known as the Ursa Major system, +which embraces primarily Beta, Gamma, Delta, +Epsilon, and Zeta Ursæ Majoris, or five of the seven +stars that mark the familiar Dipper. But as many +as eight other stars widely scattered are thought to +belong to the same system, including Sirius and +Alpha Coronæ Borealis. The absolute motion +amounts to 28.8 kilometers per second, and is approximately +parallel to the Galaxy. Turner has +made a model of the cluster, which has the form of +a flat disk.</p> + +<p>Among stars of the Orion type of spectrum are +several examples of moving clusters. The Pleiades +together with many fainter stars form another moving +cluster; as also do the brighter stars of Orion, +together with the faint cloudlike extensions of the +great nebula in Orion, whose radial velocity agrees +with that of the stars in the constellation. Still +another very remarkable moving cluster is in +Perseus, first detected by Eddington, and embracing +eighteen stars, the brightest of which is Alpha Persei.</p> + +<p>The further discovery of moving clusters is most +important in the future development of stellar astronomy, +because with their aid we can find out the +relative distribution, luminosity, and distance of +very remote stars. So far the stars found associated +in groups are of early types of spectrum; but the +Taurus cluster embraces several members equally +advanced in evolution with the sun, and in the more +<span class="pagenum"><a name="Page_p344" id="Page_p344">[344]</a></span> +scattered system of Ursæ Major there are three +stars of Type F.</p> + +<p>"Some of these systems," Eddington concludes, +"would thus appear to have existed for a time comparable +with the lifetime of an average star. They +are wandering through a part of space in which are +scattered stars not belonging to their system—interlopers +penetrating right among the cluster stars. +Nevertheless, the equality of motion has not been +seriously disturbed. It is scarcely possible to avoid +the conclusion that the chance attractions of stars +passing in the vicinity have no appreciable effect on +stellar motions; and that if the motions change in +course of time (as it appears they must do) this +change is due, not to the passage of individual stars, +but to the central attraction of the whole stellar universe, +which is sensibly constant over the volume of +space occupied by a moving cluster."</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p345" id="Page_p345">[345]</a></span></p> + +<h2><a name="CHAPTER_LVI" id="CHAPTER_LVI"></a>CHAPTER LVI<br /> +<br /> +THE TWO STAR STREAMS</h2> + +<p>Consider the ships on the Atlantic voyaging between +Europe and America: at any one time +there may be a hundred or more, all bound either +east or west, some moving in interpenetrating +groups, individuals frequently passing each other, +but rarely or never colliding. We might say, there +are two great streams of ships, one moving east and +the other west.</p> + +<p>Now in place of each ship, imagine a hundred +ships, and magnify their distances from each other +to the vast distances that the stars are from each +other, and all in motion in two great streams as +before. This will convey some idea of the relatively +recent discovery, called by astronomers "star-streaming."</p> + +<p>Early in this century the investigation of moving +clusters began to reveal the fact that the motions +of the stars were not at random throughout the +universe, and about 1904 Kapteyn was the first to +show that the stellar motions considered in great +groups are very far from being haphazard, but that +the stars tend to travel in two great streams, or +favored directions. This was ascertained by analyzing +the proper motions of stars in the sky, many +thousands of them, and correcting all for the effect +which the known motion of the sun would have upon +them. The corrected motion, or part that is left +<span class="pagenum"><a name="Page_p346" id="Page_p346">[346]</a></span> +over, is known as the star's own motion, or <i>motus +peculiaris</i>.</p> + +<p>This important investigation was very greatly +facilitated by the general catalogue of 6,188 stars +well distributed over the entire sky, the work of the +late Professor Boss. It was published by the Carnegie +Institution of Washington, and includes all stars +down to the sixth magnitude. Boss was very critical +in the matter of stellar positions and proper +motions and his work is the most accurate at present +available. Excluding stars of the Orion type and +the known members of moving clusters, Kapteyn's +investigation was based on 5,322 stars, which he divided +into seventeen regions of the sky, each northern +region having an antipodal one in the southern +hemisphere.</p> + +<p>Mathematical analysis of these regions showed +them all in substantial agreement, with one exception, +and enabled Kapteyn to draw the conclusion +that the stars of one stream, called Drift I, move +with a speed of thirty-two kilometers per second, +while those of the other, Drift II, travel with a speed +of eighteen kilometers per second. Their directions +are not, like those of east and west bound ships, 180 +degrees from each other, but are inclined at an +angle of 100 degrees. Drift I embraces about three-fifths +of the stars, and Drift II the remaining two-fifths. +Quite as remarkable as the drifts themselves +is the fact that the relative motion of the two is very +closely parallel to the plane of the Milky Way.</p> + +<p>This epochal research has very great significance +in all investigations of stellar motions, and it has +been verified in various ways, particularly by the +Astronomer Royal, Sir Frank Dyson, who limited the +stars under consideration to 1,924 in number, but +<span class="pagenum"><a name="Page_p347" id="Page_p347">[347]</a></span> +all having very large proper motions. In this way +the two streams are even more characteristically +marked. But radial velocity determinations afford +the ultimate and most satisfactory test, and Campbell +has this investigation in hand, classifying the +stars in their streaming according to the type.</p> + +<p>Type A stars are so far found to be confirmatory. +Turning to the question of physical differences between +the stars of the two streams, Eddington inquires +into the average magnitude of the stars in +both drifts, and their spectral type. Also whether +they are distributed at the same distance from the +sun, and in the same proportion in all parts of the +sky. His conclusion is that there is no important +difference in the magnitudes of the stars constituting +the two drifts. Regarding their spectra, stars +of early and late types are found in both streams, +with a somewhat higher proportion of late types +among the stars of Drift II than those of Drift I. +Campbell and Moore of the Lick Observatory have +investigated seventy-three planetary nebulæ which +exhibit the phenomena of star-streaming, and have +motions which are characteristic of the stars.</p> + +<p>Dealing with the very important question whether +the two streams are actually intermingled in space, +Eddington finds them nearly at the same mean distance +and thoroughly intermingled, and there is no +possible hypothesis of Drifts I and II passing one +behind the other in the same line of sight. A third +drift, to which all the Orion stars belong, is under +investigation, together with comprehensive analysis +of the drifts according to the spectral type of all the +stars included.</p> + +<p>The farther research on star-streaming is pushed, +the more it becomes evident that a third stream, +<span class="pagenum"><a name="Page_p348" id="Page_p348">[348]</a></span> +called Drift O, is necessary, especially to include B-type +stars. The farther we recede from the sun, the +more this drift is in evidence. At the average distances +of B-type stars, the observed motions are almost +completely represented by Drift O alone. Halm +of Cape Town concludes from recent investigations +that the double-drift phenomena (Drifts I and II) +is of a distinctly <i>local</i> character, and concerns chiefly +the stars in the vicinity of the solar system; while +stars at the greatest distances from the sun belong +preeminently to Drift O.</p> + +<p>The 60-inch reflector on Mount Wilson gathers +sufficient light so that the spectra of very faint stars +can be photographed, and a discussion of velocities +derived in this manner has shown that Kapteyn's +two star streams extend into space much farther +than it was possible to trace them with the nearer +stars. Star-streaming, then, may be a phenomenon +of the widest significance in reference to the entire +universe.</p> + +<p>As to the fundamental causes for the two opposite +and nearly equal star streams, it is early perhaps +to even theorize upon the subject. Eddington, however, +finds a possible explanation in the spiral +nebulæ, which are so numerous as to indicate the +certainty of an almost universal law compelling +matter to flow in these forms. Why it does so, we +cannot be said to know; but obviously matter is +either flowing into the nucleus from the branches of +the spiral, or it is flowing out from the nucleus into +the branches. Which of the two directions does not +matter, because in either case there would be currents +of matter in opposite directions at the points +where the arms merge in the central aggregation. +The currents continue through the center, because +<span class="pagenum"><a name="Page_p349" id="Page_p349">[349]</a></span> +the stars do not interfere with one another's paths. +As Eddington concludes: "There then we have an +explanation of the prevalence of motions to and fro +in a particular straight line; it is the line from which +the spiral branches start out. The two star streams +and the double-branched spirals arise from the +same cause."</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p350" id="Page_p350">[350]</a></span></p> + +<h2><a name="CHAPTER_LVII" id="CHAPTER_LVII"></a>CHAPTER LVII<br /> +<br /> +THE GALAXY OR MILKY WAY</h2> + +<p>Grandest of all the problems that have occupied +the mind of man is the distribution of the +stars throughout space. To the earliest astronomers +who knew nothing about the distances of the +stars, it was not much of a problem because they +thought all the fixed stars were attached to a revolving +sphere, and therefore all at essentially the +same distance; a very moderate distance, too. Even +Kepler held the idea that the distances of individual +stars from each other are much less than +their distances from our sun.</p> + +<p>Thomas Wright, of Durham, England, seems to +have been the first to suggest the modern theory of +the structure of the stellar universe, about the +middle of the eighteenth century. His idea was +taken up by Kant who elaborated it more fully. It +is founded on the Galaxy, the basal plane of stellar +distribution, just as the ecliptic is the fundamental +circle of reference in the solar system.</p> + +<p>What is the Galaxy or Milky Way?</p> + +<p>Here is a great poet's view of the most poetic +object in all nature:</p> + +<div class="poem"> +<div class="i4">A broad and ample road, whose dust is gold,</div> +<div class="i4">And pavement stars, as stars to thee appear</div> +<div class="i4">Seen in the Galaxy, that Milky Way</div> +<div class="i4">Which nightly as a circling zone thou seest</div> +<div class="i4">Powder'd with stars.</div> +</div> + +<div style="margin-left: 55%"> +<i>Milton, P. L.</i> vii, 580. +</div> + +<p><span class="pagenum"><a name="Page_p351" id="Page_p351">[351]</a></span> +Were the earth transparent as crystal, so that we +could see downward through it and outward in all +directions to the celestial sphere, the Galaxy or +Milky Way would appear as a belt or zone of +cloudlike luminosity extending all the way round +the heavens. As the horizon cuts the celestial +sphere in two, we see at anyone time only one-half +of the Milky Way, spanning the dome of the sky as +a cloudlike arch.</p> + +<p>As the general plane of the Galaxy makes a large +angle with our equator, the Milky Way is continually +changing its angle with the horizon, so that it +rises at different elevations. One-half of the +Milky Way will always be below our horizon, and +a small region of it lies so near the south pole of +the heavens that it can never be seen from medium +northern latitudes.</p> + +<p>Galileo was the first to explain the fundamental +mystery of this belt, when he turned his telescope +upon it and found that it was not a continuous +sheet of faint light, as it seemed to be, but was +made up of countless numbers of stars, individually +too faint to be visible to the naked eye, but whose +vast number, taken in the aggregate, gave the well-known +effect which we see in the sky. In some +regions, as Perseus, the stars are more numerous +than in others, and they are gathered in close +clusters. The larger the telescope we employ, the +greater the number of stars that are seen as we +approach the Galaxy on either side; and the farther +we recede from the Galaxy and approach either of +its poles fewer and fewer stars are found. Indeed, +if all the stars visible in a 12-inch telescope could +be conceived as blotted out, nearly all the stars that +are left would be found in the Galaxy itself.</p> + +<p><span class="pagenum"><a name="Page_p352" id="Page_p352">[352]</a></span> +The naked eye readily notes the variations in +breadth and brightness of the galactic zone. +Nearly a third of it, from Scorpio to Cygnus, is +split into two divisions nearly parallel. In many +regions its light is interrupted, especially in Centaurus, +where a dark starless region exists, known +as the "coal sack." Sir John Herschel, who followed +up the stellar researches of his father, Sir +William, in great detail, places the north pole of +the Galactic plane in declination 37 degrees N., and +right ascension 12 h. 47 m. This makes the plane +of the Milky Way lie at an angle of about 60 degrees +with the ecliptic, which it intersects not far +from the solstices.</p> + +<p>Now Kant, in view of the two great facts about +the Galaxy known in his time, (1) that it wholly +encircles the heavens, and (2) that it is composed +of countless stars too faint to be individually visible +to the naked eye, drew the safe conclusions that +the system of the stars must extend much farther +in the direction of the Milky Way than in other +directions.</p> + +<p>This theory of Kant was next investigated from +an observational standpoint by Sir William Herschel, +the ultimate goal of whose researches was +always a knowledge of the construction of the +heavens. The present conclusion is that we may +regard the stellar bodies of the sidereal universe +as scattered, without much regard to uniformity, +throughout a vast space having in general the shape +of a thick watch, its thickness being perhaps one-tenth +its diameter. On both sides of this disk of +stars, and clustered about the poles of the sidereal +system are the regions occupied by vast numbers +of nebulæ. The entire visible universe, then, would +be spheroidal in general shape. The plane of the +Milky Way passes through the middle of this aggregation +of stars and nebulæ, and the solar system is +near the center of the Milky Way. Throughout the +watch-form space the stars are clustered irregularly, +in varied and sometimes fantastic forms, but +without approach to order or system. If we except +some of the star groups and star clusters and consider +only the naked-eye stars, we find them scattered +with fair approach to uniformity.</p> + +<p><span class="pagenum"><a name="Page_p352p1" id="Page_p352p1">[352i]</a></span></p> + +<div class="fig_center" style="width: 644px;"> +<img src="images/p352_1.png" width="644" height="462" alt="" /> +<div class="fig_caption"><span class="smcap">Star Clouds and Black Holes in Sagittarius.</span> The dark rifts and lanes resemble those in the nearby +Milky Way. (<i>Photo, Yerkes Observatory.</i>)</div> +</div> + +<p><span class="pagenum"><a name="Page_p352p2" id="Page_p352p2">[353i]</a></span></p> + +<div class="fig_center" style="width: 462px;"> +<img src="images/p352_2.png" width="462" height="581" alt="" /> +<div class="fig_caption"><span class="smcap">The Great Nebula of Andromeda, Largest (Apparently) of all the +Spiral Nebulæ.</span> This nebula can be seen very faintly with the naked +eye, but no telescope has yet resolved it into separate stars. (<i>Photo, +Yerkes Observatory.</i>)</div> +</div> + +<p><span class="pagenum"><a name="Page_p353" id="Page_p353">[353]</a></span> +The watch-shaped disk is not to be understood as +representing the actual form of the stellar system, +but only in general the limits within which it is +for the most part contained.</p> + +<p>A vigorous attack on the problem of the evolution +and structure of the stellar universe as a whole +is now being conducted by cooperation of many observatories +in both hemispheres. It is known as +the Kapteyn "Plan of Selected Areas," embracing +206 regions which are distributed regularly over +the entire sky. Besides this a special plan includes +forty-six additional regions, either very rich or +extremely poor in stars, or to which other interest +attaches.</p> + +<p>Of all investigators Kapteyn has gone into the +question of our precise location in the Milky Way +most thoroughly, concluding that the solar system +lies, not at the center in the exact plane, but somewhat +to the north of the Galaxy. Discussing the +Sirian stars he finds that if stars of equal brightness +are compared, the Sirians average nearly three +times more distance from the sun than those of +the solar type. So, probably, the Sirians far exceed +the Solars in intrinsic brightness. Farther, +Kapteyn concludes that the Galaxy has no connection +<span class="pagenum"><a name="Page_p354" id="Page_p354">[354]</a></span> +with our solar system, and is composed of a +vast encircling annulus or ring of stars, far exceeding +in number the stars of the great central +solar cluster, and everywhere exceedingly remote +from these stars, as well as differing from them in +physical type and constitution. So it would be +mainly the mere element of distance that makes +them appear so faint and crowded thickly together +into that gauzy girdle which we call the Galaxy.</p> + +<p>The Milky Way reveals irregularities of stellar +density and star clustering on a large scale, with +deep rifts between great clouds of stars. Modern +photographs, particularly those of Barnard in Sagittarius, +make this very apparent. Within the Milky +Way, nearly in its plane and almost central, is what +Eddington terms the inner stellar system, near the +center of which is the sun. Surrounding it and +near its plane are the masses of star clouds which +make up the Milky Way. Whether these star +clouds are isolated from the inner system or continuous +with it, is not yet ascertained.</p> + +<p>The vast masses of the Milky Way stars are very +faint, and we know nothing yet as to their proper +motions, their radial motions, or their spectra. +Probably a few stars as bright as the sixth magnitude +are actually located in the midst of the Milky +Way clusters, the fainter ninth magnitude stars +certainly begin the Milky Way proper, while the +stars of the twelfth or thirteenth magnitude carry +us into the very depths of the Galaxy.</p> + +<p>It is now pretty generally believed that many of +the dark regions of the Milky Way are due not to +actual absence of stars so much as to the absorption +of light by intervening tracts of nebulous matter +on the hither side of the Galactic aggregations +<span class="pagenum"><a name="Page_p355" id="Page_p355">[355]</a></span> +and, probably in fact, within the oblate inner stellar +system itself. Easton has made many hundred +counts of stars in galactic regions of Cygnus and +Aquila where the range of intensity of the light is +very marked; in fact, the star density of the bright +patches of the Galaxy is so far in excess of the +density adjacent and just outside the Milky Way, +that the conclusion is inevitable that this excess is +due to the star clouds.</p> + +<p>Of the distance of the Milky Way we have very +little knowledge. It is certainly not less than 1,000 +parsecs, and more likely 5,000 parsecs, a distance +over which light would travel in about 16,000 years. +Quite certainly all parts of the Galaxy are not at +the same distance, and probably there are branches +in some regions that lie behind one another. While +the general regions of the nebulæ are remote from +the Galactic plane, the large irregular nebulæ, as +the Trifid, the Keyhole, and the Omega nebulæ, are +found chiefly in the Milky Way.</p> + +<p>In addition to the irregular nebulæ many types +of stellar objects appear to be strongly condensed +toward the Milky Way, but this may be due to the +inner stellar system, rather than a real relation to +Galactic formations. Quite different are the Magellanic +clouds, which contain many gaseous nebulæ and +are unique objects of the sky, having no resemblance +to the true spiral nebulæ which, as a rule, avoid the +Galactic regions. Worthy of note also is the theory +of Easton that the Milky Way has itself the form +of a double-branched spiral, which explains the +visible features quite well, but is incapable of either +disproof or verification. The central nucleus he +locates in the rich Galactic region of Cygnus, with +the sun well outside the nucleus itself. By combining +<span class="pagenum"><a name="Page_p356" id="Page_p356">[356]</a></span> +the available photographs of the Galaxy, he has +produced a chart which indicates in a general way +how the stellar aggregations might all be arrayed +so as to give the effect of the Galaxy as we see it.</p> + +<p>Shapley, at Mount Wilson, has studied the structure +of the Galactic system, in which he has been +aided by Mrs. Shapley. An interesting part of this +work relates to the distribution of the spiral +nebulæ, and to certain properties of their systematic +recessional motion, suggesting that the entire +Galactic system may be rapidly moving through +space. Apparently the spiral nebulæ are not distant +stellar organizations or "island universes," but truly +nebular structures of vast volume which in general +are actively repelled from stellar systems. A tentative +cosmogonic hypothesis has been formulated to +account for the motions, distribution, and observed +structure of clusters and spiral nebulæ.</p> + +<p>An additional great problem of the Galaxy is a +purely dynamical one. Doubtless it is in some sort +of equilibrium, according to Eddington, that is to +say, the individual stars do not oscillate to and fro +across the stellar system in a period of 300 million +years, but remain concentrated in clusters as at +present. Poincaré has considered the entire Milky +Way as in stately rotation, and on the assumption +that the total mass of the inner stellar system is +1,000,000,000 times the sun's mass, and that the distance +of the Milky Way is 2,000 parsecs, the angular +velocity for equilibrium comes out 0".5 per century. +That is to say, a complete revolution would +take place in about 250 million years.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p357" id="Page_p357">[357]</a></span></p> + +<h2><a name="CHAPTER_LVIII" id="CHAPTER_LVIII"></a>CHAPTER LVIII</h2> + +<h2>STAR CLOUDS AND NEBULÆ</h2> + +<p>From star clusters to nebulæ, only a century ago, +the transition was thought to be easy and immediate. +Accuracy in determining the distances of +stars was just beginning to be reached, the clusters +were obviously of all degrees of closeness following +to the verge of irresolvability, and it was but natural +to jump to the conclusion that the mystery of the +nebulæ consisted in nothing but their vaster distance +than that of clusters, and it was believed that +all nebulæ would prove resolvable into stars whenever +telescopes of sufficiently great power could be +constructed.</p> + +<p>But the development of the spectroscope soon +showed the error of this hypothesis, by revealing +bright lines in the nebular spectra showing that +many nebulæ emit light that comes from glowing incandescent +gas, not from an infinitude of small stars.</p> + +<p>In pre-telescope days nothing was known about +the nebulæ. The great nebula in Andromeda, and +possibly the great nebula in Orion, are alone visible +to the naked eye, but as thus seen they are the +merest wisps of light, the same as the larger clusters +are. Galileo, Huygens and other early users of +the telescope made observations of nebulæ, but long-focus +telescopes were not well adapted to this work. +Simon Mayer has left us the first drawing of a +nebula, the Orion nebula as he saw it in 1612. The +<span class="pagenum"><a name="Page_p358" id="Page_p358">[358]</a></span> +vast light-gathering power of the reflectors built by +Sir William Herschel first afforded glimpses of the +structure of the nebulæ, and if his drawings are +critically compared with modern ones, no case of +motion with reference to the stars or of change in +the filaments of the nebulæ themselves has been +satisfactorily made out.</p> + +<p>Only very recently has the distance of a nebula +been determined, and the few that have been measured +seem to indicate that the nebulæ are at distances +comparable with the stars. Of all celestial +objects the nebulæ fill the greatest angles, so that +we are forced to conclude, with regard to the actual +size of the greater nebulæ as they exist in space, +that they far surpass all other objects in bulk.</p> + +<p>Photography invaded the realm of the nebulæ +in 1880, when Dr. Henry Draper secured the first +photograph of the nebula of Orion. Theoretically +photography ought to help greatly in the study of +the nebulæ, and enable us in the lapse of centuries +to ascertain the exact nature of the changes which +must be going on. The differences of photographic +processes, of plates, of exposure and development +produce in the finished photograph vastly greater +differences than any actual changes that might be +going on, so that we must rely rather on optical +drawings made with the telescope, or on drawings +made by expert artists from photographs with many +lengths of exposure on the same object.</p> + +<p>The great work on nebulæ and star clusters recently +concluded by Bigourdan of the Paris Observatory +and published in five volumes received the +award of the gold medal of the Royal Astronomical +Society. While D'Arrest measured about 2,000 +nebulæ, and Sir John Herschel about double that +<span class="pagenum"><a name="Page_p359" id="Page_p359">[359]</a></span> +number in both hemispheres, Bigourdan has measured +about 7,000. His work forms an invaluable +lexicon of information concerning the nebulæ.</p> + +<p>Classification of the nebulæ is not very satisfactory, +if made by their shapes alone. There are +perhaps fifteen thousand nebulæ in all that have +been catalogued, described, and photographed. +Dreyer's new general catalogue (N.G.C.) is the best +and most useful. Many of the nebulæ, especially +the large ones, can only be classified as irregular +nebulæ. The Orion nebula is the principal one of +this class, revealing an enormous amount of complicated +detail, with exceptional brilliancy of many +regions and filaments. An extraordinary multiple +star, Theta Orionis, occupies a very prominent position +in the nebula, and photographs by Pickering +have brought to light curved filaments, very faint +and optically invisible, in the outlying regions which +give the Orion nebula in part a spiral character. +But the delicate optical wisps of this nebula are well +seen, even in very small telescopes. Its spectrum +yields hydrogen, helium and nitrogen. The Orion +nebula is receding from the earth about eleven miles +in every second. Keeler and Campbell have shown +that nearly every line of the nebular spectrum is a +counterpart of a prominent dark line in the spectrum +of the brighter stars of the constellation of +Orion. A recent investigator of the distribution of +luminosity in the great nebula of Orion finds that +radiations from nebulium are confined chiefly to the +Huygenian region of the nebula and its immediate +neighborhood.</p> + +<p>Photography has revealed another extraordinary +nebula or group of nebulæ surrounding the stars +in the Pleiades, which the deft manipulation of +<span class="pagenum"><a name="Page_p360" id="Page_p360">[360]</a></span> +Barnard has brought to light. All the stars and the +nebula are so interrelated that they are obviously +bound together physically, as the common proper +motion of the stars also appears to show. Also in +the constellation Cygnus, Barnard has discovered +very extensive nebulosities of a delicate filmy cloudlike +nature which are wholly invisible with telescopes, +but very obvious on highly sensitive plates +with long exposures.</p> + +<p>Another class of these objects are the annular and +elliptic nebulæ which are not very abundant. The +southern constellation Grus, the crane, contains a +fine one, but by far the best example is in the constellation +Lyra. It is a nearly perfect ring, elliptic +in figure, exceedingly faint in small telescopes; but +large instruments reveal many stars within the annulus, +one near the center which, although very faint +to the eye, is always an easy object on the photographic +plate, because it is rich in blue and violet +rays. The parallax of the ring nebula in Lyra comes +out only one-sixth of that of the planetary nebulæ, +and the least greatest diameters of this huge continuous +ring are 250 and 330 times the orbit of +Neptune.</p> + +<p>Planetary nebulæ and nebulous stars are yet +another class of nebulæ, for the most part faint and +small, resembling in some measure a planetary disk +or a star with nebulous outline. Practically all are +gaseous in composition, and have large radial velocities. +Probably they are located within our own stellar +system. The parallaxes of several of them have +been measured by Van Maanen: one of the very small +angle 0".023, which enables us to calculate the diameter +of this faint but interesting object as equal to +nineteen times the orbit of Neptune.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p361" id="Page_p361">[361]</a></span></p> + +<h2><a name="CHAPTER_LIX" id="CHAPTER_LIX"></a>CHAPTER LIX</h2> + +<h2>THE SPIRAL NEBULÆ</h2> + +<p>Last and most important of all are the spiral +nebulæ. The finest example is in the constellation +Canes Venatici, and its spiral configuration was +first noted by Lord Rosse, an epoch-making discovery. +The convolutions of its spiral are filled with +numerous starlike condensations, themselves engulfed +in nebulosity. Photography possesses a vast +advantage over the eye in revealing the marvelous +character of this object, an inconceivably vast celestial +whirlpool. Naturally the central regions of the +whorl would revolve most swiftly, but no comparison +of drawings and photographs, separated by intervals +of many years, has yet revealed even a trace of any +such motion.</p> + +<p>The number of large spiral nebulæ is not very +great; the largest of all is the great nebula of Andromeda, +whose length stretches over an arc of seven +times the breadth of the moon, and its width about +half as great. This nebula is a naked-eye object near +Eta Andromedæ, and it is often mistaken for a +comet. Optically it was always a puzzle, but photographs +by Roberts of England first revealed the true +spiral, with ringlike formations partially distinct, +and knots of condensing nebulosity as of companion +stars in the making. While its spectrum shows the +nongaseous constitution of this nebula, no telescope +has yet resolved it into component stars.</p> + +<p><span class="pagenum"><a name="Page_p362" id="Page_p362">[362]</a></span> +Systematic search for spiral nebulæ by Keeler, +and later continued by Perrine, at the Lick Observatory, +with the 36-inch Crossley reflector, disclosed +the existence of vast numbers of these objects, +in fact many hundreds of thousands by estimation; +so that, next to the stars, the spiral nebulæ are by +far the most abundant of all objects in the sky. They +present every phase according to the angle of their +plane with the line of sight, and the convolutions +of the open ones are very perfectly marked. Many +are filled with stars in all degrees of condensation, +and the appearance is strongly as if stars are here +caught in every step of the process of making.</p> + +<p>The vast multitude of the spiral nebulæ indicates +clearly their importance in the theory of the cosmogony, +or science of the development of the material +universe. Curtis of the Lick Observatory has +lately extended the estimated number of these objects +to 700,000. He has also photographed with the +Crossley reflector many nebulæ with lanes or dark +streaks crossing them longitudinally through or +near the center. These remarkable streaks appear +as if due to opaque matter between us and the luminous +matter of the nebula beyond. Perhaps a dark +ring of absorptive or occulting matter encircles the +nebula in nearly the same plane with the luminous +whorls. Duncan has employed the 60-inch Mount +Wilson reflector in photographing bright nebulæ +and star clusters in the very interesting regions of +Sagittarius. One of these shows unmistakable dark +rifts or lanes in all parts of the nebula, resembling +the dark regions of the neighboring Milky Way.</p> + +<p>Pease of Mount Wilson has recently employed the +60-inch and the 100-inch reflectors of the Mount +Wilson Observatory to good advantage in photographing +<span class="pagenum"><a name="Page_p363" id="Page_p363">[363]</a></span> +several hundred of the fainter nebulæ. +Many of these are spirals, and others present very +intricate and irregular forms. A search was made +for additional spirals among the smaller nebulæ +along the Galaxy, but without success. Several of +the supposedly variable nebulæ are found to be unchanging. +Many nights in each month when the +moon is absent are devoted to a systematic survey +of the smaller nebulæ and their spectra by photography. +The visible spiral figure of all these objects +is a double-branched curve, its two arms joining +on the nucleus in opposing points, and coiling +round in the same geometrical direction. The spiral +nebulæ, as to their distribution, are remote from +the Galaxy, and the north Galactic polar region contains +a greater aggregation than the south. The +distances of the spiral nebulæ are exceedingly great. +They lie far beyond the planetary and irregular +gaseous nebulæ, like that of Orion, which are closely +related to the stars forming part of our own +system. Possibly the spiral nebulæ are exterior or +separate "island universes." If so, they must be inconceivably +vast in size, and would develop, not into +solar systems, but into stellar clusters. The enormous +radial velocities of the spiral nebulæ, averaging 300 +to 400 kilometers per second, or twenty-fold that of +the stars, tend to sustain the view that they may be +"island universes," each comparable in extent with +the universe of stars to which our sun belongs.</p> + +<p>Recent spectroscopic observations of the nebulæ +applying the principle of Doppler have revealed high +velocities of rotation. Slipher of the Lowell Observatory +made the first discovery of this sort and Van +Maanen of Mount Wilson has detected in the great +Ursa Major spiral, No. 101 in Messier's catalogue, +<span class="pagenum"><a name="Page_p364" id="Page_p364">[364]</a></span> +a speed of rotation at five minutes of arc from the +center that would correspond to a complete period +in 85,000 years. As was to be expected, the nebula +does not rotate as a rigid body, but the nearer the +center the greater the angular velocity, and Van +Maanen finds evidence of motion along the arms and +away from the center.</p> + +<p>These great velocities appear to belong to the +spiral nebulæ as a class, and not to other nebulæ. +Thirteen nebulæ investigated by Keeler are as a +whole almost at rest relatively to our system, as are +the large irregular objects in Orion, and the Trifid +nebula. This would seem to indicate that the spiral +nebulæ form systems outside our own and independent +of it.</p> + +<p>Quite different from the spirals in their distribution +through space are the planetary nebulæ. The +spirals follow the early general law of nebulæ arrangement, +that is, they are concentrated toward +the poles of the Galaxy; but the planetary nebulæ, +on the other hand, are very few near the poles and +show a marked frequency toward the Galactic plane. +Campbell and Moore have found spectroscopic evidence +of internal rotatory motion in a large proportion +of the planetary nebulæ.</p> + +<p>The distribution of the nebulæ throughout space, +like that of the stars, is still under critical investigation, +but the location of vast numbers of the more +compact nebulæ on the celestial sphere is very extraordinary. +The Milky Way appears to be the +determining plane in both cases; the nearer we +approach it the more numerous the stars become, +whereas this is the general region of fewest nebulæ +and they increase in number outward in both directions +from the Galaxy, and toward both poles of the +<span class="pagenum"><a name="Page_p365" id="Page_p365">[365]</a></span> +Galactic circle. Obviously this relation, or contra-relation +of stars and nebulæ on such a vast scale is +not accidental, and it also must be duly accounted +for in the true theory of the cosmogony. The +nebulæ which are found principally in and near the +Milky Way are the large irregular nebulæ, and vast +nebulous backgrounds, like those photographed by +Barnard in Scorpio, Taurus and elsewhere, as well +as the Keyhole, Omega, and Trifid nebulæ. Allied +to these backgrounds are doubtless some of the dark +Galactic spaces, radiating little or no intrinsic light, +and absorbing the light of the fainter stars beyond +them. A peculiar veiled or tinted appearance has +been remarked in some cases visually, and examination +of the photographs strongly confirms the existence +of absorbing nebulosity.</p> + +<p>The spiral nebulæ are so abundant, and so much +attention is now being given to them, both by observers +and mathematicians, that their precise relation +to the stellar systems must soon be known; that +is, whether they are comparatively small objects +belonging to the stellar system, or independent +systems on the borders of the stellar system, or as +seems more likely, vast and exceedingly remote +galaxies comparable with that of the Milky Way +itself. Our knowledge of the motions of the spirals, +both radial and angular, is increasing rapidly, and +must soon permit accurate general conclusions to +be drawn.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p366" id="Page_p366">[366]</a></span></p> + +<h2><a name="CHAPTER_LX" id="CHAPTER_LX"></a>CHAPTER LX<br /> +<br /> +COSMOGONY</h2> + +<p>Down to the middle of the last century and later, +it was commonly believed that in the beginning +the cosmos came into being by divine fiat +substantially as it is. Previously the earth had +been "without form and void," as in the Scripture. +Had it not been for the growth and gradual +acceptance of the doctrine of evolution, and +its reactionary effect upon human thought, it is +conceivable that the early view might have persisted +to the present day; but now it is universally held +that everything in the heavens above and the earth +beneath is subject more or less to secular change, +and is the result of an orderly development throughout +indefinite past ages, a progressive evolution +which will continue through indefinite aeons of the +future.</p> + +<p>In the writings of the Greek philosophers, and +down through the Middle Ages we find the idea of +an original "chaos" prevailing, with no indication +whatever of the modern view of the process by which +the cosmos came to be what they saw it and as it is +to-day. If we go still farther back, there is no glimmer +of any ideas that will bear investigation by +scientific method, however interesting they may be +as purely philosophical conceptions. Many ancient +philosophers, among them Anaxagoras, Democritus, +and Anaximenes, regarded the earth as the product +<span class="pagenum"><a name="Page_p367" id="Page_p367">[367]</a></span> +of diffused matter in a state of the original chaos +having fallen together haphazard, and they even +presumed to predict its future career and ultimate +destiny.</p> + +<p>In Anaximander and Anaximenes alone do we find +any conception of possible progress; their thought +was that as the world had taken time to become what +it is, so in time it would pass, and as the entire universe +had undergone alternate renewal and destruction +in the past, that would be its history in the +future. Aristotle, Ptolemy, and others appear to +have held the curious notion that although everything +terrestrial is evanescent, nevertheless the +cosmos beyond the orbit of the moon is imperishable +and eternal.</p> + +<p>By tracing the history of the intellectual development +of Europe we may find why it was that scientific +speculation on the cosmogony was delayed until +the 18th century, and then undertaken quite independently +by three philosophers in three different +countries. Swedenborg, the theologian, set down in +due form many of the principles that underlie the +modern nebular hypothesis. Thomas Wright of +Durham whose early theory of the arrangement of +stars in the Galaxy we have already mentioned, +speculated also on the origin and development of +the universe, and his writings were known to Kant, +who is now regarded as the author of the modern +nebular hypothesis. This presents a definite mechanical +explanation of the development and formation +of the heavenly bodies, and in particular those +composing the solar system.</p> + +<p>Kant was illustrious as a metaphysician, but he +was a great physicist or natural philosopher as well, +and he set down his ideas regarding the cosmogony +<span class="pagenum"><a name="Page_p368" id="Page_p368">[368]</a></span> +with precision. Learned in the philosophy of the +ancients, he did not follow their speculative conceptions, +but merely assumed that all the materials +from which the bodies of the solar system have been +fashioned were resolved into their original elements +at the beginning, and filled all that part of +space in which they now move. True, this is pretty +near the chaos of the Greeks, but Kant knew of the +operation of the Newtonian law of gravitation, +which the Greeks did not.</p> + +<p>As a natural result of gravitative processes, Kant +inferred that the denser portions of the original +mass would draw upon themselves the less dense +portions, whirling motions would be everywhere +set up, and the process would continue until many +spherical bodies, each with a gaseous exterior in +process of condensation, had taken the place of the +original elements which filled space. In this manner +Kant would explain the sameness in direction +of motion, both orbital and axial, of all the planets +and satellites of our system. But many philosophers +are of the opinion that Kant's hypothesis +would result, not in the formation of such a collection +of bodies as the solar system is, but rather in a +single central sun formed by common gravitation +toward a single center.</p> + +<p>From quite another viewpoint the work of the +elder Herschel is important here. No one knew the +nebulæ from actual observation better than he did; +but, while his ideas about their composition were +wrong, he nevertheless conceived of them as gradually +condensing into stars or clusters of stars. And +it was this speculative aspect of the nebulæ, not as +a possible means of accounting for the birth and +development of the solar system, which constitutes +<span class="pagenum"><a name="Page_p369" id="Page_p369">[369]</a></span> +Herschel's chief contribution to the nebular hypothesis. +Classifying the nebulæ which he had carefully +studied with his great telescopes, it seemed +obvious to him that they were actually in all the +different stages of condensation, and subsequent +research has strongly tended to substantiate the +Herschelian view.</p> + +<p>Then came Laplace, who took up the great hypothesis +where Kant and Herschel had left it, added +new and important conceptions in the light of his +mature labors as mathematician and astronomer, +and put the theory in definitive form, such that it +has ever since been known under the name of Laplacian +nebular hypothesis. For reasons like those +that prevailed with Kant, he began the evolution of +the solar system with the sun already formed as +the center, but surrounded by a vast incandescent +atmosphere that filled all the space which the sun's +family of planets now occupy. This entire mass, +sun, atmosphere, and all, he conceived to have a +stately rotation about its axis. With rotation of +the mass and slow reduction of temperature in its +outer regions, there would be contraction toward +the solar center, and an increase in velocity of rotation +until the whole mass had been much reduced +in diameter at its poles and proportionately expanded +at its equator.</p> + +<p>When the centrifugal force of the outer equatorial +masses finally became equal to the gravitational +forces of the central mass, then these conjoined +outer portions would be left behind as a +ring, still revolving at the velocity it had acquired +when detached. The revolution of the entire inner +mass goes on, its velocity accelerating until a similar +equilibration of forces is again reached, when +<span class="pagenum"><a name="Page_p370" id="Page_p370">[370]</a></span> +a second rotating ring is left behind. Laplace conceived +the process as repeated until as many rings +had been detached as there are individual planets, +all central about the sun, or nearly so.</p> + +<p>In all, then, we should have nine gaseous rings; +the outer ones preceding the inner in formation, +but not all existing as rings at the same time. Radiation +from the ring on all sides would lead to rapid +contraction of its mass, so that many nuclei of condensation +would form, of various sizes, all revolving +round the central sun in practically the same +period. Laplace conceived the evolution of the ring +to proceed still farther till the largest aggregation +in it had drawn to itself all the other separate +nuclei in the ring.</p> + +<p>This, then, was the planet in embryo, in effect a +diminutive sun, a secondary incandescent mass endowed +with axial rotation in the same direction as +the parent nebula. With reduction of temperature +by radiation, polar contraction and equatorial expansion +go on, and planetary rings are detached +from this secondary mass in exactly the same way +as from the original sun nebula. And these planetary +rings are, in the Laplacian hypothesis, the embryo +moons or planetary satellites, all revolving +round their several planets in the same direction +that the planets revolve about the sun.</p> + +<p>In the case of one of the planetary rings, its +formation was so nearly homogeneous throughout +that no aggregation into a single satellite was possible; +all portions of the ring being of equal density, +there was no denser region to attract the less dense +regions, and in this manner the rings of Saturn +were formed, in lieu of condensation into a separate +satellite. Similarly in the case of the primal +<span class="pagenum"><a name="Page_p371" id="Page_p371">[371]</a></span> +solar ring that was detached next after the Jovian +ring; there was such a nice balancing of masses +and densities that, instead of a single major planet, +we have the well-known asteroidal ring, composed +of innumerable discrete minor planets.</p> + +<p>This, then, in bare outline, is the Laplacian nebular +hypothesis, and it accounted very well for the +solar system as known in his day; the fairly regular +progression of planetary distances; their orbits +round the sun all nearly circular and approximately +in a single plane; the planetary and satellite revolutions +in orbit all in the same direction; the axial +rotations of planets in the same direction as their +orbital revolutions; and the plane of orbital revolution +of the satellites practically coinciding with the +plane of the planet's axial rotation. But the principle +of conservation of energy was, of course, unknown +to Laplace, nor had the mechanical equivalence +of heat with other forms of energy been +established in his day.</p> + +<p>In 1870, Lane of Washington first demonstrated +the remarkable law that a gaseous sphere, in +process of losing heat by radiation and contraction +because of its own gravity, actually grows hotter +instead of cooler, as long as it continues to be gaseous, +and not liquid or solid. So there is no need of +postulating with Laplace an excessively high temperature +of the original nebula. The chief objection +to Laplace's hypothesis by modern theorists is +that the detachment of rings, though possible, +would likely be a rare occurrence; protuberances or +lumps on the equatorial exterior of a swiftly revolving +mass would be more likely, and it is much +easier to see how such masses would ultimately become +planets than it is to follow the disruption of +<span class="pagenum"><a name="Page_p372" id="Page_p372">[372]</a></span> +a possible ring and the necessary steps of the +process by which it would condense into a final +planet. The continued progress of research in many +departments of astronomy has had important bearing +on the nebular hypothesis, and we may rest +assured that this hypothesis in somewhat modified +form can hardly fail of ultimate acceptance, though +not in every essential as its great originator left it.</p> + +<p>Lord Rosse's discovery of spiral nebulæ, followed +up by Keeler's photographic search for these bodies, +revealing their actual existence in the heavens by +the hundreds of thousands, has led to another +criticism of the Laplacian theory. Could Laplace +have known of the existence of these objects +in such vast numbers, his hypothesis would no +doubt have been suitably modified to account for +their formation and development. It is generally +considered that the ring of Saturn suggested to +Laplace the ring feature in his scheme of origin of +planets and satellites; so far as we know, the Saturnian +ring is unique, the only object of its kind in the +heavens. Whereas, next to the star itself, the spiral +nebula is the type object which occurs most frequently. +A theory, therefore, which will satisfactorily +account for the origin and development of +spiral nebulæ must command recognition as of great +importance in the cosmogony.</p> + +<p>Such a theory has been set forth by Chamberlin +and Moulton in their planetesimal hypothesis, according +to which the genesis of spiral nebulæ +happens when two giant suns approach each other +so closely that tide-producing effects take place on a +vast scale. These suns need not be luminous; they +may perhaps belong to the class of dark or extinguished +suns. The evidences of the existence of +<span class="pagenum"><a name="Page_p373" id="Page_p373">[373]</a></span> +such in vast numbers throughout the universe is +thought to be well established.</p> + +<p>Now, on close approach, what happens? There +will be huge tides, and the nearer the bodies come +to each other, the vaster the scale on which tides +will be formed. If the bodies are liquid or gaseous, +they will be distorted by the force of gravitation, and +the figure of both bodies will become ellipsoidal; and +at last under greater stress, the restraining shell of +both bodies will burst asunder on opposite sides in +streams of matter from the interior. In this +manner the arms of the spiral are formed.</p> + +<p>As Chamberlin puts it: "If, with these potent +forces thus nearly balanced, the sun closely approaches +another sun, or body of like magnitude … +the gravity which restrains this enormous elastic +power will be reduced along the line of mutual attraction. +At the same time the pressure transverse +to this line of relief will be increased. Such localized +relief and intensified pressure must bring into +action corresponding portions of the sun's elastic +potency, resulting in protuberances of corresponding +mass and high velocity."</p> + +<p>Only a fraction of one per cent of the sun's mass +ejected in this fashion would be sufficient to generate +the entire planetary system. Nuclei or knots in +the arms of the spiral gradually grew by accretion, +the four interior knots forming Mercury, Venus, the +Earth, and Mars. The earth knot was a double one, +which developed into the earth-moon system. The +absence of a dominating nucleus beyond Mars accounts +for the zone of the asteroids remaining in +some sense in the original planetesimal condition. +The vaster nuclei beyond Mars gradually condensed +into Jupiter, Saturn, Uranus, and Neptune; and +<span class="pagenum"><a name="Page_p374" id="Page_p374">[374]</a></span> +lesser nuclei related to the larger ones form the +systems of moons or satellites.</p> + +<p>The orbits of the planetesimals and the planetary +and satellite nuclei would be very eccentric, forming +a confusion of ellipses with frequently crossing +paths. Collisions would occur, and the nuclei would +inevitably grow by accretion. Each planet, then, +would clear up the planetesimals of its zone; and +Moulton shows that this process would give rise to +axial revolution of the planet in the same direction +as its orbital revolution. The eccentricities would +finally disappear, and the entire mass would revolve +in a nearly circular orbit.</p> + +<p>Rotation twists the streams into the spiral form, +and the huge amounts of wreckage from the near-collision +are thrown into eddies. The fragments or +particles (planetesimals) which have given the +name to the theory, begin their motion round +their central sun in elliptical paths as required by +gravitation. The form of the spiral is preserved by +the orbital motion of its particles. There is a +gradual gathering together of the planetesimals at +points or nodes of intersection, and these become +aggregations of matter, nuclei that will perhaps become +planets, though more likely other stars. The +appulse or near approach is but one of the methods +by which the spiral nebulæ may have come into existence. +The planetesimal hypothesis would seem +to account for the formation of many of these objects +as we see them in the sky, though perhaps it is +hardly competent to replace entirely the Laplacian +hypothesis of the formation of the solar system, +which would appear to be a special case by itself.</p> + +<p>It will be observed that while the Laplacian hypothesis +is concerned in the main with the progressive +<span class="pagenum"><a name="Page_p375" id="Page_p375">[375]</a></span> +development of the solar system, and +systems of a like order surrounding other stellar +centers, whose existence is highly probable, the +origin and development of the stellar universe is a +vaster problem which can only be undertaken and +completed in its broadest bearings when the structure +of the stellar universe has been ascertained.</p> + +<p>Darwin's important investigations in 1877-1878 +on tidal friction may be here related. Before his day +acceptance of the ring-theory of development of the +moon from the earth had scarcely been questioned; +but his recondite mathematical researches on the +tidal reaction between a central yielding mass and +a body revolving round it brought to light the unsuspected +effect of tides raised upon both bodies by +their mutual attraction. The type of tides here +meant is not the usual rise and fall of the waters of +the ocean, but primeval tides in the plastic material +of which the earth in its early history was composed. +The Newtonian law of gravitation afforded a complete +explanation of the rise and fall of the waters +of the oceans, but as applied to the motions of +planets and satellites by the Lagrangian formulæ, it +presupposed that all these bodies are rigid and unyielding. +However, mutual tides of phenomenal +height in their early plastic substances must have +been a necessary consequence of the action of the +Newtonian law, and they gradually drew upon the +earth's rotational moment of momentum.</p> + +<p>In its very early history, before there was any +moon to produce tides, the earth rotated much more +rapidly, that is, the day was very much shorter than +now, probably about five or six hours long. And +with the rapid whirling, it was not a Laplacian +ring that was detached, but a huge globular mass +<span class="pagenum"><a name="Page_p376" id="Page_p376">[376]</a></span> +was separated from the plastic earth's equator. +Darwin shows that the gravitative interaction of the +two bodies immediately began to raise tides of extraordinary +height in both, therefore tending to +slow down the rotational periods of both bodies. +Action and reaction being equal, the reaction at once +began driving the moon away from the earth and +thereby lengthening its period of revolution. So +small was the mass of the moon and so near was it +to the earth, that its relative rotational energy was +in time completely used up, and the moon has ever +since turned her constant face toward us. Tides +of sun and moon in the plastic earth, acting through +the ages, slowed down the earth's rotation to its +present period, or the length of the day.</p> + +<p>Moulton, however, has investigated the tidal theory +of the origin of the moon in the light of the +planetesimal hypothesis, concluding that the moon +never was part of the earth and separated therefrom +by too rapid rotation of the earth, but that +the distance of the two bodies has always been the +same as now. The more massive earth has in its +development throughout time robbed the less massive +moon in the gradual process of accretion. So +the moon has never acquired either an ocean or atmosphere, +and this view is acceptable to geologists +who have studied the sheer lunar surface, Shaler of +Harvard among the first, and laid the foundations +for a separate science of selenology.</p> + +<p>Tidal friction has also been operant in producing +sun-raised tides upon the early plastic substances +which composed the planets: more powerfully in the +case of planets nearer the sun; less rapidly if the +planet's mass is large; also less completely if the +planet has solidified earlier on account of its small +<span class="pagenum"><a name="Page_p377" id="Page_p377">[377]</a></span> +dimensions. So Darwin would account for the +present rotation periods of all the planets: both +Mercury and Venus powerfully acted on by the sun +on account of their nearness to him, and their rotational +energy completely exhausted, so that they now +and for all time turn a constant face toward him, +as the moon does to the earth; earth and possibly +Mars even yet undergoing a very slight lengthening +of their day; Jupiter and Saturn, also Uranus and +probably Neptune, still exhibiting relatively swift +axial rotation, because of their great mass and +great original moment of momentum, and also by +reason of their vast distances from the central tide-raising +body, the sun.</p> + +<p>By applying to stellar systems the principles developed +by Darwin, See accounted for the fact, to +which he was the first to direct attention, that the +great eccentricity of the binary orbits is a necessary +result of the secular action of tidal friction. The +double stars, then, were double nebulæ, originally +single, but separated by a process allied to that +known as "fission" in protozoans. Indeed, Poincaré +proved mathematically that a swiftly revolving +nebula, in consequence of contraction, first undergoes +distortion into a pear-shaped or hour-glass +figure, the two masses ultimately separating entirely; +and the observations of the Herschels, Lord +Rosse and others, with the recent photographic +plates at the Lick and Mount Wilson observatories, +afford immediate confirmation in a multitude of +double nebulæ, widely scattered throughout the +nebular regions of the heavens.</p> + +<p>Jeans of Cambridge, England, among the most +recent of mathematical investigators of the cosmogony, +balances the advantages and disadvantages of +<span class="pagenum"><a name="Page_p378" id="Page_p378">[378]</a></span> +the differing cosmogonic systems as follows, in his +"Problems of Cosmogony and Stellar Dynamics": +"Some hundreds of millions of years ago all the +stars within our Galactic universe formed a single +mass of excessively tenuous gas in slow rotation. +As imagined by Laplace, this mass contracted owing +to loss of energy by radiation, and so increased its +angular velocity until it assumed a lenticular shape…. +After this, further contraction was a sheer +mathematical impossibility and the system had to +expand. The mechanism of expansion was provided +by matter being thrown off from the sharp +edge of the lenticular figure, the lenticular center +now forming the nucleus, and the thrown-off matter +forming the arms, of a spiral nebula of the normal +type. The long filaments of matter which constituted +the arms, being gravitationally unstable, first +formed into chains of condensation about nuclei, +and ultimately formed detached masses of gas. +With continued shrinkage, the temperature of these +masses increased until they attained to incandescence, +and shone as luminous stars. At the same +time their velocity of rotation increased until a +large proportion of them broke up by fission into +binary systems. The majority of the stars broke +away from their neighbors and so formed a cluster +of irregularly moving stars—our present Galactic +universe, in which the flattened shape of the original +nebula may still be traced in the concentration +about the Galactic plane, while the original motion +along the nebular arms still persists in the form of +'star-streaming.' In some cases a pair or small +group of stars failed to get clear of one another's +gravitational attractions and remain describing orbits +about one another as wide binaries or multiple +<span class="pagenum"><a name="Page_p379" id="Page_p379">[379]</a></span> +stars. The stars which were formed last, the present +B-type stars, have been unusually immune from +disturbance by their neighbors, partly because they +were born when adjacent stars had almost ceased +to interfere with one another, partly because their +exceptionally large mass minimized the effect of +such interference as may have occurred; consequently +they remain moving in the plane in which +they were formed, many of them still constituting +closely associated groups of stars—the moving star +clusters.</p> + +<p>"At intervals it must have happened that two +stars passed relatively near to one another in their +motion through the universe. We conjecture that +something like 300 million years ago our sun experienced +an encounter of this kind, a large star +passing within a distance of about the sun's diameter +from its surface. The effect of this, as we have +seen, would be the ejection of a stream of gas toward +the passing star. At this epoch the sun is +supposed to have been dark and cold, its density +being so low that its radius was perhaps comparable +with the present radius of Neptune's orbit. The +ejected stream of matter, becoming still colder by +radiation, may have condensed into liquid near its +ends and perhaps partially also near its middle. +Such a jet of matter would be longitudinally unstable +and would condense into detached nuclei +which would ultimately form planets."</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_p380" id="Page_p380">[380]</a></span></p> + +<h2><a name="CHAPTER_LXI" id="CHAPTER_LXI"></a>CHAPTER LXI<br /> +<br /> +COSMOGONY IN TRANSITION</h2> + +<p>We have seen how Wright in 1750 initiated a +theory of evolution, not only of the solar +system, but of all the stars and nebulæ as well; how +Kant in 1752 by elaborating this theory sought to +develop the details of evolution of the solar system +on the basis of the Newtonian law, though weakened, +as we know, by serious errors in applying +physical laws; how Laplace in 1796 put forward his +nebular hypothesis of origin and development of +the solar system, by contraction from an original +gaseous nebula in accord with the Newtonian law; +how Sir William Herschel in 1810 saw in all nebulæ +merely the stuff that stars are made of; how Lord +Rosse in 1845 discovered spiral nebulæ; how Helmholtz +in 1854 put forward his contraction theory of +maintenance of the solar heat, seemingly reinforcing +the Laplacian theory; how Lane in 1870 proved +that a contracting gaseous star might rise in temperature; +how Roche in 1873 in attempting to +modify the Laplacian hypothesis, pointed out the +conditions under which a satellite would be broken +up by tidal strains; how Darwin in 1879 showed +that the theory of tidal evolution of non-rigid bodies +might account for the formation of the moon, and +binary stars might originate by fission; how Keeler +in 1900 discovered the vast numbers of spiral nebulæ; +how Chamberlin and Moulton in 1903 put forward +<span class="pagenum"><a name="Page_p381" id="Page_p381">[381]</a></span> +the planetesimal hypothesis of formation of +the spiral nebulæ, showing also how that hypothesis +might account for the evolution of the solar system; +and how Jeans in 1916 advocated the median ground +in evolution of the arms of the spiral nebulæ, showing +that they will break up into nuclei, if +sufficiently massive.</p> + +<p>In all these theories, truth and error, or lack +of complete knowledge, appear to be intermingled +in varying proportions. Is it not early yet to say, +either that any one of them must be abandoned as +totally wrong, or on the other hand that any one of +them, or indeed any single hypothesis, can explain +all the evolutionary processes of the universe?</p> + +<p>Clearly the great problems cannot all be solved by +the kinetic theory of gases and the law of gravitation +alone. Recent physical researches into sub-atomic +energy and the structure and properties of +matter, appear to point in the direction where we +must next look for more light on such questions as +the origin and maintenance of the sun's heat, the +complex phenomena of variable stars and the progressive +evolution of the myriad bodies of the stellar +universe. Because we have actually seen one star +turn into a nebula we should not jump to the conclusion +that all nebulæ are formed from stars, even +if this might seem a direct inference from the high +radial velocities of planetary nebulæ.</p> + +<p>Quite as obviously many of the spiral nebulæ are +in a stage of transition into local universes of stars—even +more obvious from the marvelous photographs +in our day than the evolution of stars from +nebulæ of all types was to Herschel in his day.</p> + +<p>The physicist must further investigate such questions +as the building up of heavy atomic elements +<span class="pagenum"><a name="Page_p382" id="Page_p382">[382]</a></span> +by gravitative condensation of such lighter ones as +compose the nebulæ; and laboratory investigation +must elucidate further the process of development of +energy from atomic disintegration under very high +pressures. This leads to a reclassification of the +stars on a temperature basis.</p> + +<p>Equally important is the inquiry into the mechanism +of radiative equilibrium in sun and stars. +Not impossibly the process of the earth's upper atmosphere +in maintaining a terrestrial equilibrium +may afford some clue. What this physical mechanism +may be is very incompletely known, but it +is now open to further research through recent progress +of aeronautics, which will afford the investigator +a "ceiling" of 50,000 feet and probably more. +Beneath this level, perhaps even below 40,000 feet, +lie all the strata, including the inversion layer, +where the sun's heat is conserved and an equilibrium +maintained.</p> + +<p>Even ten years ago, had an astronomer been +asked about the physical condition of the interior of +the stars, he would have replied that information of +this character could only be had on visiting the +stars themselves—and perhaps not even then. But +at the Cardiff meeting of the British Association in +1920, Eddington, the president of Section A, delivered +an address on the internal constitution of the +stars. He cites the recent investigations of Russell +and others on truly gaseous stars, like Aldebaran, +Arcturus, Antares and Canopus, which are in a +diffuse state and are the most powerful light-givers, +and thus are to be distinguished from the denser +stars like our Sun. The term <i>giants</i> is applied to +the former, and <i>dwarfs</i> to the latter, in accord with +Russell's theory.</p> + +<p><span class="pagenum"><a name="Page_p383" id="Page_p383">[383]</a></span> +As density increases through contraction, these +terms represent the progressive stages, from earlier +to later, in a star's history. A red or M-type star +begins its history as a giant of comparatively low +temperature. Contracting, according to Lane's law, +its temperature must rise until its density becomes +such that it no longer behaves as a perfect gas. +Much depends on the star's mass; but after its +maximum temperature is attained, the star, which +has shrunk to the proportions of a dwarf, goes on +cooling and contracts still further.</p> + +<p>Each temperature-level is reached and passed +twice, once during the ascending stage and once +again in descending—once as a giant, and once as a +dwarf. Thus there are vast differences in luminosity: +the huge giant, having a far larger surface +than the shrunken dwarf, radiates an amount of +light correspondingly greater.</p> + +<p>The physicist recognizes heat in two forms—the +energy of motion of material atoms, and the energy +of ether waves. In hot bodies with which we are +familiar, the second form is quite insignificant; but +in the giant stars, the two forms are present in about +equal proportions. The super-heated conditions of +the interior of the stars can only be estimated in +millions of degrees; and the problem is not one of +convection currents, as formerly thought, bringing +hot masses to the surface from the highly heated +interior, but how can the heat of the interior be +barred against leakage and reduced to the relatively +small radiation emitted by the stars. +"Smaller stars have to manufacture the radiant +heat which they emit, living from hand to mouth; +the giant stars merely leak radiant heat from their +store."</p> + +<p><span class="pagenum"><a name="Page_p384" id="Page_p384">[384]</a></span> +So a radioactive type of equilibrium must be established, +rather than a convective one. Laboratory +investigations of the very short waves are now in +progress, bearing on the transparency of stellar material +to the radiation traversing it; and the penetrating +power of the star's radiation is much like +that of X-rays. The opacity is remarkably high, +explaining why the star is so nearly "heat-tight."</p> + +<p>Opacity being constant, the total radiation of a +giant star depends on its mass only, and is quite +independent of its temperature or state of diffuseness. +So that the total radiation of a star which is +measured roughly by its luminosity, may readily remain +constant during the entire 'giant' stage of its +history. As Russell originally pointed out, giant +stars of every spectral type have nearly the same +luminosity. From the range of luminosity of the +giant stars, then, we may infer their range of +masses: they come out much alike, agreeing well +with results obtained by double-star investigation.</p> + +<p>These studies of radiation and internal condition +of the stars again bring up the question of the original +source of that supply of radiant energy continually +squandered by all self-luminous bodies. +The giant stars are especially prodigal, and radiate +at least a hundredfold faster than the sun.</p> + +<p>"A star is drawing on some vast reservoir of +energy," says Eddington, "by means unknown to +us. This reservoir can scarcely be other than the +sub-atomic energy which, it is known, exists abundantly +in all matter; we sometimes dream that man +will one day learn how to release it and use it for his +service. The store is well-nigh inexhaustible, if only +it could be tapped. There is sufficient in the sun to +maintain its output of heat for fifteen billion years."</p> + + +<hr class="tb" /> + + +<div class="trans_notes"><h2>Transcriber's Notes:</h2> + + +<p>Obvious punctuation errors repaired. Hyphenation and spelling was +standardized by using the most prevalent form.</p> + +<table summary="Typos"> +<tr> + <td class="brdbt2">Page</td> + <td> </td> + <td class="brdbt2">Correction</td> +</tr> +<tr> + <td>20</td> + <td> </td> + <td>Aa ⇒ Aya</td> +</tr> +<tr> + <td>39</td> + <td> </td> + <td>Ulugh Begh ⇒ Ulugh Beg</td> +</tr> +<tr> + <td>46</td> + <td> </td> + <td>Astronomiæ Instaurata Mecanica ⇒ Astronomiæ Instauratæ Mechanica</td> +</tr> +<tr> + <td>58</td> + <td> </td> + <td>Oscillatorium Horologium ⇒ Horologium Oscillatorium</td> +</tr> +<tr> + <td>225</td> + <td> </td> + <td>seceded ⇒ succeeded</td> +</tr> +<tr> + <td>226</td> + <td> </td> + <td>areoplane ⇒ aeroplane</td> +</tr> +<tr> + <td>320</td> + <td> </td> + <td>Plate 2 - Vulpeculæ ⇒ Vulpecula</td> +</tr> +</table> +<br /> +</div> + + + + + + + + + +<pre> + + + + + +End of the Project Gutenberg EBook of Astronomy, by David Todd + +*** END OF THIS PROJECT GUTENBERG EBOOK ASTRONOMY *** + +***** This file should be named 39142-h.htm or 39142-h.zip ***** +This and all associated files of various formats will be found in: + http://www.gutenberg.org/3/9/1/4/39142/ + +Produced by K Nordquist, Tom Cosmas, Brenda Lewis and the +Online Distributed Proofreading Team at http://www.pgdp.net +(This file was produced from images generously made +available by The Internet Archive/American Libraries.) + + +Updated editions will replace the previous one--the old editions +will be renamed. + +Creating the works from public domain print editions means that no +one owns a United States copyright in these works, so the Foundation +(and you!) can copy and distribute it in the United States without +permission and without paying copyright royalties. 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