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diff --git a/8172-h/8172-h.htm b/8172-h/8172-h.htm new file mode 100644 index 0000000..7e1ee00 --- /dev/null +++ b/8172-h/8172-h.htm @@ -0,0 +1,6437 @@ +<!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=utf-8" /> +<meta http-equiv="Content-Style-Type" content="text/css" /> +<title>History of Astronomy, by George Forbes</title> +<link rel="coverpage" href="images/cover.jpg" /> +<style type="text/css"> + +body { margin-left: 20%; + margin-right: 20%; + text-align: justify; } + +h1, h2, h3, h4, h5 {text-align: center; font-style: normal; font-weight: +normal; line-height: 1.5; margin-top: .5em; margin-bottom: .5em;} + +h1 {font-size: 300%; + margin-top: 0.6em; + margin-bottom: 0.6em; + letter-spacing: 0.12em; + word-spacing: 0.2em; + text-indent: 0em;} +h2 {font-size: 150%; margin-top: 2em; margin-bottom: 1em;} +h3 {font-size: 150%; margin-top: 2em;} +h4 {font-size: 120%;} +h5 {font-size: 110%;} + +hr {width: 80%; margin-top: 2em; margin-bottom: 2em;} + +div.chapter {page-break-before: always; margin-top: 4em;} + +p {text-indent: 1em; + margin-top: 0.25em; + margin-bottom: 0.25em; } + +.p2 {margin-top: 2em;} + +p.poem {text-indent: 0%; + margin-left: 10%; + font-size: 90%; + margin-top: 1em; + margin-bottom: 1em; } + +p.letter {text-indent: 0%; + margin-left: 10%; + margin-right: 10%; + margin-top: 1em; + margin-bottom: 1em; } + +p.noindent {text-indent: 0% } + +p.center {text-align: center; + text-indent: 0em; + margin-top: 1em; + margin-bottom: 1em; } + +p.right {text-align: right; + margin-right: 10%; + margin-top: 1em; + margin-bottom: 1em; } + +p.footnote {font-size: 90%; + text-indent: 0%; + margin-left: 0%; + margin-right: 10%; + margin-top: 1em; + margin-bottom: 1em; } + +sup { vertical-align: top; font-size: 0.6em; } + +div.fig { display:block; + margin:0 auto; + text-align:center; + margin-top: 1em; + margin-bottom: 1em;} + +a:link {color:blue; text-decoration:none} +a:visited {color:blue; text-decoration:none} +a:hover {color:red} + +</style> + +</head> + +<body> + +<pre> +The Project Gutenberg EBook of History of Astronomy, by George Forbes + +This eBook is for the use of anyone anywhere in the United States and most +other parts of the world 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. If you are not located in the United States, you'll have +to check the laws of the country where you are located before using this ebook. + +Title: History of Astronomy + +Author: George Forbes + +Release Date: June 25, 2003 [EBook #8172] +[Most recently updated: March 21, 2020] + +Language: English + +Character set encoding: UTF-8 + +*** START OF THIS PROJECT GUTENBERG EBOOK HISTORY OF ASTRONOMY *** + + + + +Produced by Jonathan Ingram, Dave Maddock, Charles Franks +and the Online Distributed Proofreading Team. + + + + + + +</pre> + +<div class="fig" style="width:60%;"> +<a name="illus01"></a> +<img src="images/001.jpg" style="width:100%;" alt="SIR ISAAC NEWTON +(From the bust by Roubiliac In Trinity College, Cambridge.)" /> +<p class="caption">S<small>IR</small> I<small>SAAC</small> +N<small>EWTON</small><br/>(From the bust by Roubiliac In Trinity College, +Cambridge.)</p> +</div> + +<h1>HISTORY OF ASTRONOMY</h1> + +<h3>BY</h3> + +<h2>GEORGE FORBES,<br/> +M.A., F.R.S., M. INST. C. E.,</h2> + +<p><b>(FORMERLY PROFESSOR OF NATURAL PHILOSOPHY, ANDERSON’S +COLLEGE, GLASGOW)</b></p> + +<p>AUTHOR OF “THE TRANSIT OF VENUS,” RENDU’S +“THEORY OF THE GLACIERS OF SAVOY,” ETC., ETC.</p> + +<p><br/><br/></p> + +<h2>CONTENTS</h2> + +<table summary="" style=""> + +<tr> +<td> <a href="#preface">PREFACE</a><br/><br/></td> +</tr> + +<tr> +<td> <a href="#book01"><b>BOOK I. THE GEOMETRICAL PERIOD</b></a></td> +</tr> + +<tr> +<td> <a href="#1">1. PRIMITIVE ASTRONOMY AND ASTROLOGY</a></td> +</tr> + +<tr> +<td> <a href="#2">2. ANCIENT ASTRONOMY—CHINESE AND CHALDÆANS</a></td> +</tr> + +<tr> +<td> <a href="#3">3. ANCIENT GREEK ASTRONOMY</a></td> +</tr> + +<tr> +<td> <a href="#4">4. THE REIGN OF EPICYCLES—FROM PTOLEMY TO COPERNICUS</a><br/><br/></td> +</tr> + +<tr> +<td> <a href="#book02"><b>BOOK II. THE DYNAMICAL PERIOD</b></a></td> +</tr> + +<tr> +<td> <a href="#5">5. DISCOVERY OF THE TRUE SOLAR SYSTEM—TYCHO BRAHE—KEPLER</a></td> +</tr> + +<tr> +<td> <a href="#6">6. GALILEO AND THE TELESCOPE—NOTIONS OF GRAVITY BY HORROCKS, ETC.</a></td> +</tr> + +<tr> +<td> <a href="#7">7. SIR ISAAC NEWTON—LAW OF UNIVERSAL GRAVITATION</a></td> +</tr> + +<tr> +<td> <a href="#8">8. NEWTON’S SUCCESSORS—HALLEY, EULER, LAGRANGE, +LAPLACE, ETC.</a></td> +</tr> + +<tr> +<td> <a href="#9">9. DISCOVERY OF NEW PLANETS—HERSCHEL, PIAZZI, ADAMS, +AND LE VERRIER</a><br/><br/></td> +</tr> + +<tr> +<td> <a href="#book03"><b>BOOK III. OBSERVATION</b></a></td> +</tr> + +<tr> +<td> <a href="#10">10. INSTRUMENTS OF PRECISION—SIZE OF THE SOLAR SYSTEM</a></td> +</tr> + +<tr> +<td> <a href="#11">11. HISTORY OF THE TELESCOPE—SPECTROSCOPE</a><br/><br/></td> +</tr> + +<tr> +<td> <a href="#book04"><b>BOOK IV. THE PHYSICAL PERIOD</b></a></td> +</tr> + +<tr> +<td> <a href="#12">12. THE SUN</a></td> +</tr> + +<tr> +<td> <a href="#13">13. THE MOON AND PLANETS</a></td> +</tr> + +<tr> +<td> <a href="#14">14. COMETS AND METEORS</a></td> +</tr> + +<tr> +<td> <a href="#15">15. THE STARS AND NEBULÆ</a><br/><br/></td> +</tr> + +<tr> +<td> <a href="#16">ILLUSTRATIONS</a></td> +</tr> + +<tr> +<td> <a href="#index">INDEX</a></td> +</tr> +</table> + +<hr /> + +<div class="chapter"> + +<h2><a name="preface"></a>PREFACE</h2> + +<p> +An attempt has been made in these pages to trace the evolution of intellectual +thought in the progress of astronomical discovery, and, by recognising the +different points of view of the different ages, to give due credit even to the +ancients. No one can expect, in a history of astronomy of limited size, to find +a treatise on “practical” or on “theoretical +astronomy,” nor a complete “descriptive astronomy,” and still +less a book on “speculative astronomy.” Something of each of these +is essential, however, for tracing the progress of thought and knowledge which +it is the object of this History to describe. +</p> + +<p> +The progress of human knowledge is measured by the increased habit of looking +at facts from new points of view, as much as by the accumulation of facts. The +mental capacity of one age does not seem to differ from that of other ages; but +it is the imagination of new points of view that gives a wider scope to that +capacity. And this is cumulative, and therefore progressive. Aristotle viewed +the solar system as a geometrical problem; Kepler and Newton converted the +point of view into a dynamical one. Aristotle’s mental capacity to +understand the meaning of facts or to criticise a train of reasoning may have +been equal to that of Kepler or Newton, but the point of view was different. +</p> + +<p> +Then, again, new points of view are provided by the invention of new methods in +that system of logic which we call mathematics. All that mathematics can do is +to assure us that a statement A is equivalent to statements B, C, D, or is one +of the facts expressed by the statements B, C, D; so that we may know, if B, C, +and D are true, then A is true. To many people our inability to understand all +that is contained in statements B, C, and D, without the cumbrous process of a +mathematical demonstration, proves the feebleness of the human mind as a +logical machine. For it required the new point of view imagined by +Newton’s analysis to enable people to see that, so far as planetary +orbits are concerned, Kepler’s three laws (B, C, D) were identical with +Newton’s law of gravitation (A). No one recognises more than the +mathematical astronomer this feebleness of the human intellect, and no one is +more conscious of the limitations of the logical process called mathematics, +which even now has not solved directly the problem of only three bodies. +</p> + +<p> +These reflections, arising from the writing of this History, go to explain the +invariable humility of the great mathematical astronomers. Newton’s +comparison of himself to the child on the seashore applies to them all. As each +new discovery opens up, it may be, boundless oceans for investigation, for +wonder, and for admiration, the great astronomers, refusing to accept mere +hypotheses as true, have founded upon these discoveries a science as exact in +its observation of facts as in theories. So it is that these men, who have +built up the most sure and most solid of all the sciences, refuse to invite +others to join them in vain speculation. The writer has, therefore, in this +short History, tried to follow that great master, Airy, whose pupil he was, and +the key to whose character was exactness and accuracy; and he recognises that +Science is impotent except in her own limited sphere. +</p> + +<p> +It has been necessary to curtail many parts of the History in the +attempt—perhaps a hopeless one—to lay before the reader in a +limited space enough about each age to illustrate its tone and spirit, the +ideals of the workers, the gradual addition of new points of view and of new +means of investigation. +</p> + +<p> +It would, indeed, be a pleasure to entertain the hope that these pages might, +among new recruits, arouse an interest in the greatest of all the sciences, or +that those who have handled the theoretical or practical side might be led by +them to read in the original some of the classics of astronomy. Many students +have much compassion for the schoolboy of to-day, who is not allowed the luxury +of learning the art of reasoning from him who still remains pre-eminently its +greatest exponent, Euclid. These students pity also the man of to-morrow, who +is not to be allowed to read, in the original Latin of the brilliant Kepler, +how he was able—by observations taken from a moving platform, the earth, +of the directions of a moving object, Mars—to deduce the exact shape of +the path of each of these planets, and their actual positions on these paths at +any time. Kepler’s masterpiece is one of the most interesting books that +was ever written, combining wit, imagination, ingenuity, and certainty. +</p> + +<p> +Lastly, it must be noted that, as a History of England cannot deal with the +present Parliament, so also the unfinished researches and untested hypotheses +of many well-known astronomers of to-day cannot be included among the records +of the History of Astronomy. The writer regrets the necessity that thus arises +of leaving without mention the names of many who are now making history in +astronomical work. +</p> + +<p class="right"> +G. F. +</p> + +<p> +<i>August</i> 1<i>st</i>, 1909. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="book01"></a>BOOK I. THE GEOMETRICAL PERIOD</h2> + +</div><!--end chapter--> + +<div class="chapter"> + +<h3><a name="1"></a>1. PRIMITIVE ASTRONOMY AND ASTROLOGY.</h3> + +<p> +The growth of intelligence in the human race has its counterpart in that of the +individual, especially in the earliest stages. Intellectual activity and the +development of reasoning powers are in both cases based upon the accumulation +of experiences, and on the comparison, classification, arrangement, and +nomenclature of these experiences. During the infancy of each the succession of +events can be watched, but there can be no <i>à priori</i> anticipations. +Experience alone, in both cases, leads to the idea of cause and effect as a +principle that seems to dominate our present universe, as a rule for predicting +the course of events, and as a guide to the choice of a course of action. This +idea of cause and effect is the most potent factor in developing the history of +the human race, as of the individual. +</p> + +<p> +In no realm of nature is the principle of cause and effect more conspicuous +than in astronomy; and we fall into the habit of thinking of its laws as not +only being unchangeable in our universe, but necessary to the conception of any +universe that might have been substituted in its place. The first inhabitants +of the world were compelled to accommodate their acts to the daily and annual +alternations of light and darkness and of heat and cold, as much as to the +irregular changes of weather, attacks of disease, and the fortune of war. They +soon came to regard the influence of the sun, in connection with light and +heat, as a cause. This led to a search for other signs in the heavens. If the +appearance of a comet was sometimes noted simultaneously with the death of a +great ruler, or an eclipse with a scourge of plague, these might well be looked +upon as causes in the same sense that the veering or backing of the wind is +regarded as a cause of fine or foul weather. +</p> + +<p> +For these reasons we find that the earnest men of all ages have recorded the +occurrence of comets, eclipses, new stars, meteor showers, and remarkable +conjunctions of the planets, as well as plagues and famines, floods and +droughts, wars and the deaths of great rulers. Sometimes they thought they +could trace connections which might lead them to say that a comet presaged +famine, or an eclipse war. +</p> + +<p> +Even if these men were sometimes led to evolve laws of cause and effect which +now seem to us absurd, let us be tolerant, and gratefully acknowledge that +these astrologers, when they suggested such “working hypotheses,” +were laying the foundations of observation and deduction. +</p> + +<p> +If the ancient Chaldæans gave to the planetary conjunctions an influence over +terrestrial events, let us remember that in our own time people have searched +for connection between terrestrial conditions and periods of unusual prevalence +of sun spots; while De la Rue, Loewy, and Balfour Stewart<a href="#linknote-1" name="linknoteref-1" id="linknoteref-1"><sup>[1]</sup></a> thought they found a connection between sun-spot displays +and the planetary positions. Thus we find scientific men, even in our own time, +responsible for the belief that storms in the Indian Ocean, the fertility of +German vines, famines in India, and high or low Nile-floods in Egypt follow the +planetary positions. +</p> + +<p> +And, again, the desire to foretell the weather is so laudable that we cannot +blame the ancient Greeks for announcing the influence of the moon with as much +confidence as it is affirmed in Lord Wolseley’s <i>Soldier’s Pocket +Book</i>. +</p> + +<p> +Even if the scientific spirit of observation and deduction (astronomy) has +sometimes led to erroneous systems for predicting terrestrial events +(astrology), we owe to the old astronomer and astrologer alike the deepest +gratitude for their diligence in recording astronomical events. For, out of the +scanty records which have survived the destructive acts of fire and flood, of +monarchs and mobs, we have found much that has helped to a fuller knowledge of +the heavenly motions than was possible without these records. +</p> + +<p> +So Hipparchus, about 150 B.C., and Ptolemy a little later, were able to use the +observations of Chaldæan astrologers, as well as those of Alexandrian +astronomers, and to make some discoveries which have helped the progress of +astronomy in all ages. So, also, Mr. Cowell<a href="#linknote-2" name="linknoteref-2" id="linknoteref-2"><sup>[2]</sup></a> has +examined the marks made on the baked bricks used by the Chaldæans for recording +the eclipses of 1062 B.C. and 762 B.C.; and has thereby been enabled, in the +last few years, to correct the lunar tables of Hansen, and to find a more +accurate value for the secular acceleration of the moon’s longitude and +the node of her orbit than any that could be obtained from modern observations +made with instruments of the highest precision. +</p> + +<p> +So again, Mr. Hind<a href="#linknote-3" name="linknoteref-3" id="linknoteref-3"><sup>[3]</sup></a> was enabled to trace back the +period during which Halley’s comet has been a member of the solar system, +and to identify it in the Chinese observations of comets as far back as 12 B.C. +Cowell and Cromellin extended the date to 240 B.C. In the same way the comet +1861.i. has been traced back in the Chinese records to 617 A.D.<a href="#linknote-4" name="linknoteref-4" id="linknoteref-4"><sup>[4]</sup></a> +</p> + +<p> +The theoretical views founded on Newton’s great law of universal +gravitation led to the conclusion that the inclination of the earth’s +equator to the plane of her orbit (the obliquity of the ecliptic) has been +diminishing slowly since prehistoric times; and this fact has been confirmed by +Egyptian and Chinese observations on the length of the shadow of a vertical +pillar, made thousands of years before the Christian era, in summer and winter. +</p> + +<p> +There are other reasons why we must be tolerant of the crude notions of the +ancients. The historian, wishing to give credit wherever it may be due, is met +by two difficulties. Firstly, only a few records of very ancient astronomy are +extant, and the authenticity of many of these is open to doubt. Secondly, it is +very difficult to divest ourselves of present knowledge, and to appreciate the +originality of thought required to make the first beginnings. +</p> + +<p> +With regard to the first point, we are generally dependent upon histories +written long after the events. The astronomy of Egyptians, Babylonians, and +Assyrians is known to us mainly through the Greek historians, and for +information about the Chinese we rely upon the researches of travellers and +missionaries in comparatively recent times. The testimony of the Greek writers +has fortunately been confirmed, and we now have in addition a mass of facts +translated from the original sculptures, papyri, and inscribed bricks, dating +back thousands of years. +</p> + +<p> +In attempting to appraise the efforts of the beginners we must remember that it +was natural to look upon the earth (as all the first astronomers did) as a +circular plane, surrounded and bounded by the heaven, which was a solid vault, +or hemisphere, with its concavity turned downwards. The stars seemed to be +fixed on this vault; the moon, and later the planets, were seen to crawl over +it. It was a great step to look on the vault as a hollow sphere carrying the +sun too. It must have been difficult to believe that at midday the stars are +shining as brightly in the blue sky as they do at night. It must have been +difficult to explain how the sun, having set in the west, could get back to +rise in the east without being seen <i>if</i> it was always the same sun. It +was a great step to suppose the earth to be spherical, and to ascribe the +diurnal motions to its rotation. Probably the greatest step ever made in +astronomical theory was the placing of the sun, moon, and planets at different +distances from the earth instead of having them stuck on the vault of heaven. +It was a transition from “flatland” to a space of three dimensions. +</p> + +<p> +Great progress was made when systematic observations began, such as following +the motion of the moon and planets among the stars, and the inferred motion of +the sun among the stars, by observing their <i>heliacal risings</i>—i.e., +the times of year when a star would first be seen to rise at sunrise, and when +it could last be seen to rise at sunset. The grouping of the stars into +constellations and recording their places was a useful observation. The +theoretical prediction of eclipses of the sun and moon, and of the motions of +the planets among the stars, became later the highest goal in astronomy. +</p> + +<p> +To not one of the above important steps in the progress of astronomy can we +assign the author with certainty. Probably many of them were independently +taken by Chinese, Indian, Persian, Tartar, Egyptian, Babylonian, Assyrian, +Phoenician, and Greek astronomers. And we have not a particle of information +about the discoveries, which may have been great, by other peoples—by the +Druids, the Mexicans, and the Peruvians, for example. +</p> + +<p> +We do know this, that all nations required to have a calendar. The solar year, +the lunar month, and the day were the units, and it is owing to their +incommensurability that we find so many calendars proposed and in use at +different times. The only object to be attained by comparing the chronologies +of ancient races is to fix the actual dates of observations recorded, and this +is not a part of a history of astronomy. +</p> + +<p> +In conclusion, let us bear in mind the limited point of view of the ancients +when we try to estimate their merit. Let us remember that the first astronomy +was of two dimensions; the second astronomy was of three dimensions, but still +purely geometrical. Since Kepler’s day we have had a dynamical astronomy. +</p> + +<hr /> + +<p> +<b>FOOTNOTES:</b> +</p> + +<p class="footnote"> +<a name="linknote-1" id="linknote-1"></a> <a href="#linknoteref-1">[1]</a> +Trans. R. S. E., xxiii. 1864, p. 499, <i>On Sun +Spots</i>, <i>etc</i>., by B. Stewart. Also Trans. R. S. 1860-70. Also Prof. +Ernest Brown, in <i>R. A. S. Monthly Notices</i>, 1900. +</p> + +<p class="footnote"> +<a name="linknote-2" id="linknote-2"></a> <a href="#linknoteref-2">[2]</a> +<i>R. A. S. Monthly Notices</i>, Sup.; 1905. +</p> + +<div class="fig" style="width:60%;"> +<a name="illus02"></a> +<img src="images/002.jpg" style="width:100%;" alt="CHALDÆAN BAKED BRICK OR +TABLET" /> +<p class="caption">C<small>HALDÆAN</small> B<small>AKED</small> B<small>RICK +OR</small> T<small>ABLET</small>,<br/> +<i>Obverse and reverse sides</i>,<br/> +Containing record of solar eclipse, 1062 B.C., used lately by Cowell for +rendering the lunar theory more accurate than was possible by finest modern +observations. (British Museum collection, No. 35908.) +</p> +</div> + +<p class="footnote"> +<a name="linknote-3" id="linknote-3"></a> <a href="#linknoteref-3">[3]</a> +<i>R. A. S. Monthly Notices</i>, vol. x., p. 65. +</p> + +<p class="footnote"> +<a name="linknote-4" id="linknote-4"></a> <a href="#linknoteref-4">[4]</a> +R. S. E. Proc., vol. x., 1880. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h3><a name="2"></a>2. ANCIENT ASTRONOMY—THE CHINESE AND CHALDÆANS.</h3> + +<p> +The last section must have made clear the difficulties the way of assigning to +the ancient nations their proper place in the development of primitive notions +about astronomy. The fact that some alleged observations date back to a period +before the Chinese had invented the art of writing leads immediately to the +question how far tradition can be trusted. +</p> + +<p> +Our first detailed knowledge was gathered in the far East by travellers, and by +the Jesuit priests, and was published in the eighteenth century. The Asiatic +Society of Bengal contributed translations of Brahmin literature. The two +principal sources of knowledge about Chinese astronomy were supplied, first by +Father Souciet, who in 1729 published <i>Observations Astronomical, +Geographical, Chronological, and Physical</i>, drawn from ancient Chinese +books; and later by Father Moyriac-de-Mailla, who in 1777-1785 published +<i>Annals of the Chinese Empire, translated from Tong-Kien-Kang-Mou</i>. +</p> + +<p> +Bailly, in his <i>Astronomie Ancienne</i> (1781), drew, from these and other +sources, the conclusion that all we know of the astronomical learning of the +Chinese, Indians, Chaldæans, Assyrians, and Egyptians is but the remnant of a +far more complete astronomy of which no trace can be found. +</p> + +<p> +Delambre, in his <i>Histoire de l’Astronomie Ancienne</i> (1817), +ridicules the opinion of Bailly, and considers that the progress made by all of +these nations is insignificant. +</p> + +<p> +It will be well now to give an idea of some of the astronomy of the ancients +not yet entirely discredited. China and Babylon may be taken as typical +examples. +</p> + +<p> +<i>China</i>.—It would appear that Fohi, the first emperor, reigned about +2952 B.C., and shortly afterwards Yu-Chi made a sphere to represent the motions +of the celestial bodies. It is also mentioned, in the book called Chu-King, +supposed to have been written in 2205 B.C., that a similar sphere was made in +the time of Yao (2357 B.C.).<a href="#linknote-5" name="linknoteref-5" id="linknoteref-5"><sup>[1]</sup></a> It is said that the +Emperor Chueni (2513 B.C.) saw five planets in conjunction the same day that +the sun and moon were in conjunction. This is discussed by Father Martin (MSS. +of De Lisle); also by M. Desvignolles (Mem. Acad. Berlin, vol. iii., p. 193), +and by M. Kirsch (ditto, vol. v., p. 19), who both found that Mars, Jupiter, +Saturn, and Mercury were all between the eleventh and eighteenth degrees of +Pisces, all visible together in the evening on February 28th 2446 B.C., while +on the same day the sun and moon were in conjunction at 9 a.m., and that on +March 1st the moon was in conjunction with the other four planets. But this +needs confirmation. +</p> + +<p> +Yao, referred to above, gave instructions to his astronomers to determine the +positions of the solstices and equinoxes, and they reported the names of the +stars in the places occupied by the sun at these seasons, and in 2285 B.C. he +gave them further orders. If this account be true, it shows a knowledge that +the vault of heaven is a complete sphere, and that stars are shining at +mid-day, although eclipsed by the sun’s brightness. +</p> + +<p> +It is also asserted, in the book called <i>Chu-King</i>, that in the time of +Yao the year was known to have 365¼ days, and that he adopted 365 days and +added an intercalary day every four years (as in the Julian Calendar). This may +be true or not, but the ancient Chinese certainly seem to have divided the +circle into 365 degrees. To learn the length of the year needed only patient +observation—a characteristic of the Chinese; but many younger nations got +into a terrible mess with their calendar from ignorance of the year’s +length. +</p> + +<p> +It is stated that in 2159 B.C. the royal astronomers Hi and Ho failed to +predict an eclipse. It probably created great terror, for they were executed in +punishment for their neglect. If this account be true, it means that in the +twenty-second century B.C. some rule for calculating eclipses was in use. Here, +again, patient observation would easily lead to the detection of the +eighteen-year cycle known to the Chaldeans as the <i>Saros</i>. It consists of +235 lunations, and in that time the pole of the moon’s orbit revolves +just once round the pole of the ecliptic, and for this reason the eclipses in +one cycle are repeated with very slight modification in the next cycle, and so +on for many centuries. +</p> + +<p> +It may be that the neglect of their duties by Hi and Ho, and their punishment, +influenced Chinese astronomy; or that the succeeding records have not been +available to later scholars; but the fact remains that—although at long +intervals observations were made of eclipses, comets, and falling stars, and of +the position of the solstices, and of the obliquity of the +ecliptic—records become rare, until 776 B.C., when eclipses began to be +recorded once more with some approach to continuity. Shortly afterwards notices +of comets were added. Biot gave a list of these, and Mr. John Williams, in +1871, published <i>Observations of Comets from 611 B.C. to 1640 A.D., Extracted +from the Chinese Annals</i>. +</p> + +<p> +With regard to those centuries concerning which we have no astronomical Chinese +records, it is fair to state that it is recorded that some centuries before the +Christian era, in the reign of Tsin-Chi-Hoang, all the classical and scientific +books that could be found were ordered to be destroyed. If true, our loss +therefrom is as great as from the burning of the Alexandrian library by the +Caliph Omar. He burnt all the books because he held that they must be either +consistent or inconsistent with the Koran, and in the one case they were +superfluous, in the other case objectionable. +</p> + +<p> +<i>Chaldæans</i>.—Until the last half century historians were accustomed +to look back upon the Greeks, who led the world from the fifth to the third +century B.C., as the pioneers of art, literature, and science. But the +excavations and researches of later years make us more ready to grant that in +science as in art the Greeks only developed what they derived from the +Egyptians, Babylonians, and Assyrians. The Greek historians said as much, in +fact; and modern commentators used to attribute the assertion to undue modesty. +Since, however, the records of the libraries have been unearthed it has been +recognised that the Babylonians were in no way inferior in the matter of +original scientific investigation to other races of the same era. +</p> + +<p> +The Chaldæans, being the most ancient Babylonians, held the same station and +dignity in the State as did the priests in Egypt, and spent all their time in +the study of philosophy and astronomy, and the arts of divination and +astrology. They held that the world of which we have a conception is an eternal +world without any beginning or ending, in which all things are ordered by rules +supported by a divine providence, and that the heavenly bodies do not move by +chance, nor by their own will, but by the determinate will and appointment of +the gods. They recorded these movements, but mainly in the hope of tracing the +will of the gods in mundane affairs. Ptolemy (about 130 A.D.) made use of +Babylonian eclipses in the eighth century B.C. for improving his solar and +lunar tables. +</p> + +<p> +Fragments of a library at Agade have been preserved at Nineveh, from which we +learn that the star-charts were even then divided into constellations, which +were known by the names which they bear to this day, and that the signs of the +zodiac were used for determining the courses of the sun, moon, and of the five +planets Mercury, Venus, Mars, Jupiter, and Saturn. +</p> + +<p> +We have records of observations carried on under Asshurbanapal, who sent +astronomers to different parts to study celestial phenomena. Here is +one:— +</p> + +<p> +To the Director of Observations,—My Lord, his humble servant +Nabushum-iddin, Great Astronomer of Nineveh, writes thus: “May Nabu and +Marduk be propitious to the Director of these Observations, my Lord. The +fifteenth day we observed the Node of the moon, and the moon was +eclipsed.” +</p> + +<p> +The Phoenicians are supposed to have used the stars for navigation, but there +are no records. The Egyptian priests tried to keep such astronomical knowledge +as they possessed to themselves. It is probable that they had arbitrary rules +for predicting eclipses. All that was known to the Greeks about Egyptian +science is to be found in the writings of Diodorus Siculus. But confirmatory +and more authentic facts have been derived from late explorations. Thus we +learn from E. B. Knobel<a href="#linknote-6" name="linknoteref-6" id="linknoteref-6"><sup>[2]</sup></a> about the Jewish calendar +dates, on records of land sales in Aramaic papyri at Assuan, translated by +Professor A. H. Sayce and A. E. Cowley, (1) that the lunar cycle of nineteen +years was used by the Jews in the fifth century B.C. [the present reformed +Jewish calendar dating from the fourth century A.D.], a date a “little +more than a century after the grandfathers and great-grandfathers of those +whose business is recorded had fled into Egypt with Jeremiah” (Sayce); +and (2) that the order of intercalation at that time was not dissimilar to that +in use at the present day. +</p> + +<p> +Then again, Knobel reminds us of “the most interesting discovery a few +years ago by Father Strassmeier of a Babylonian tablet recording a partial +lunar eclipse at Babylon in the seventh year of Cambyses, on the fourteenth day +of the Jewish month Tammuz.” Ptolemy, in the Almagest (Suntaxis), says it +occurred in the seventh year of Cambyses, on the night of the seventeenth and +eighteenth of the Egyptian month Phamenoth. Pingré and Oppolzer fix the +date July 16th, 533 B.C. Thus are the relations of the chronologies of Jews and +Egyptians established by these explorations. +</p> + +<hr /> + +<p> +<b>FOOTNOTES:</b> +</p> + +<p class="footnote"> +<a name="linknote-5" id="linknote-5"></a> <a href="#linknoteref-5">[1]</a> +These ancient dates are uncertain. +</p> + +<p class="footnote"> +<a name="linknote-6" id="linknote-6"></a> <a href="#linknoteref-6">[2]</a> +<i>R. A. S. Monthly Notices</i>, vol. lxviii., No. 5, March, 1908. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h3><a name="3"></a>3. ANCIENT GREEK ASTRONOMY.</h3> + +<p> +We have our information about the earliest Greek astronomy from Herodotus (born +480 B.C.). He put the traditions into writing. Thales (639-546 B.C.) is said to +have predicted an eclipse, which caused much alarm, and ended the battle +between the Medes and Lydians. Airy fixed the date May 28th, 585 B.C. But other +modern astronomers give different dates. Thales went to Egypt to study science, +and learnt from its priests the length of the year (which was kept a profound +secret!), and the signs of the zodiac, and the positions of the solstices. He +held that the sun, moon, and stars are not mere spots on the heavenly vault, +but solids; that the moon derives her light from the sun, and that this fact +explains her phases; that an eclipse of the moon happens when the earth cuts +off the sun’s light from her. He supposed the earth to be flat, and to +float upon water. He determined the ratio of the sun’s diameter to its +orbit, and apparently made out the diameter correctly as half a degree. He left +nothing in writing. +</p> + +<p> +His successors, Anaximander (610-547 B.C.) and Anaximenes (550-475 B.C.), held +absurd notions about the sun, moon, and stars, while Heraclitus (540-500 B.C.) +supposed that the stars were lighted each night like lamps, and the sun each +morning. Parmenides supposed the earth to be a sphere. +</p> + +<p> +Pythagoras (569-470 B.C.) visited Egypt to study science. He deduced his +system, in which the earth revolves in an orbit, from fantastic first +principles, of which the following are examples: “The circular motion is +the most perfect motion,” “Fire is more worthy than earth,” +“Ten is the perfect number.” He wrote nothing, but is supposed to +have said that the earth, moon, five planets, and fixed stars all revolve round +the sun, which itself revolves round an imaginary central fire called the +Antichthon. Copernicus in the sixteenth century claimed Pythagoras as the +founder of the system which he, Copernicus, revived. +</p> + +<p> +Anaxagoras (born 499 B.C.) studied astronomy in Egypt. He explained the return +of the sun to the east each morning by its going under the flat earth in the +night. He held that in a solar eclipse the moon hides the sun, and in a lunar +eclipse the moon enters the earth’s shadow—both excellent opinions. +But he entertained absurd ideas of the vortical motion of the heavens whisking +stones into the sky, there to be ignited by the fiery firmament to form stars. +He was prosecuted for this unsettling opinion, and for maintaining that the +moon is an inhabited earth. He was defended by Pericles (432 B.C.). +</p> + +<p> +Solon dabbled, like many others, in reforms of the calendar. The common year of +the Greeks originally had 360 days—twelve months of thirty days. +Solon’s year was 354 days. It is obvious that these erroneous years +would, before long, remove the summer to January and the winter to July. To +prevent this it was customary at regular intervals to intercalate days or +months. Meton (432 B.C.) introduced a reform based on the nineteen-year cycle. +This is not the same as the Egyptian and Chaldean eclipse cycle called +<i>Saros</i> of 223 lunations, or a little over eighteen years. The Metonic +cycle is 235 lunations or nineteen years, after which period the sun and moon +occupy the same position relative to the stars. It is still used for fixing the +date of Easter, the number of the year in Melon’s cycle being the golden +number of our prayer-books. Melon’s system divided the 235 lunations into +months of thirty days and omitted every sixty-third day. Of the nineteen years, +twelve had twelve months and seven had thirteen months. +</p> + +<p> +Callippus (330 B.C.) used a cycle four times as long, 940 lunations, but one +day short of Melon’s seventy-six years. This was more correct. +</p> + +<p> +Eudoxus (406-350 B.C.) is said to have travelled with Plato in Egypt. He made +astronomical observations in Asia Minor, Sicily, and Italy, and described the +starry heavens divided into constellations. His name is connected with a +planetary theory which as generally stated sounds most fanciful. He imagined +the fixed stars to be on a vault of heaven; and the sun, moon, and planets to +be upon similar vaults or spheres, twenty-six revolving spheres in all, the +motion of each planet being resolved into its components, and a separate sphere +being assigned for each component motion. Callippus (330 B.C.) increased the +number to thirty-three. It is now generally accepted that the real existence of +these spheres was not suggested, but the idea was only a mathematical +conception to facilitate the construction of tables for predicting the places +of the heavenly bodies. +</p> + +<p> +Aristotle (384-322 B.C.) summed up the state of astronomical knowledge in his +time, and held the earth to be fixed in the centre of the world. +</p> + +<p> +Nicetas, Heraclides, and Ecphantes supposed the earth to revolve on its axis, +but to have no orbital motion. +</p> + +<p> +The short epitome so far given illustrates the extraordinary deductive methods +adopted by the ancient Greeks. But they went much farther in the same +direction. They seem to have been in great difficulty to explain how the earth +is supported, just as were those who invented the myth of Atlas, or the Indians +with the tortoise. Thales thought that the flat earth floated on water. +Anaxagoras thought that, being flat, it would be buoyed up and supported on the +air like a kite. Democritus thought it remained fixed, like the donkey between +two bundles of hay, because it was equidistant from all parts of the containing +sphere, and there was no reason why it should incline one way rather than +another. Empedocles attributed its state of rest to centrifugal force by the +rapid circular movement of the heavens, as water is stationary in a pail when +whirled round by a string. Democritus further supposed that the inclination of +the flat earth to the ecliptic was due to the greater weight of the southern +parts owing to the exuberant vegetation. +</p> + +<p> +For further references to similar efforts of imagination the reader is referred +to Sir George Cornwall Lewis’s <i>Historical Survey of the Astronomy of +the Ancients</i>; London, 1862. His list of authorities is very complete, but +some of his conclusions are doubtful. At p. 113 of that work he records the +real opinions of Socrates as set forth by Xenophon; and the reader will, +perhaps, sympathise with Socrates in his views on contemporary +astronomy:— +</p> + +<p> +With regard to astronomy he [Socrates] considered a knowledge of it desirable +to the extent of determining the day of the year or month, and the hour of the +night, ... but as to learning the courses of the stars, to be occupied with the +planets, and to inquire about their distances from the earth, and their orbits, +and the causes of their motions, he strongly objected to such a waste of +valuable time. He dwelt on the contradictions and conflicting opinions of the +physical philosophers, ... and, in fine, he held that the speculators on the +universe and on the laws of the heavenly bodies were no better than madmen +(<i>Xen. Mem</i>, i. 1, 11-15). +</p> + +<p> +Plato (born 429 B.C.), the pupil of Socrates, the fellow-student of Euclid, and +a follower of Pythagoras, studied science in his travels in Egypt and +elsewhere. He was held in so great reverence by all learned men that a problem +which he set to the astronomers was the keynote to all astronomical +investigation from this date till the time of Kepler in the sixteenth century. +He proposed to astronomers <i>the problem of representing the courses of the +planets by circular and uniform motions</i>. +</p> + +<p> +Systematic observation among the Greeks began with the rise of the Alexandrian +school. Aristillus and Timocharis set up instruments and fixed the positions of +the zodiacal stars, near to which all the planets in their orbits pass, thus +facilitating the determination of planetary motions. Aristarchus (320-250 B.C.) +showed that the sun must be at least nineteen times as far off as the moon, +which is far short of the mark. He also found the sun’s diameter, +correctly, to be half a degree. Eratosthenes (276-196 B.C.) measured the +inclination to the equator of the sun’s apparent path in the +heavens—i.e., he measured the obliquity of the ecliptic, making it +23° 51’, confirming our knowledge of its continuous diminution +during historical times. He measured an arc of meridian, from Alexandria to +Syene (Assuan), and found the difference of latitude by the length of a shadow +at noon, summer solstice. He deduced the diameter of the earth, 250,000 stadia. +Unfortunately, we do not know the length of the stadium he used. +</p> + +<p> +Hipparchus (190-120 B.C.) may be regarded as the founder of observational +astronomy. He measured the obliquity of the ecliptic, and agreed with +Eratosthenes. He altered the length of the tropical year from 365 days, 6 hours +to 365 days, 5 hours, 53 minutes—still four minutes too much. He measured +the equation of time and the irregular motion of the sun; and allowed for this +in his calculations by supposing that the centre, about which the sun moves +uniformly, is situated a little distance from the fixed earth. He called this +point the <i>excentric</i>. The line from the earth to the +“excentric” was called the <i>line of apses</i>. A circle having +this centre was called the <i>equant</i>, and he supposed that a radius drawn +to the sun from the excentric passes over equal arcs on the equant in equal +times. He then computed tables for predicting the place of the sun. +</p> + +<p> +He proceeded in the same way to compute Lunar tables. Making use of Chaldæan +eclipses, he was able to get an accurate value of the moon’s mean motion. +[Halley, in 1693, compared this value with his own measurements, and so +discovered the acceleration of the moon’s mean motion. This was +conclusively established, but could not be explained by the Newtonian theory +for quite a long time.] He determined the plane of the moon’s orbit and +its inclination to the ecliptic. The motion of this plane round the pole of the +ecliptic once in eighteen years complicated the problem. He located the +moon’s excentric as he had done the sun’s. He also discovered some +of the minor irregularities of the moon’s motion, due, as Newton’s +theory proves, to the disturbing action of the sun’s attraction. +</p> + +<p> +In the year 134 B.C. Hipparchus observed a new star. This upset every notion +about the permanence of the fixed stars. He then set to work to catalogue all +the principal stars so as to know if any others appeared or disappeared. Here +his experiences resembled those of several later astronomers, who, when in +search of some special object, have been rewarded by a discovery in a totally +different direction. On comparing his star positions with those of Timocharis +and Aristillus he found no stars that had appeared or disappeared in the +interval of 150 years; but he found that all the stars seemed to have changed +their places with reference to that point in the heavens where the ecliptic is +90° from the poles of the earth—i.e., the equinox. He found that +this could be explained by a motion of the equinox in the direction of the +apparent diurnal motion of the stars. This discovery of <i>precession of the +equinoxes</i>, which takes place at the rate of 52".1 every year, was necessary +for the progress of accurate astronomical observations. It is due to a steady +revolution of the earth’s pole round the pole of the ecliptic once in +26,000 years in the opposite direction to the planetary revolutions. +</p> + +<p> +Hipparchus was also the inventor of trigonometry, both plane and spherical. He +explained the method of using eclipses for determining the longitude. +</p> + +<p> +In connection with Hipparchus’ great discovery it may be mentioned that +modern astronomers have often attempted to fix dates in history by the effects +of precession of the equinoxes. (1) At about the date when the Great Pyramid +may have been built γ Draconis was near to the pole, and must have been +used as the pole-star. In the north face of the Great Pyramid is the entrance +to an inclined passage, and six of the nine pyramids at Gizeh possess the same +feature; all the passages being inclined at an angle between 26° and +27° to the horizon and in the plane of the meridian. It also appears that +4,000 years ago—i.e., about 2100 B.C.—an observer at the lower end +of the passage would be able to see γ Draconis, the then pole-star, at its +lower culmination.<a href="#linknote-7" name="linknoteref-7" id="linknoteref-7"><sup>[1]</sup></a> It has been suggested that the +passage was made for this purpose. On other grounds the date assigned to the +Great Pyramid is 2123 B.C. +</p> + +<p> +(2) The Chaldæans gave names to constellations now invisible from Babylon which +would have been visible in 2000 B.C., at which date it is claimed that these +people were studying astronomy. +</p> + +<p> +(3) In the Odyssey, Calypso directs Odysseus, in accordance with Phoenician +rules for navigating the Mediterranean, to keep the Great Bear “ever on +the left as he traversed the deep” when sailing from the pillars of +Hercules (Gibraltar) to Corfu. Yet such a course taken now would land the +traveller in Africa. Odysseus is said in his voyage in springtime to have seen +the Pleiades and Arcturus setting late, which seemed to early commentators a +proof of Homer’s inaccuracy. Likewise Homer, both in the <i>Odyssey</i><a href="#linknote-8" name="linknoteref-8" id="linknoteref-8"><sup>[2]</sup></a> +(v. 272-5) and in the <i>Iliad</i> (xviii. 489), +asserts that the Great Bear never set in those latitudes. Now it has been found +that the precession of the equinoxes explains all these puzzles; shows that in +springtime on the Mediterranean the Bear was just above the horizon, near the +sea but not touching it, between 750 B.C. and 1000 B.C.; and fixes the date of +the poems, thus confirming other evidence, and establishing Homer’s +character for accuracy.<a href="#linknote-9" name="linknoteref-9" id="linknoteref-9"><sup>[3]</sup></a> +</p> + +<p> +(4) The orientation of Egyptian temples and Druidical stones is such that +possibly they were so placed as to assist in the observation of the heliacal +risings<a href="#linknote-10" name="linknoteref-10" id="linknoteref-10"><sup>[4]</sup></a> of certain stars. If the star were known, this +would give an approximate date. Up to the present the results of these +investigations are far from being conclusive. +</p> + +<p> +Ptolemy (130 A.D.) wrote the Suntaxis, or Almagest, which includes a cyclopedia +of astronomy, containing a summary of knowledge at that date. We have no +evidence beyond his own statement that he was a practical observer. He +theorised on the planetary motions, and held that the earth is fixed in the +centre of the universe. He adopted the excentric and equant of Hipparchus to +explain the unequal motions of the sun and moon. He adopted the epicycles and +deferents which had been used by Apollonius and others to explain the +retrograde motions of the planets. We, who know that the earth revolves round +the sun once in a year, can understand that the apparent motion of a planet is +only its motion relative to the earth. If, then, we suppose the earth fixed and +the sun to revolve round it once a year, and the planets each in its own +period, it is only necessary to impose upon each of these an additional +<i>annual</i> motion to enable us to represent truly the apparent motions. This +way of looking at the apparent motions shows why each planet, when nearest to +the earth, seems to move for a time in a retrograde direction. The attempts of +Ptolemy and others of his time to explain the retrograde motion in this way +were only approximate. Let us suppose each planet to have a bar with one end +centred at the earth. If at the other end of the bar one end of a shorter bar +is pivotted, having the planet at its other end, then the planet is given an +annual motion in the secondary circle (the epicycle), whose centre revolves +round the earth on the primary circle (the <i>deferent</i>), at a uniform rate +round the excentric. Ptolemy supposed the centres of the epicycles of Mercury +and Venus to be on a bar passing through the sun, and to be between the earth +and the sun. The centres of the epicycles of Mars, Jupiter, and Saturn were +supposed to be further away than the sun. Mercury and Venus were supposed to +revolve in their epicycles in their own periodic times and in the deferent +round the earth in a year. The major planets were supposed to revolve in the +deferent round the earth in their own periodic times, and in their epicycles +once in a year. +</p> + +<p> +It did not occur to Ptolemy to place the centres of the epicycles of Mercury +and Venus at the sun, and to extend the same system to the major planets. +Something of this sort had been proposed by the Egyptians (we are told by +Cicero and others), and was accepted by Tycho Brahe; and was as true a +representation of the relative motions in the solar system as when we suppose +the sun to be fixed and the earth to revolve. +</p> + +<p> +The cumbrous system advocated by Ptolemy answered its purpose, enabling him to +predict astronomical events approximately. He improved the lunar theory +considerably, and discovered minor inequalities which could be allowed for by +the addition of new epicycles. We may look upon these epicycles of Apollonius, +and the excentric of Hipparchus, as the responses of these astronomers to the +demand of Plato for uniform circular motions. Their use became more and more +confirmed, until the seventeenth century, when the accurate observations of +Tycho Brahe enabled Kepler to abolish these purely geometrical makeshifts, and +to substitute a system in which the sun became physically its controller. +</p> + +<hr /> + +<p> +<b>FOOTNOTES:</b> +</p> + +<p class="footnote"> +<a name="linknote-7" id="linknote-7"></a> <a href="#linknoteref-7">[1]</a> +<i>Phil. Mag</i>., vol. xxiv., pp. 481-4. +</p> + +<p class="footnote"> +<a name="linknote-8" id="linknote-8"></a> <a href="#linknoteref-8">[2]</a> +<br/> +Plaeiadas t’ esoronte kai opse duonta bootaen<br/> +‘Arkton th’ aen kai amaxan epiklaesin kaleousin,<br/> +‘Ae t’ autou strephetai kai t’ Oriona dokeuei,<br/> +Oin d’ammoros esti loetron Okeanoio.<br/> +<br/> +“The Pleiades and Boötes that setteth late, and the Bear, which they +likewise call the Wain, which turneth ever in one place, and keepeth watch upon +Orion, and alone hath no part in the baths of the ocean.” +</p> + +<p class="footnote"> +<a name="linknote-9" id="linknote-9"></a> <a href="#linknoteref-9">[3]</a> +See Pearson in the Camb. Phil. Soc. Proc., vol. iv., +pt. ii., p. 93, on whose authority the above statements are made. +</p> + +<p class="footnote"> +<a name="linknote-10" id="linknote-10"></a> <a href="#linknoteref-10">[4]</a> +See p. 6 for definition. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h3><a name="4"></a>4. THE REIGN OF EPICYCLES—FROM PTOLEMY TO +COPERNICUS.</h3> + +<p> +After Ptolemy had published his book there seemed to be nothing more to do for +the solar system except to go on observing and finding more and more accurate +values for the constants involved--viz., the periods of revolution, the +diameter of the deferent,<a href="#linknote-11" name="linknoteref-11" id="linknoteref-11"><sup>[1]</sup></a> and its ratio to that of the +epicycle,<a href="#linknote-12" name="linknoteref-12" id="linknoteref-12"><sup>[2]</sup></a> the distance of the excentric<a href="#linknote-13" name="linknoteref-13" id="linknoteref-13"><sup>[3]</sup></a> from the centre of the deferent, and the position of the +line of apses,<a href="#linknote-14" name="linknoteref-14" id="linknoteref-14"><sup>[4]</sup></a> besides the inclination and position of +the plane of the planet’s orbit. The only object ever aimed at in those +days was to prepare tables for predicting the places of the planets. It was not +a mechanical problem; there was no notion of a governing law of forces. +</p> + +<p> +From this time onwards all interest in astronomy seemed, in Europe at least, to +sink to a low ebb. When the Caliph Omar, in the middle of the seventh century, +burnt the library of Alexandria, which had been the centre of intellectual +progress, that centre migrated to Baghdad, and the Arabs became the leaders of +science and philosophy. In astronomy they made careful observations. In the +middle of the ninth century Albategnius, a Syrian prince, improved the value of +excentricity of the sun’s orbit, observed the motion of the moon’s +apse, and thought he detected a smaller progression of the sun’s apse. +His tables were much more accurate than Ptolemy’s. Abul Wefa, in the +tenth century, seems to have discovered the moon’s +“variation.” Meanwhile the Moors were leaders of science in the +west, and Arzachel of Toledo improved the solar tables very much. Ulugh Begh, +grandson of the great Tamerlane the Tartar, built a fine observatory at +Samarcand in the fifteenth century, and made a great catalogue of stars, the +first since the time of Hipparchus. +</p> + +<p> +At the close of the fifteenth century King Alphonso of Spain employed computers +to produce the Alphonsine Tables (1488 A.D.), Purbach translated +Ptolemy’s book, and observations were carried out in Germany by +Müller, known as Regiomontanus, and Waltherus. +</p> + +<p> +Nicolai Copernicus, a Sclav, was born in 1473 at Thorn, in Polish Prussia. He +studied at Cracow and in Italy. He was a priest, and settled at Frauenberg. He +did not undertake continuous observations, but devoted himself to simplifying +the planetary systems and devising means for more accurately predicting the +positions of the sun, moon, and planets. He had no idea of framing a solar +system on a dynamical basis. His great object was to increase the accuracy of +the calculations and the tables. The results of his cogitations were printed +just before his death in an interesting book, <i>De Revolutionibus Orbium +Celestium</i>. It is only by careful reading of this book that the true +position of Copernicus can be realised. He noticed that Nicetas and others had +ascribed the apparent diurnal rotation of the heavens to a real daily rotation +of the earth about its axis, in the opposite direction to the apparent motion +of the stars. Also in the writings of Martianus Capella he learnt that the +Egyptians had supposed Mercury and Venus to revolve round the sun, and to be +carried with him in his annual motion round the earth. He noticed that the same +supposition, if extended to Mars, Jupiter, and Saturn, would explain easily why +they, and especially Mars, seem so much brighter in opposition. For Mars would +then be a great deal nearer to the earth than at other times. It would also +explain the retrograde motion of planets when in opposition. +</p> + +<p> +We must here notice that at this stage Copernicus was actually confronted with +the system accepted later by Tycho Brahe, with the earth fixed. But he now +recalled and accepted the views of Pythagoras and others, according to which +the sun is fixed and the earth revolves; and it must be noted that, +geometrically, there is no difference of any sort between the Egyptian or +Tychonic system and that of Pythagoras as revived by Copernicus, except that on +the latter theory the stars ought to seem to move when the earth changes its +position—a test which failed completely with the rough means of +observation then available. The radical defect of all solar systems previous to +the time of Kepler (1609 A.D.) was the slavish yielding to Plato’s dictum +demanding uniform circular motion for the planets, and the consequent evolution +of the epicycle, which was fatal to any conception of a dynamical theory. +</p> + +<p> +Copernicus could not sever himself from this obnoxious tradition.<a href="#linknote-15" name="linknoteref-15" id="linknoteref-15"><sup>[5]</sup></a> It is true that neither the Pythagorean nor the +Egypto-Tychonic system required epicycles for explaining retrograde motion, as +the Ptolemaic theory did. Furthermore, either system could use the excentric of +Hipparchus to explain the irregular motion known as the equation of the centre. +But Copernicus remarked that he could also use an epicycle for this purpose, or +that he could use both an excentric and an epicycle for each planet, and so +bring theory still closer into accord with observation. And this he proceeded +to do.<a href="#linknote-16" name="linknoteref-16" id="linknoteref-16"><sup>[6]</sup></a> Moreover, observers had found irregularities in +the moon’s motion, due, as we now know, to the disturbing attraction of +the sun. To correct for these irregularities Copernicus introduced epicycle on +epicycle in the lunar orbit. +</p> + +<p> +This is in its main features the system propounded by Copernicus. But attention +must, to state the case fully, be drawn to two points to be found in his first +and sixth books respectively. The first point relates to the seasons, and it +shows a strange ignorance of the laws of rotating bodies. To use the words of +Delambre,<a href="#linknote-17" name="linknoteref-17" id="linknoteref-17"><sup>[7]</sup></a> in drawing attention to the strange +conception, +</p> + +<p class="letter"> he imagined that the earth, revolving round the sun, ought +always to show to it the same face; the contrary phenomena surprised him: to +explain them he invented a third motion, and added it to the two real motions +(rotation and orbital revolution). By this third motion the earth, he held, +made a revolution on itself and on the poles of the ecliptic once a year ... +Copernicus did not know that motion in a straight line is the natural motion, +and that motion in a curve is the resultant of several movements. He believed, +with Aristotle, that circular motion was the natural one. +</p> + +<p> +Copernicus made this rotation of the earth’s axis about the pole of the +ecliptic retrograde (i.e., opposite to the orbital revolution), and by making +it perform more than one complete revolution in a year, the added part being +1/26000 of the whole, he was able to include the precession of the equinoxes in +his explanation of the seasons. His explanation of the seasons is given on leaf +10 of his book (the pages of this book are not all numbered, only alternate +pages, or leaves). +</p> + +<p> +In his sixth book he discusses the inclination of the planetary orbits to the +ecliptic. In regard to this the theory of Copernicus is unique; and it will be +best to explain this in the words of Grant in his great work.<a href="#linknote-18" name="linknoteref-18" id="linknoteref-18"><sup>[8]</sup></a> He says:— +</p> + +<p class="letter"> Copernicus, as we have already remarked, did not attack the +principle of the epicyclical theory: he merely sought to make it more simple by +placing the centre of the earth’s orbit in the centre of the universe. +This was the point to which the motions of the planets were referred, for the +planes of their orbits were made to pass through it, and their points of least +and greatest velocities were also determined with reference to it. By this +arrangement the sun was situate mathematically near the centre of the planetary +system, but he did not appear to have any physical connexion with the planets +as the centre of their motions. +</p> + +<p> +According to Copernicus’ sixth book, the planes of the planetary orbits +do not pass through the sun, and the lines of apses do not pass through to the +sun. +</p> + +<p> +Such was the theory advanced by Copernicus: The earth moves in an epicycle, on +a deferent whose centre is a little distance from the sun. The planets move in +a similar way on epicycles, but their deferents have no geometrical or physical +relation to the sun. The moon moves on an epicycle centred on a second +epicycle, itself centred on a deferent, excentric to the earth. The +earth’s axis rotates about the pole of the ecliptic, making one +revolution and a twenty-six thousandth part of a revolution in the sidereal +year, in the opposite direction to its orbital motion. +</p> + +<p> +In view of this fanciful structure it must be noted, in fairness to Copernicus, +that he repeatedly states that the reader is not obliged to accept his system +as showing the real motions; that it does not matter whether they be true, even +approximately, or not, so long as they enable us to compute tables from which +the places of the planets among the stars can be predicted.<a href="#linknote-19" name="linknoteref-19" id="linknoteref-19"><sup>[9]</sup></a> He says that whoever is not satisfied with this +explanation must be contented by being told that “mathematics are for +mathematicians” (Mathematicis mathematica scribuntur). +</p> + +<p> +At the same time he expresses his conviction over and over again that the earth +is in motion. It is with him a pious belief, just as it was with Pythagoras and +his school and with Aristarchus. “But” (as Dreyer says in his most +interesting book, <i>Tycho Brahe</i>) “proofs of the physical truth of +his system Copernicus had given none, and could give none,” any more than +Pythagoras or Aristarchus. +</p> + +<p> +There was nothing so startlingly simple in his system as to lead the cautious +astronomer to accept it, as there was in the later Keplerian system; and the +absence of parallax in the stars seemed to condemn his system, which had no +physical basis to recommend it, and no simplification at all over the +Egypto-Tychonic system, to which Copernicus himself drew attention. It has been +necessary to devote perhaps undue space to the interesting work of Copernicus, +because by a curious chance his name has become so widely known. He has been +spoken of very generally as the founder of the solar system that is now +accepted. This seems unfair, and on reading over what has been written about +him at different times it will be noticed that the astronomers—those who +have evidently read his great book—are very cautious in the words with +which they eulogise him, and refrain from attributing to him the foundation of +our solar system, which is entirely due to Kepler. It is only the more popular +writers who give the idea that a revolution had been effected when +Pythagoras’ system was revived, and when Copernicus supported his view +that the earth moves and is not fixed. +</p> + +<p> +It may be easy to explain the association of the name of Copernicus with the +Keplerian system. But the time has long passed when the historian can support +in any way this popular error, which was started not by astronomers acquainted +with Kepler’s work, but by those who desired to put the Church in the +wrong by extolling Copernicus. +</p> + +<p> +Copernicus dreaded much the abuse he expected to receive from philosophers for +opposing the authority of Aristotle, who had declared that the earth was fixed. +So he sought and obtained the support of the Church, dedicating his great work +to Pope Paul III. in a lengthy explanatory epistle. The Bishop of Cracow set up +a memorial tablet in his honour. +</p> + +<p> +Copernicus was the most refined exponent, and almost the last representative, +of the Epicyclical School. As has been already stated, his successor, Tycho +Brahe, supported the same use of epicycles and excentrics as Copernicus, though +he held the earth to be fixed. But Tycho Brahe was eminently a practical +observer, and took little part in theory; and his observations formed so +essential a portion of the system of Kepler that it is only fair to include his +name among these who laid the foundations of the solar system which we accept +to-day. +</p> + +<p> +In now taking leave of the system of epicycles let it be remarked that it has +been held up to ridicule more than it deserves. On reading Airy’s account +of epicycles, in the beautifully clear language of his <i>Six Lectures on +Astronomy</i>, the impression is made that the jointed bars there spoken of for +describing the circles were supposed to be real. This is no more the case than +that the spheres of Eudoxus and Callippus were supposed to be real. Both were +introduced only to illustrate the mathematical conception upon which the solar, +planetary, and lunar tables were constructed. The epicycles represented nothing +more nor less than the first terms in the Fourier series, which in the last +century has become a basis of such calculations, both in astronomy and physics +generally. +</p> + +<div class="fig" style="width:50%;"> +<a name="illus03"></a> +<img src="images/003.jpg" style="width:100%;" alt="“QUADRANS MURALIS SIVE +TICHONICUS.”" /> +<p class="caption">“Q<small>UADRANS</small> M<small>URALIS SIVE</small> +T<small>ICHONICUS</small>.”<br/> With portrait of Tycho Brahe, +instruments, etc., painted on the wall; showing assistants using the sight, +watching the clock, and recording. (From the author’s copy of the +<i>Astronomiæ Instauratæ Mechanica</i>.) +</p> +</div> + +<hr /> + +<p> +<b>FOOTNOTES:</b> +</p> + +<p class="footnote"> +<a name="linknote-11" id="linknote-11"></a> <a href="#linknoteref-11">[1]</a> +For definition see p. 22. +</p> + +<p class="footnote"> +<a name="linknote-12" id="linknote-12"></a> <a href="#linknoteref-12">[2]</a> +<i>Ibid</i>. +</p> + +<p class="footnote"> +<a name="linknote-13" id="linknote-13"></a> <a href="#linknoteref-13">[3]</a> +For definition see p. 18. +</p> + +<p class="footnote"> +<a name="linknote-14" id="linknote-14"></a> <a href="#linknoteref-14">[4]</a> +For definition see p. 18. +</p> + +<p class="footnote"> +<a name="linknote-15" id="linknote-15"></a> <a href="#linknoteref-15">[5]</a> +In his great book Copernicus says: “The movement of the heavenly bodies +is uniform, circular, perpetual, or else composed of circular movements.” +In this he proclaimed himself a follower of Pythagoras (see p. 14), as also +when he says: “The world is spherical because the sphere is, of all +figures, the most perfect” (Delambre, <i>Ast. Mod. Hist</i>., pp. 86, +87). +</p> + +<p class="footnote"> +<a name="linknote-16" id="linknote-16"></a> <a href="#linknoteref-16">[6]</a> +Kepler tells us that Tycho Brahe was pleased with this device, and adapted it +to his own system. +</p> + +<p class="footnote"> +<a name="linknote-17" id="linknote-17"></a> <a href="#linknoteref-17">[7]</a> +<i>Hist. Ast.</i>, vol. i., p. 354. +</p> + +<p class="footnote"> +<a name="linknote-18" id="linknote-18"></a> <a href="#linknoteref-18">[8]</a> +<i>Hist. of Phys. Ast.</i>, p. vii. +</p> + +<p class="footnote"> +<a name="linknote-19" id="linknote-19"></a> <a href="#linknoteref-19">[9]</a> +“Est enim Astronomi proprium, historiam motuum coelestium diligenti et +artificiosa observatione colligere. Deinde causas earundem, seu hypotheses, cum +veras assequi nulla ratione possit ... Neque enim necesse est, eas hypotheses +esse veras, imo ne verisimiles quidem, sed sufficit hoc usum, si calculum +observationibus congruentem exhibeant.” +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="book02"></a>BOOK II. THE DYNAMICAL PERIOD</h2> + +</div><!--end chapter--> + +<div class="chapter"> + +<h3><a name="5"></a>5. DISCOVERY OF THE TRUE SOLAR SYSTEM—TYCHO +BRAHE—KEPLER.</h3> + +<p> +During the period of the intellectual and aesthetic revival, at the beginning +of the sixteenth century, the “spirit of the age” was fostered by +the invention of printing, by the downfall of the Byzantine Empire, and the +scattering of Greek fugitives, carrying the treasures of literature through +Western Europe, by the works of Raphael and Michael Angelo, by the Reformation, +and by the extension of the known world through the voyages of Spaniards and +Portuguese. During that period there came to the front the founder of accurate +observational astronomy. Tycho Brahe, a Dane, born in 1546 of noble parents, +was the most distinguished, diligent, and accurate observer of the heavens +since the days of Hipparchus, 1,700 years before. +</p> + +<p> +Tycho was devoted entirely to his science from childhood, and the opposition of +his parents only stimulated him in his efforts to overcome difficulties. He +soon grasped the hopelessness of the old deductive methods of reasoning, and +decided that no theories ought to be indulged in until preparations had been +made by the accumulation of accurate observations. We may claim for him the +title of founder of the inductive method. +</p> + +<p> +For a complete life of this great man the reader is referred to Dreyer’s +<i>Tycho Brahe</i>, Edinburgh, 1890, containing a complete bibliography. The +present notice must be limited to noting the work done, and the qualities of +character which enabled him to attain his scientific aims, and which have been +conspicuous in many of his successors. +</p> + +<p> +He studied in Germany, but King Frederick of Denmark, appreciating his great +talents, invited him to carry out his life’s work in that country. He +granted to him the island of Hveen, gave him a pension, and made him a canon of +the Cathedral of Roskilde. On that island Tycho Brahe built the splendid +observatory which he called Uraniborg, and, later, a second one for his +assistants and students, called Stjerneborg. These he fitted up with the most +perfect instruments, and never lost a chance of adding to his stock of careful +observations.<a href="#linknote-20" name="linknoteref-20" id="linknoteref-20"><sup>[1]</sup></a> +</p> + +<p> +The account of all these instruments and observations, printed at his own press +on the island, was published by Tycho Brahe himself, and the admirable and +numerous engravings bear witness to the excellence of design and the stability +of his instruments. +</p> + +<p> +His mechanical skill was very great, and in his workmanship he was satisfied +with nothing but the best. He recognised the importance of rigidity in the +instruments, and, whereas these had generally been made of wood, he designed +them in metal. His instruments included armillae like those which had been used +in Alexandria, and other armillae designed by himself—sextants, mural +quadrants, large celestial globes and various instruments for special purposes. +He lived before the days of telescopes and accurate clocks. He invented the +method of sub-dividing the degrees on the arc of an instrument by transversals +somewhat in the way that Pedro Nunez had proposed. +</p> + +<p> +He originated the true system of observation and reduction of observations, +recognising the fact that the best instrument in the world is not perfect; and +with each of his instruments he set to work to find out the errors of +graduation and the errors of mounting, the necessary correction being applied +to each observation. +</p> + +<p> +When he wanted to point his instrument exactly to a star he was confronted with +precisely the same difficulty as is met in gunnery and rifle-shooting. The +sights and the object aimed at cannot be in focus together, and a great deal +depends on the form of sight. Tycho Brahe invented, and applied to the pointers +of his instruments, an aperture-sight of variable area, like the iris diaphragm +used now in photography. This enabled him to get the best result with stars of +different brightness. The telescope not having been invented, he could not use +a telescopic-sight as we now do in gunnery. This not only removes the +difficulty of focussing, but makes the minimum visible angle smaller. Helmholtz +has defined the minimum angle measurable with the naked eye as being one minute +of arc. In view of this it is simply marvellous that, when the positions of +Tycho’s standard stars are compared with the best modern catalogues, his +probable error in right ascension is only ± 24”, 1, and in +declination only ± 25”, 9. +</p> + +<p> +Clocks of a sort had been made, but Tycho Brahe found them so unreliable that +he seldom used them, and many of his position-measurements were made by +measuring the angular distances from known stars. +</p> + +<p> +Taking into consideration the absence of either a telescope or a clock, and +reading his account of the labour he bestowed upon each observation, we must +all agree that Kepler, who inherited these observations in MS., was justified, +under the conditions then existing, in declaring that there was no hope of +anyone ever improving upon them. +</p> + +<p> +In the year 1572, on November 11th, Tycho discovered in Cassiopeia a new star +of great brilliance, and continued to observe it until the end of January, +1573. So incredible to him was such an event that he refused to believe his own +eyes until he got others to confirm what he saw. He made accurate observations +of its distance from the nine principal stars in Casseiopeia, and proved that +it had no measurable parallax. Later he employed the same method with the +comets of 1577, 1580, 1582, 1585, 1590, 1593, and 1596, and proved that they +too had no measurable parallax and must be very distant. +</p> + +<p> +The startling discovery that stars are not necessarily permanent, that new +stars may appear, and possibly that old ones may disappear, had upon him +exactly the same effect that a similar occurrence had upon Hipparchus 1,700 +years before. He felt it his duty to catalogue all the principal stars, so that +there should be no mistake in the future. During the construction of his +catalogue of 1,000 stars he prepared and used accurate tables of refraction +deduced from his own observations. Thus he eliminated (so far as naked eye +observations required) the effect of atmospheric refraction which makes the +altitude of a star seem greater than it really is. +</p> + +<p> +Tycho Brahe was able to correct the lunar theory by his observations. +Copernicus had introduced two epicycles on the lunar orbit in the hope of +obtaining a better accordance between theory and observation; and he was not +too ambitious, as his desire was to get the tables accurate to ten minutes. +Tycho Brahe found that the tables of Copernicus were in error as much as two +degrees. He re-discovered the inequality called “variation” by +observing the moon in all phases—a thing which had not been attended to. +[It is remarkable that in the nineteenth century Sir George Airy established an +altazimuth at Greenwich Observatory with this special object, to get +observations of the moon in all phases.] He also discovered other lunar +equalities, and wanted to add another epicycle to the moon’s orbit, but +he feared that these would soon become unmanageable if further observations +showed more new inequalities. +</p> + +<p> +But, as it turned out, the most fruitful work of Tycho Brahe was on the motions +of the planets, and especially of the planet Mars, for it was by an examination +of these results that Kepler was led to the discovery of his immortal laws. +</p> + +<p> +After the death of King Frederick the observatories of Tycho Brahe were not +supported. The gigantic power and industry displayed by this determined man +were accompanied, as often happens, by an overbearing manner, intolerant of +obstacles. This led to friction, and eventually the observatories were +dismantled, and Tycho Brahe was received by the Emperor Rudolph II., who placed +a house in Prague at his disposal. Here he worked for a few years, with Kepler +as one of his assistants, and he died in the year 1601. +</p> + +<p> +It is an interesting fact that Tycho Brahe had a firm conviction that mundane +events could be predicted by astrology, and that this belief was supported by +his own predictions. +</p> + +<p> +It has already been stated that Tycho Brahe maintained that observation must +precede theory. He did not accept the Copernican theory that the earth moves, +but for a working hypothesis he used a modification of an old Egyptian theory, +mathematically identical with that of Copernicus, but not involving a stellar +parallax. He says (<i>De Mundi</i>, <i>etc</i>.) that +</p> + +<p class="letter">the Ptolemean system was too complicated, and the new one +which that great man Copernicus had proposed, following in the footsteps of +Aristarchus of Samos, though there was nothing in it contrary to mathematical +principles, was in opposition to those of physics, as the heavy and sluggish +earth is unfit to move, and the system is even opposed to the authority of +Scripture. The absence of annual parallax further involves an incredible +distance between the outermost planet and the fixed stars. +</p> + +<p> +We are bound to admit that in the circumstances of the case, so long as there +was no question of dynamical forces connecting the members of the solar system, +his reasoning, as we should expect from such a man, is practical and sound. It +is not surprising, then, that astronomers generally did not readily accept the +views of Copernicus, that Luther (Luther’s <i>Tischreden</i>, pp. 22, 60) +derided him in his usual pithy manner, that Melancthon (<i>Initia doctrinae +physicae</i>) said that Scripture, and also science, are against the +earth’s motion; and that the men of science whose opinion was asked for +by the cardinals (who wished to know whether Galileo was right or wrong) looked +upon Copernicus as a weaver of fanciful theories. +</p> + +<p> +Johann Kepler is the name of the man whose place, as is generally agreed, would +have been the most difficult to fill among all those who have contributed to +the advance of astronomical knowledge. He was born at Wiel, in the Duchy of +Wurtemberg, in 1571. He held an appointment at Gratz, in Styria, and went to +join Tycho Brahe in Prague, and to assist in reducing his observations. These +came into his possession when Tycho Brahe died, the Emperor Rudolph entrusting +to him the preparation of new tables (called the Rudolphine tables) founded on +the new and accurate observations. He had the most profound respect for the +knowledge, skill, determination, and perseverance of the man who had reaped +such a harvest of most accurate data; and though Tycho hardly recognised the +transcendent genius of the man who was working as his assistant, and although +there were disagreements between them, Kepler held to his post, sustained by +the conviction that, with these observations to test any theory, he would be in +a position to settle for ever the problem of the solar system. +</p> + +<div class="fig" style="width:60%;"> +<a name="illus04"></a> +<img src="images/004.jpg" style="width:100%;" alt="PORTRAIT OF JOHANNES +KEPLER." /> +<p class="caption">P<small>ORTRAIT OF</small> J<small>OHANNES</small> +K<small>EPLER</small>.<br/> By F. Wanderer, from Reitlinger’s +“Johannes Kepler”<br/> (original in Strassburg). +</p> +</div> + +<p> +It has seemed to many that Plato’s demand for uniform circular motion +(linear or angular) was responsible for a loss to astronomy of good work during +fifteen hundred years, for a hundred ill-considered speculative cosmogonies, +for dissatisfaction, amounting to disgust, with these <i>à priori</i> +guesses, and for the relegation of the science to less intellectual races than +Greeks and other Europeans. Nobody seemed to dare to depart from this fetish of +uniform angular motion and circular orbits until the insight, boldness, and +independence of Johann Kepler opened up a new world of thought and of +intellectual delight. +</p> + +<p> +While at work on the Rudolphine tables he used the old epicycles and deferents +and excentrics, but he could not make theory agree with observation. His +instincts told him that these apologists for uniform motion were a fraud; and +he proved it to himself by trying every possible variation of the elements and +finding them fail. The number of hypotheses which he examined and rejected was +almost incredible (for example, that the planets turn round centres at a little +distance from the sun, that the epicycles have centres at a little distance +from the deferent, and so on). He says that, after using all these devices to +make theory agree with Tycho’s observations, he still found errors +amounting to eight minutes of a degree. Then he said boldly that it was +impossible that so good an observer as Tycho could have made a mistake of eight +minutes, and added: “Out of these eight minutes we will construct a new +theory that will explain the motions of all the planets.” And he did it, +with elliptic orbits having the sun in a focus of each.<a href="#linknote-21" name="linknoteref-21" id="linknoteref-21"><sup>[2]</sup></a> +</p> + +<p> +It is often difficult to define the boundaries between fancies, imagination, +hypothesis, and sound theory. This extraordinary genius was a master in all +these modes of attacking a problem. His analogy between the spaces occupied by +the five regular solids and the distances of the planets from the sun, which +filled him with so much delight, was a display of pure fancy. His demonstration +of the three fundamental laws of planetary motion was the most strict and +complete theory that had ever been attempted. +</p> + +<p> +It has been often suggested that the revival by Copernicus of the notion of a +moving earth was a help to Kepler. No one who reads Kepler’s great book +could hold such an opinion for a moment. In fact, the excellence of +Copernicus’s book helped to prolong the life of the epicyclical theories +in opposition to Kepler’s teaching. +</p> + +<p> +All of the best theories were compared by him with observation. These were the +Ptolemaic, the Copernican, and the Tychonic. The two latter placed all of the +planetary orbits concentric with one another, the sun being placed a little +away from their common centre, and having no apparent relation to them, and +being actually outside the planes in which they move. Kepler’s first +great discovery was that the planes of all the orbits pass through the sun; his +second was that the line of apses of each planet passes through the sun; both +were contradictory to the Copernican theory. +</p> + +<p> +He proceeds cautiously with his propositions until he arrives at his great +laws, and he concludes his book by comparing observations of Mars, of all +dates, with his theory. +</p> + +<p> +His first law states that the planets describe ellipses with the sun at a focus +of each ellipse. +</p> + +<p> +His second law (a far more difficult one to prove) states that a line drawn +from a planet to the sun sweeps over equal areas in equal times. These two laws +were published in his great work, <i>Astronomia Nova, sen. Physica Coelestis +tradita commentariis de Motibus Stelloe; Martis</i>, Prague, 1609. +</p> + +<p> +It took him nine years more<a href="#linknote-22" name="linknoteref-22" id="linknoteref-22"><sup>[3]</sup></a> to discover his third law, +that the squares of the periodic times are proportional to the cubes of the +mean distances from the sun. +</p> + +<p> +These three laws contain implicitly the law of universal gravitation. They are +simply an alternative way of expressing that law in dealing with planets, not +particles. Only, the power of the greatest human intellect is so utterly feeble +that the meaning of the words in Kepler’s three laws could not be +understood until expounded by the logic of Newton’s dynamics. +</p> + +<p> +The joy with which Kepler contemplated the final demonstration of these laws, +the evolution of which had occupied twenty years, can hardly be imagined by us. +He has given some idea of it in a passage in his work on <i>Harmonics</i>, +which is not now quoted, only lest someone might say it was egotistical—a +term which is simply grotesque when applied to such a man with such a +life’s work accomplished. +</p> + +<p> +The whole book, <i>Astronomia Nova</i>, is a pleasure to read; the mass of +observations that are used, and the ingenuity of the propositions, contrast +strongly with the loose and imperfectly supported explanations of all his +predecessors; and the indulgent reader will excuse the devotion of a few lines +to an example of the ingenuity and beauty of his methods. +</p> + +<p> + +It may seem a hopeless task to find out the true paths of Mars and the earth +(at that time when their shape even was not known) from the observations giving +only the relative direction from night to night. Now, Kepler had twenty years +of observations of Mars to deal with. This enabled him to use a new method, to +find the earth’s orbit. Observe the date at any time when Mars is in +opposition. The earth’s position E at that date gives the longitude of +Mars M. His period is 687 days. Now choose dates before and after the principal +date at intervals of 687 days and its multiples. Mars is in each case in the +same position. Now for any date when Mars is at M and the earth at +E<sub>3</sub> the date of the year gives the angle E<sub>3</sub>SM. And the +observation of Tycho gives the direction of Mars compared with the sun, +SE<sub>3</sub>M. So all the angles of the triangle SEM in any of these +positions of E are known, and also the ratios of SE<sub>1</sub>, +SE<sub>2</sub>, SE<sub>3</sub>, SE<sub>4</sub> to SM and to each other. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/006.jpg" width="300" height="274" alt="" /> +</div> + +<p> +For the orbit of Mars observations were chosen at intervals of a year, when the +earth was always in the same place. +</p> + +<p> +But Kepler saw much farther than the geometrical facts. He realised that the +orbits are followed owing to a force directed to the sun; and he guessed that +this is the same force as the gravity that makes a stone fall. He saw the +difficulty of gravitation acting through the void space. He compared universal +gravitation to magnetism, and speaks of the work of Gilbert of Colchester. +(Gilbert’s book, <i>De Mundo Nostro Sublunari, Philosophia Nova</i>, +Amstelodami, 1651, containing similar views, was published forty-eight years +after Gilbert’s death, and forty-two years after Kepler’s book and +reference. His book <i>De Magnete</i> was published in 1600.) +</p> + +<p> +A few of Kepler’s views on gravitation, extracted from the Introduction +to his <i>Astronomia Nova</i>, may now be mentioned:— +</p> + +<p> +1. Every body at rest remains at rest if outside the attractive power of other +bodies. +</p> + +<p> +2. Gravity is a property of masses mutually attracting in such manner that the +earth attracts a stone much more than a stone attracts the earth. +</p> + +<p> +3. Bodies are attracted to the earth’s centre, not because it is the +centre of the universe, but because it is the centre of the attracting +particles of the earth. +</p> + +<p> +4. If the earth be not round (but spheroidal?), then bodies at different +latitudes will not be attracted to its centre, but to different points in the +neighbourhood of that centre. +</p> + +<p> +5. If the earth and moon were not retained in their orbits by vital force +(<i>aut alia aligua aequipollenti</i>), the earth and moon would come together. +</p> + +<p> +6. If the earth were to cease to attract its waters, the oceans would all rise +and flow to the moon. +</p> + +<p> +7. He attributes the tides to lunar attraction. Kepler had been appointed +Imperial Astronomer with a handsome salary (on paper), a fraction of which was +doled out to him very irregularly. He was led to miserable makeshifts to earn +enough to keep his family from starvation; and proceeded to Ratisbon in 1630 to +represent his claims to the Diet. He arrived worn out and debilitated; he +failed in his appeal, and died from fever, contracted under, and fed upon, +disappointment and exhaustion. Those were not the days when men could adopt as +a profession the “research of endowment.” +</p> + +<p> +Before taking leave of Kepler, who was by no means a man of one idea, it ought +to be here recorded that he was the first to suggest that a telescope made with +both lenses convex (not a Galilean telescope) can have cross wires in the +focus, for use as a pointer to fix accurately the positions of stars. An +Englishman, Gascoigne, was the first to use this in practice. +</p> + +<p> +From the all too brief epitome here given of Kepler’s greatest book, it +must be obvious that he had at that time some inkling of the meaning of his +laws—universal gravitation. From that moment the idea of universal +gravitation was in the air, and hints and guesses were thrown out by many; and +in time the law of gravitation would doubtless have been discovered, though +probably not by the work of one man, even if Newton had not lived. But, if +Kepler had not lived, who else could have discovered his laws? +</p> + +<hr /> + +<p> +<b>FOOTNOTES:</b> +</p> + +<p class="footnote"> +<a name="linknote-20" id="linknote-20"></a> <a href="#linknoteref-20">[1]</a> +When the writer visited M. D’Arrest, the astronomer, at Copenhagen, in +1872, he was presented by D’Arrest with one of several bricks collected +from the ruins of Uraniborg. This was one of his most cherished possessions +until, on returning home after a prolonged absence on astronomical work, he +found that his treasure had been tidied away from his study. +</p> + +<p class="footnote"> +<a name="linknote-21" id="linknote-21"></a> <a href="#linknoteref-21">[2]</a> +An ellipse is one of the plane, sections of a cone. It is an oval curve, which +may be drawn by fixing two pins in a sheet of paper at S and H, fastening a +string, SPH, to the two pins, and stretching it with a pencil point at P, and +moving the pencil point, while the string is kept taut, to trace the oval +ellipse, APB. S and H are the <i>foci</i>. Kepler found the sun to be in one +focus, say S. AB is the <i>major axis</i>. DE is the <i>minor axis</i>. C is +the <i>centre</i>. The direction of AB is the <i>line of apses</i>. The ratio +of CS to CA is the <i>excentricity</i>. The position of the planet at A is the +<i>perihelion</i> (nearest to the sun). The position of the planet at B is the +<i>aphelion</i> (farthest from the sun). The angle ASP is the <i>anomaly</i> +when the planet is at P. CA or a line drawn from S to D is the <i>mean +distance</i> of the planet from the sun. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/005.jpg" width="300" height="252" alt="" /> +</div> + +<p class="footnote"> +<a name="linknote-22" id="linknote-22"></a> <a href="#linknoteref-22">[3]</a> +The ruled logarithmic paper we now use was not then to be had by going into a +stationer’s shop. Else he would have accomplished this in five minutes. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h3><a name="6"></a> 6. GALILEO AND THE TELESCOPE—NOTIONS OF GRAVITY +BY HORROCKS, ETC.</h3> + +<p> +It is now necessary to leave the subject of dynamical astronomy for a short +time in order to give some account of work in a different direction originated +by a contemporary of Kepler’s, his senior in fact by seven years. Galileo +Galilei was born at Pisa in 1564. The most scientific part of his work dealt +with terrestrial dynamics; but one of those fortunate chances which happen only +to really great men put him in the way of originating a new branch of +astronomy. +</p> + +<p> +The laws of motion had not been correctly defined. The only man of +Galileo’s time who seems to have worked successfully in the same +direction as himself was that Admirable Crichton of the Italians, Leonardo da +Vinci. Galileo cleared the ground. It had always been noticed that things tend +to come to rest; a ball rolled on the ground, a boat moved on the water, a shot +fired in the air. Galileo realised that in all of these cases a resisting force +acts to stop the motion, and he was the first to arrive at the not very obvious +law that the motion of a body will never stop, nor vary its speed, nor change +its direction, except by the action of some force. +</p> + +<p> +It is not very obvious that a light body and a heavy one fall at the same speed +(except for the resistance of the air). Galileo proved this on paper, but to +convince the world he had to experiment from the leaning tower of Pisa. +</p> + +<p> +At an early age he discovered the principle of isochronism of the pendulum, +which, in the hands of Huyghens in the middle of the seventeenth century, led +to the invention of the pendulum clock, perhaps the most valuable astronomical +instrument ever produced. +</p> + +<p> +These and other discoveries in dynamics may seem very obvious now; but it is +often the most every-day matters which have been found to elude the inquiries +of ordinary minds, and it required a high order of intellect to unravel the +truth and discard the stupid maxims scattered through the works of Aristotle +and accepted on his authority. A blind worship of scientific authorities has +often delayed the progress of human knowledge, just as too much +“instruction” of a youth often ruins his “education.” +Grant, in his history of Physical Astronomy, has well said that “the +sagacity and skill which Galileo displays in resolving the phenomena of motion +into their constituent elements, and hence deriving the original principles +involved in them, will ever assure to him a distinguished place among those who +have extended the domains of science.” +</p> + +<p> +But it was work of a different kind that established Galileo’s popular +reputation. In 1609 Galileo heard that a Dutch spectacle-maker had combined a +pair of lenses so as to magnify distant objects. Working on this hint, he +solved the same problem, first on paper and then in practice. So he came to +make one of the first telescopes ever used in astronomy. No sooner had he +turned it on the heavenly bodies than he was rewarded by such a shower of +startling discoveries as forthwith made his name the best known in Europe. He +found curious irregular black spots on the sun, revolving round it in +twenty-seven days; hills and valleys on the moon; the planets showing discs of +sensible size, not points like the fixed stars; Venus showing phases according +to her position in relation to the sun; Jupiter accompanied by four moons; +Saturn with appendages that he could not explain, but unlike the other planets; +the Milky Way composed of a multitude of separate stars. +</p> + +<p> +His fame flew over Europe like magic, and his discoveries were much +discussed—and there were many who refused to believe. Cosmo de Medici +induced him to migrate to Florence to carry on his observations. He was +received by Paul V., the Pope, at Rome, to whom he explained his discoveries. +</p> + +<p> +He thought that these discoveries proved the truth of the Copernican theory of +the Earth’s motion; and he urged this view on friends and foes alike. +Although in frequent correspondence with Kepler, he never alluded to the New +Astronomy, and wrote to him extolling the virtue of epicycles. He loved to +argue, never shirked an encounter with any number of disputants, and laughed as +he broke down their arguments. +</p> + +<p> +Through some strange course of events, not easy to follow, the Copernican +theory, whose birth was welcomed by the Church, had now been taken up by +certain anti-clerical agitators, and was opposed by the cardinals as well as by +the dignitaries of the Reformed Church. Galileo—a good Catholic—got +mixed up in these discussions, although on excellent terms with the Pope and +his entourage. At last it came about that Galileo was summoned to appear at +Rome, where he was charged with holding and teaching heretical opinions about +the movement of the earth; and he then solemnly abjured these opinions. There +has been much exaggeration and misstatement about his trial and punishment, and +for a long time there was a great deal of bitterness shown on both sides. But +the general verdict of the present day seems to be that, although Galileo +himself was treated with consideration, the hostility of the Church to the +views of Copernicus placed it in opposition also to the true Keplerian system, +and this led to unprofitable controversies. From the time of Galileo onwards, +for some time, opponents of religion included the theory of the Earth’s +motion in their disputations, not so much for the love, or knowledge, of +astronomy, as for the pleasure of putting the Church in the wrong. This created +a great deal of bitterness and intolerance on both sides. Among the sufferers +was Giordano Bruno, a learned speculative philosopher, who was condemned to be +burnt at the stake. +</p> + +<p> +Galileo died on Christmas Day, 1642—the day of Newton’s birth. The +further consideration of the grand field of discovery opened out by Galileo +with his telescopes must be now postponed, to avoid discontinuity in the +history of the intellectual development of this period, which lay in the +direction of dynamical, or physical, astronomy. +</p> + +<p> +Until the time of Kepler no one seems to have conceived the idea of universal +physical forces controlling terrestrial phenomena, and equally applicable to +the heavenly bodies. The grand discovery by Kepler of the true relationship of +the Sun to the Planets, and the telescopic discoveries of Galileo and of those +who followed him, spread a spirit of inquiry and philosophic thought throughout +Europe, and once more did astronomy rise in estimation; and the irresistible +logic of its mathematical process of reasoning soon placed it in the position +it has ever since occupied as the foremost of the exact sciences. +</p> + +<p> +The practical application of this process of reasoning was enormously +facilitated by the invention of logarithms by Napier. He was born at +Merchistoun, near Edinburgh, in 1550, and died in 1617. By this system the +tedious arithmetical operations necessary in astronomical calculations, +especially those dealing with the trigonometrical functions of angles, were so +much simplified that Laplace declared that by this invention the life-work of +an astronomer was doubled. +</p> + +<p> +Jeremiah Horrocks (born 1619, died 1641) was an ardent admirer of Tycho Brahe +and Kepler, and was able to improve the Rudolphine tables so much that he +foretold a transit of Venus, in 1639, which these tables failed to indicate, +and was the only observer of it. His life was short, but he accomplished a +great deal, and rightly ascribed the lunar inequality called <i>evection</i> to +variations in the value of the eccentricity and in the direction of the line of +apses, at the same time correctly assigning <i>the disturbing force of the +Sun</i> as the cause. He discovered the errors in Jupiter’s calculated +place, due to what we now know as the long inequality of Jupiter and Saturn, +and measured with considerable accuracy the acceleration at that date of +Jupiter’s mean motion, and indicated the retardation of Saturn’s +mean motion. +</p> + +<p> +Horrocks’ investigations, so far as they could be collected, were +published posthumously in 1672, and seldom, if ever, has a man who lived only +twenty-two years originated so much scientific knowledge. +</p> + +<p> +At this period British science received a lasting impetus by the wise +initiation of a much-abused man, Charles II., who founded the Royal Society of +London, and also the Royal Observatory of Greeenwich, where he established +Flamsteed as first Astronomer Royal, especially for lunar and stellar +observations likely to be useful for navigation. At the same time the French +Academy and the Paris Observatory were founded. All this within fourteen years, +1662-1675. +</p> + +<p> +Meanwhile gravitation in general terms was being discussed by Hooke, Wren, +Halley, and many others. All of these men felt a repugnance to accept the idea +of a force acting across the empty void of space. Descartes (1596-1650) +proposed an ethereal medium whirling round the sun with the planets, and having +local whirls revolving with the satellites. As Delambre and Grant have said, +this fiction only retarded the progress of pure science. It had no sort of +relation to the more modern, but equally misleading, “nebular +hypothesis.” While many were talking and guessing, a giant mind was +needed at this stage to make things clear. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h3><a name="7"></a>7. SIR ISAAC NEWTON—LAW OF UNIVERSAL +GRAVITATION.</h3> + +<p> +We now reach the period which is the culminating point of interest in the +history of dynamical astronomy. Isaac Newton was born in 1642. Pemberton states +that Newton, having quitted Cambridge to avoid the plague, was residing at +Wolsthorpe, in Lincolnshire, where he had been born; that he was sitting one +day in the garden, reflecting upon the force which prevents a planet from +flying off at a tangent and which draws it to the sun, and upon the force which +draws the moon to the earth; and that he saw in the case of the planets that +the sun’s force must clearly be unequal at different distances, for the +pull out of the tangential line in a minute is less for Jupiter than for Mars. +He then saw that the pull of the earth on the moon would be less than for a +nearer object. It is said that while thus meditating he saw an apple fall from +a tree to the ground, and that this fact suggested the questions: Is the force +that pulled that apple from the tree the same as the force which draws the moon +to the earth? Does the attraction for both of them follow the same law as to +distance as is given by the planetary motions round the sun? It has been stated +that in this way the first conception of universal gravitation arose.<a href="#linknote-23" name="linknoteref-23" id="linknoteref-23"><sup>[1]</sup></a> +</p> + +<p> +Quite the most important event in the whole history of physical astronomy was +the publication, in 1687, of Newton’s <i>Principia (Philosophiae +Naturalis Principia Mathematica)</i>. In this great work Newton started from +the beginning of things, the laws of motion, and carried his argument, step by +step, into every branch of physical astronomy; giving the physical meaning of +Kepler’s three laws, and explaining, or indicating the explanation of, +all the known heavenly motions and their irregularities; showing that all of +these were included in his simple statement about the law of universal +gravitation; and proceeding to deduce from that law new irregularities in the +motions of the moon which had never been noticed, and to discover the oblate +figure of the earth and the cause of the tides. These investigations occupied +the best part of his life; but he wrote the whole of his great book in fifteen +months. +</p> + +<p> +Having developed and enunciated the true laws of motion, he was able to show +that Kepler’s second law (that equal areas are described by the line from +the planet to the sun in equal times) was only another way of saying that the +centripetal force on a planet is always directed to the sun. Also that +Kepler’s first law (elliptic orbits with the sun in one focus) was only +another way of saying that the force urging a planet to the sun varies +inversely as the square of the distance. Also (if these two be granted) it +follows that Kepler’s third law is only another way of saying that the +sun’s force on different planets (besides depending as above on distance) +is proportional to their masses. +</p> + +<p> +Having further proved the, for that day, wonderful proposition that, with the +law of inverse squares, the attraction by the separate particles of a sphere of +uniform density (or one composed of concentric spherical shells, each of +uniform density) acts as if the whole mass were collected at the centre, he was +able to express the meaning of Kepler’s laws in propositions which have +been summarised as follows:— +</p> + +<p> +The law of universal gravitation.—<i>Every particle of matter in the +universe attracts every other particle with a force varying inversely as the +square of the distance between them, and directly as the product of the masses +of the two particles</i>.<a href="#linknote-24" name="linknoteref-24" id="linknoteref-24"><sup>[2]</sup></a> +</p> + +<p> +But Newton did not commit himself to the law until he had answered that +question about the apple; and the above proposition now enabled him to deal +with the Moon and the apple. Gravity makes a stone fall 16.1 feet in a second. +The moon is 60 times farther from the earth’s centre than the stone, so +it ought to be drawn out of a straight course through 16.1 feet in a minute. +Newton found the distance through which she is actually drawn as a fraction of +the earth’s diameter. But when he first examined this matter he proceeded +to use a wrong diameter for the earth, and he found a serious discrepancy. +This, for a time, seemed to condemn his theory, and regretfully he laid that +part of his work aside. Fortunately, before Newton wrote the <i>Principia</i> +the French astronomer Picard made a new and correct measure of an arc of the +meridian, from which he obtained an accurate value of the earth’s +diameter. Newton applied this value, and found, to his great joy, that when the +distance of the moon is 60 times the radius of the earth she is attracted out +of the straight course 16.1 feet per minute, and that the force acting on a +stone or an apple follows the same law as the force acting upon the heavenly +bodies.<a href="#linknote-25" name="linknoteref-25" id="linknoteref-25"><sup>[3]</sup></a> +</p> + +<p> +The universality claimed for the law—if not by Newton, at least by his +commentators—was bold, and warranted only by the large number of cases in +which Newton had found it to apply. Its universality has been under test ever +since, and so far it has stood the test. There has often been a suspicion of a +doubt, when some inequality of motion in the heavenly bodies has, for a time, +foiled the astronomers in their attempts to explain it. But improved +mathematical methods have always succeeded in the end, and so the seeming doubt +has been converted into a surer conviction of the universality of the law. +</p> + +<p> +Having once established the law, Newton proceeded to trace some of its +consequences. He saw that the figure of the earth depends partly on the mutual +gravitation of its parts, and partly on the centrifugal tendency due to the +earth’s rotation, and that these should cause a flattening of the poles. +He invented a mathematical method which he used for computing the ratio of the +polar to the equatorial diameter. +</p> + +<p> +He then noticed that the consequent bulging of matter at the equator would be +attracted by the moon unequally, the nearest parts being most attracted; and so +the moon would tend to tilt the earth when in some parts of her orbit; and the +sun would do this to a less extent, because of its great distance. Then he +proved that the effect ought to be a rotation of the earth’s axis over a +conical surface in space, exactly as the axis of a top describes a cone, if the +top has a sharp point, and is set spinning and displaced from the vertical. He +actually calculated the amount; and so he explained the cause of the precession +of the equinoxes discovered by Hipparchus about 150 B.C. +</p> + +<p> +One of his grandest discoveries was a method of weighing the heavenly bodies by +their action on each other. By means of this principle he was able to compare +the mass of the sun with the masses of those planets that have moons, and also +to compare the mass of our moon with the mass of the earth. +</p> + +<p> +Thus Newton, after having established his great principle, devoted his splendid +intellect to the calculation of its consequences. He proved that if a body be +projected with any velocity in free space, subject only to a central force, +varying inversely as the square of the distance, the body must revolve in a +curve which may be any one of the sections of a cone—a circle, ellipse, +parabola, or hyperbola; and he found that those comets of which he had +observations move in parabolae round the Sun, and are thus subject to the +universal law. +</p> + +<p> +Newton realised that, while planets and satellites are chiefly controlled by +the central body about which they revolve, the new law must involve +irregularities, due to their mutual action—such, in fact, as Horrocks had +indicated. He determined to put this to a test in the case of the moon, and to +calculate the sun’s effect, from its mass compared with that of the +earth, and from its distance. He proved that the average effect upon the plane +of the orbit would be to cause the line in which it cuts the plane of the +ecliptic (i.e., the line of nodes) to revolve in the ecliptic once in about +nineteen years. This had been a known fact from the earliest ages. He also +concluded that the line of apses would revolve in the plane of the lunar orbit +also in about nineteen years; but the observed period is only ten years. For a +long time this was the one weak point in the Newtonian theory. It was not till +1747 that Clairaut reconciled this with the theory, and showed why +Newton’s calculation was not exact. +</p> + +<p> +Newton proceeded to explain the other inequalities recognised by Tycho Brahe +and older observers, and to calculate their maximum amounts as indicated by his +theory. He further discovered from his calculations two new inequalities, one +of the apogee, the other of the nodes, and assigned the maximum value. Grant +has shown the values of some of these as given by observation in the tables of +Meyer and more modern tables, and has compared them with the values assigned by +Newton from his theory; and the comparison is very remarkable. +</p> + +<pre> + Newton. Modern Tables. + ° ’ " ° ’ " +Mean monthly motion of Apses 1.31.28 3.4.0 +Mean annual motion of nodes 19.18.1,23 19.21.22,50 +Mean value of “variation” 36.10 35.47 +Annual equation 11.51 11.14 +Inequality of mean motion of apogee 19.43 22.17 +Inequality of mean motion of nodes 9.24 9.0 +</pre> + +<p> +The only serious discrepancy is the first, which has been already mentioned. +Considering that some of these perturbations had never been discovered, that +the cause of none of them had ever been known, and that he exhibited his +results, if he did not also make the discoveries, by the synthetic methods of +geometry, it is simply marvellous that he reached to such a degree of accuracy. +He invented the infinitesimal calculus which is more suited for such +calculations, but had he expressed his results in that language he would have +been unintelligible to many. +</p> + +<p> +Newton’s method of calculating the precession of the equinoxes, already +referred to, is as beautiful as anything in the <i>Principia</i>. He had +already proved the regression of the nodes of a satellite moving in an orbit +inclined to the ecliptic. He now said that the nodes of a ring of satellites +revolving round the earth’s equator would consequently all regress. And +if joined into a solid ring its node would regress; and it would do so, only +more slowly, if encumbered by the spherical part of the earth’s mass. +Therefore the axis of the equatorial belt of the earth must revolve round the +pole of the ecliptic. Then he set to work and found the amount due to the moon +and that due to the sun, and so he solved the mystery of 2,000 years. +</p> + +<p> +When Newton applied his law of gravitation to an explanation of the tides he +started a new field for the application of mathematics to physical problems; +and there can be little doubt that, if he could have been furnished with +complete tidal observations from different parts of the world, his +extraordinary powers of analysis would have enabled him to reach a satisfactory +theory. He certainly opened up many mines full of intellectual gems; and his +successors have never ceased in their explorations. This has led to improved +mathematical methods, which, combined with the greater accuracy of observation, +have rendered physical astronomy of to-day the most exact of the sciences. +</p> + +<p> +Laplace only expressed the universal opinion of posterity when he said that to +the <i>Principia</i> is assured “a pre-eminence above all the other +productions of the human intellect.” +</p> + +<p> +The name of Flamsteed, First Astronomer Royal, must here be mentioned as having +supplied Newton with the accurate data required for completing the theory. +</p> + +<p> +The name of Edmund Halley, Second Astronomer Royal, must ever be held in +repute, not only for his own discoveries, but for the part he played in urging +Newton to commit to writing, and present to the Royal Society, the results of +his investigations. But for his friendly insistence it is possible that the +<i>Principia</i> would never have been written; and but for his generosity in +supplying the means the Royal Society could not have published the book. +</p> + +<div class="fig" style="width:50%;"> +<a name="illus05"></a> +<img src="images/007.jpg" style="width:100%;" alt="DEATH MASK OF SIR ISAAC +NEWTON." /> +<p class="caption">D<small>EATH</small> M<small>ASK OF</small> +S<small>IR</small> I<small>SAAC</small> N<small>EWTON</small>.<br/> +Photographed specially for this work from the original, by kind permission of +the Royal Society, London.</p> +</div> + +<p> +Sir Isaac Newton died in 1727, at the age of eighty-five. His body lay in state +in the Jerusalem Chamber, and was buried in Westminster Abbey. +</p> + +<hr /> + +<p> +<b>FOOTNOTES:</b> +</p> + +<p class="footnote"> +<a name="linknote-23" id="linknote-23"></a> <a href="#linknoteref-23">[1]</a> +The writer inherited from his father (Professor J. D. Forbes) a small box +containing a bit of wood and a slip of paper, which had been presented to him +by Sir David Brewster. On the paper Sir David had written these words: +“If there be any truth in the story that Newton was led to the theory of +gravitation by the fall of an apple, this bit of wood is probably a piece of +the apple tree from which Newton saw the apple fall. When I was on a pilgrimage +to the house in which Newton was born, I cut it off an ancient apple tree +growing in his garden.” When lecturing in Glasgow, about 1875, the writer +showed it to his audience. The next morning, when removing his property from +the lecture table, he found that his precious relic had been stolen. It would +be interesting to know who has got it now! +</p> + +<p class="footnote"> +<a name="linknote-24" id="linknote-24"></a> <a href="#linknoteref-24">[2]</a> +It must be noted that these words, in which the laws of gravitation are always +summarised in histories and text-books, do not appear in the <i>Principia</i>; +but, though they must have been composed by some early commentator, it does not +appear that their origin has been traced. Nor does it appear that Newton ever +extended the law beyond the Solar System, and probably his caution would have +led him to avoid any statement of the kind until it should be proved.<br/> + With this exception the above statement of the law of universal gravitation +contains nothing that is not to be found in the <i>Principia</i>; and the +nearest approach to that statement occurs in the Seventh Proposition of Book +III.:—<br/> + Prop.: That gravitation occurs in all bodies, and that it is proportional to +the quantity of matter in each.<br/> + Cor. I.: The total attraction of gravitation on a planet arises, and is +composed, out of the attraction on the separate parts.<br/> + Cor. II.: The attraction on separate equal particles of a body is reciprocally +as the square of the distance from the particles. +</p> + +<p class="footnote"> +<a name="linknote-25" id="linknote-25"></a> <a href="#linknoteref-25">[3]</a> +It is said that, when working out this final result, the probability of its +confirming that part of his theory which he had reluctantly abandoned years +before excited him so keenly that he was forced to hand over his calculations +to a friend, to be completed by him. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h3><a name="8"></a>8. NEWTON’S SUCCESSORS—HALLEY, EULER, LAGRANGE, +LAPLACE, ETC.</h3> + +<p> +Edmund Halley succeeded Flamsteed as Second Astronomer Royal in 1721. Although +he did not contribute directly to the mathematical proofs of Newton’s +theory, yet his name is closely associated with some of its greatest successes. +</p> + +<p> +He was the first to detect the acceleration of the moon’s mean motion. +Hipparchus, having compared his own observations with those of more ancient +astronomers, supplied an accurate value of the moon’s mean motion in his +time. Halley similarly deduced a value for modern times, and found it sensibly +greater. He announced this in 1693, but it was not until 1749 that Dunthorne +used modern lunar tables to compute a lunar eclipse observed in Babylon 721 +B.C., another at Alexandria 201 B.C., a solar eclipse observed by Theon 360 +A.D., and two later ones up to the tenth century. He found that to explain +these eclipses Halley’s suggestion must be adopted, the acceleration +being 10” in one century. In 1757 Lalande again fixed it at 10.” +</p> + +<p> +The Paris Academy, in 1770, offered their prize for an investigation to see if +this could be explained by the theory of gravitation. Euler won the prize, but +failed to explain the effect, and said: “It appears to be established by +indisputable evidence that the secular inequality of the moon’s mean +motion cannot be produced by the forces of gravitation.” +</p> + +<p> +The same subject was again proposed for a prize which was shared by Lagrange<a href="#linknote-26" name="linknoteref-26" id="linknoteref-26"><sup>[1]</sup></a> and Euler, neither finding a solution, while the latter +asserted the existence of a resisting medium in space. +</p> + +<p> +Again, in 1774, the Academy submitted the same subject, a third time, for the +prize; and again Lagrange failed to detect a cause in gravitation. +</p> + +<p> +Laplace<a href="#linknote-27" name="linknoteref-27" id="linknoteref-27"><sup>[2]</sup></a> now took the matter in hand. He tried the +effect of a non-instantaneous action of gravity, to no purpose. But in 1787 he +gave the true explanation. The principal effect of the sun on the moon’s +orbit is to diminish the earth’s influence, thus lengthening the period +to a new value generally taken as constant. But Laplace’s calculations +showed the new value to depend upon the excentricity of the earth’s +orbit, which, according; to theory, has a periodical variation of enormous +period, and has been continually diminishing for thousands of years. Thus the +solar influence has been diminishing, and the moon’s mean motion +increased. Laplace computed the amount at 10” in one century, agreeing +with observation. (Later on Adams showed that Laplace’s calculation was +wrong, and that the value he found was too large; so, part of the acceleration +is now attributed by some astronomers to a lengthening of the day by tidal +friction.) +</p> + +<p> +Another contribution by Halley to the verification of Newton’s law was +made when he went to St. Helena to catalogue the southern stars. He measured +the change in length of the second’s pendulum in different latitudes due +to the changes in gravity foretold by Newton. +</p> + +<p> +Furthermore, he discovered the long inequality of Jupiter and Saturn, whose +period is 929 years. For an investigation of this also the Academy of Sciences +offered their prize. This led Euler to write a valuable essay disclosing a new +method of computing perturbations, called the instantaneous ellipse with +variable elements. The method was much developed by Lagrange. +</p> + +<p> +But again it was Laplace who solved the problem of the inequalities of Jupiter +and Saturn by the theory of gravitation, reducing the errors of the tables from +20’ down to 12”, thus abolishing the use of empirical corrections +to the planetary tables, and providing another glorious triumph for the law of +gravitation. As Laplace justly said: “These inequalities appeared +formerly to be inexplicable by the law of gravitation—they now form one +of its most striking proofs.” +</p> + +<p> +Let us take one more discovery of Halley, furnishing directly a new triumph for +the theory. He noticed that Newton ascribed parabolic orbits to the comets +which he studied, so that they come from infinity, sweep round the sun, and go +off to infinity for ever, after having been visible a few weeks or months. He +collected all the reliable observations of comets he could find, to the number +of twenty-four, and computed their parabolic orbits by the rules laid down by +Newton. His object was to find out if any of them really travelled in elongated +ellipses, practically undistinguishable, in the visible part of their paths, +from parabolæ, in which case they would be seen more than once. He found two +old comets whose orbits, in shape and position, resembled the orbit of a comet +observed by himself in 1682. Apian observed one in 1531; Kepler the other in +1607. The intervals between these appearances is seventy-five or seventy-six +years. He then examined and found old records of similar appearance in 1456, +1380, and 1305. It is true, he noticed, that the intervals varied by a year and +a-half, and the inclination of the orbit to the ecliptic diminished with +successive apparitions. But he knew from previous calculations that this might +easily be due to planetary perturbations. Finally, he arrived at the conclusion +that all of these comets were identical, travelling in an ellipse so elongated +that the part where the comet was seen seemed to be part of a parabolic orbit. +He then predicted its return at the end of 1758 or beginning of 1759, when he +should be dead; but, as he said, “if it should return, according to our +prediction, about the year 1758, impartial posterity will not refuse to +acknowledge that this was first discovered by an Englishman.”<a href="#linknote-28" name="linknoteref-28" id="linknoteref-28"><sup>[3]</sup></a> [<i>Synopsis Astronomiae Cometicae</i>, 1749.] +</p> + +<p> +Once again Halley’s suggestion became an inspiration for the mathematical +astronomer. Clairaut, assisted by Lalande, found that Saturn would retard the +comet 100 days, Jupiter 518 days, and predicted its return to perihelion on +April 13th, 1759. In his communication to the French Academy, he said that a +comet travelling into such distant regions might be exposed to the influence of +forces totally unknown, and “even of some planet too far removed from the +sun to be ever perceived.” +</p> + +<p> +The excitement of astronomers towards the end of 1758 became intense; and the +honour of first catching sight of the traveller fell to an amateur in Saxony, +George Palitsch, on Christmas Day, 1758. It reached perihelion on March 13th, +1759. +</p> + +<p> +This fact was a startling confirmation of the Newtonian theory, because it was +a new kind of calculation of perturbations, and also it added a new member to +the solar system, and gave a prospect of adding many more. +</p> + +<p> +When Halley’s comet reappeared in 1835, Pontecoulant’s computations +for the date of perihelion passage were very exact, and afterwards he showed +that, with more exact values of the masses of Jupiter and Saturn, his +prediction was correct within two days, after an invisible voyage of +seventy-five years! +</p> + +<p> +Hind afterwards searched out many old appearances of this comet, going back to +11 B.C., and most of these have been identified as being really Halley’s +comet by the calculations of Cowell and Cromellin<a href="#linknote-29" name="linknoteref-29" id="linknoteref-29"><sup>[4]</sup></a> (of +Greenwich Observatory), who have also predicted its next perihelion passage for +April 8th to 16th, 1910, and have traced back its history still farther, to 240 +B.C. +</p> + +<p> +Already, in November, 1907, the Astronomer Royal was trying to catch it by the +aid of photography. +</p> + +<hr /> + +<p> +<b>FOOTNOTES:</b> +</p> + +<p class="footnote"> +<a name="linknote-26" id="linknote-26"></a> <a href="#linknoteref-26">[1]</a> +Born 1736; died 1813. +</p> + +<p class="footnote"> +<a name="linknote-27" id="linknote-27"></a> <a href="#linknoteref-27">[2]</a> +Born 1749; died 1827. +</p> + +<p class="footnote"> +<a name="linknote-28" id="linknote-28"></a> <a href="#linknoteref-28">[3]</a> +This sentence does not appear in the original memoir communicated to the Royal +Society, but was first published in a posthumous reprint. +</p> + +<p class="footnote"> +<a name="linknote-29" id="linknote-29"></a> <a href="#linknoteref-29">[4]</a> +<i>R. A. S. Monthly Notices</i>, 1907-8. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h3><a name="9"></a>9. DISCOVERY OF NEW PLANETS—HERSCHEL, PIAZZI, ADAMS, +AND LE VERRIER.</h3> + +<p> +It would be very interesting, but quite impossible in these pages, to discuss +all the exquisite researches of the mathematical astronomers, and to inspire a +reverence for the names connected with these researches, which for two hundred +years have been establishing the universality of Newton’s law. The lunar +and planetary theories, the beautiful theory of Jupiter’s satellites, the +figure of the earth, and the tides, were mathematically treated by Maclaurin, +D’Alembert, Legendre, Clairaut, Euler, Lagrange, Laplace, Walmsley, +Bailly, Lalande, Delambre, Mayer, Hansen, Burchardt, Binet, Damoiseau, Plana, +Poisson, Gauss, Bessel, Bouvard, Airy, Ivory, Delaunay, Le Verrier, Adams, and +others of later date. +</p> + +<p> +By passing over these important developments it is possible to trace some of +the steps in the crowning triumph of the Newtonian theory, by which the planet +Neptune was added to the known members of the solar system by the independent +researches of Professor J.C. Adams and of M. Le Verrier, in 1846. +</p> + +<p> +It will be best to introduce this subject by relating how the eighteenth +century increased the number of known planets, which was then only six, +including the earth. +</p> + +<p> +On March 13th, 1781, Sir William Herschel was, as usual, engaged on examining +some small stars, and, noticing that one of them appeared to be larger than the +fixed stars, suspected that it might be a comet. To test this he increased his +magnifying power from 227 to 460 and 932, finding that, unlike the fixed stars +near it, its definition was impaired and its size increased. This convinced him +that the object was a comet, and he was not surprised to find on succeeding +nights that the position was changed, the motion being in the ecliptic. He gave +the observations of five weeks to the Royal Society without a suspicion that +the object was a new planet. +</p> + +<p> +For a long time people could not compute a satisfactory orbit for the supposed +comet, because it seemed to be near the perihelion, and no comet had ever been +observed with a perihelion distance from the sun greater than four times the +earth’s distance. Lexell was the first to suspect that this was a new +planet eighteen times as far from the sun as the earth is. In January, 1783, +Laplace published the elliptic elements. The discoverer of a planet has a right +to name it, so Herschel called it Georgium Sidus, after the king. But Lalande +urged the adoption of the name Herschel. Bode suggested Uranus, and this was +adopted. The new planet was found to rank in size next to Jupiter and Saturn, +being 4.3 times the diameter of the earth. +</p> + +<p> +In 1787 Herschel discovered two satellites, both revolving in nearly the same +plane, inclined 80° to the ecliptic, and the motion of both was +retrograde. +</p> + +<p> +In 1772, before Herschel’s discovery, Bode<a href="#linknote-30" name="linknoteref-30" id="linknoteref-30"><sup>[1]</sup></a> had +discovered a curious arbitrary law of planetary distances. Opposite each +planet’s name write the figure 4; and, in succession, add the numbers 0, +3, 6, 12, 24, 48, 96, <i>etc</i>., to the 4, always doubling the last numbers. +You then get the planetary distances. +</p> + +<pre> + Mercury, dist.-- 4 4 + 0 = 4 + Venus " 7 4 + 3 = 7 + Earth " 10 4 + 6 = 10 + Mars " 15 4 + 12 = 16 + -- 4 + 24 = 28 + Jupiter dist. 52 4 + 48 = 52 + Saturn " 95 4 + 96 = 100 + (Uranus) " 192 4 + 192 = 196 + -- 4 + 384 = 388 +</pre> + +<p> +All the five planets, and the earth, fitted this rule, except that there was a +blank between Mars and Jupiter. When Uranus was discovered, also fitting the +rule, the conclusion was irresistible that there is probably a planet between +Mars and Jupiter. An association of twenty-four astronomers was now formed in +Germany to search for the planet. Almost immediately afterwards the planet was +discovered, not by any member of the association, but by Piazzi, when engaged +upon his great catalogue of stars. On January 1st, 1801, he observed a star +which had changed its place the next night. Its motion was retrograde till +January 11th, direct after the 13th. Piazzi fell ill before he had enough +observations for computing the orbit with certainty, and the planet disappeared +in the sun’s rays. Gauss published an approximate ephemeris of probable +positions when the planet should emerge from the sun’s light. There was +an exciting hunt, and on December 31st (the day before its birthday) De Zach +captured the truant, and Piazzi christened it Ceres. +</p> + +<p> +The mean distance from the sun was found to be 2.767, agreeing with the 2.8 +given by Bode’s law. Its orbit was found to be inclined over 10° to +the ecliptic, and its diameter was only 161 miles. +</p> + +<p> +On March 28th, 1802, Olbers discovered a new seventh magnitude star, which +turned out to be a planet resembling Ceres. It was called Pallas. Gauss found +its orbit to be inclined 35° to the ecliptic, and to cut the orbit of +Ceres; whence Olbers considered that these might be fragments of a broken-up +planet. He then commenced a search for other fragments. In 1804 Harding +discovered Juno, and in 1807 Olbers found Vesta. The next one was not +discovered until 1845, from which date asteroids, or minor planets (as these +small planets are called), have been found almost every year. They now number +about 700. +</p> + +<p> +It is impossible to give any idea of the interest with which the first +additions since prehistoric times to the planetary system were received. All of +those who showered congratulations upon the discoverers regarded these +discoveries in the light of rewards for patient and continuous labours, the +very highest rewards that could be desired. And yet there remained still the +most brilliant triumph of all, the addition of another planet like Uranus, +before it had ever been seen, when the analysis of Adams and Le Verrier gave a +final proof of the powers of Newton’s great law to explain any planetary +irregularity. +</p> + +<p> +After Sir William Herschel discovered Uranus, in 1781, it was found that +astronomers had observed it on many previous occasions, mistaking it for a +fixed star of the sixth or seventh magnitude. Altogether, nineteen observations +of Uranus’s position, from the time of Flamsteed, in 1690, had been +recorded. +</p> + +<p> +In 1790 Delambre, using all these observations, prepared tables for computing +its position. These worked well enough for a time, but at last the differences +between the calculated and observed longitudes of the planet became serious. In +1821 Bouvard undertook a revision of the tables, but found it impossible to +reconcile all the observations of 130 years (the period of revolution of Uranus +is eighty-four years). So he deliberately rejected the old ones, expressing the +opinion that the discrepancies might depend upon “some foreign and +unperceived cause which may have been acting upon the planet.” In a few +years the errors even of these tables became intolerable. In 1835 the error of +longitude was 30”; in 1838, 50”; in 1841, 70”; and, by +comparing the errors derived from observations made before and after +opposition, a serious error of the distance (radius vector) became apparent. +</p> + +<p> +In 1843 John Couch Adams came out Senior Wrangler at Cambridge, and was free to +undertake the research which as an undergraduate he had set himself—to +see whether the disturbances of Uranus could be explained by assuming a certain +orbit, and position in that orbit, of a hypothetical planet even more distant +than Uranus. Such an explanation had been suggested, but until 1843 no one had +the boldness to attack the problem. Bessel had intended to try, but a fatal +illness overtook him. +</p> + +<p> +Adams first recalculated all known causes of disturbance, using the latest +determinations of the planetary masses. Still the errors were nearly as great +as ever. He could now, however, use these errors as being actually due to the +perturbations produced by the unknown planet. +</p> + +<p> +In 1844, assuming a circular orbit, and a mean distance agreeing with +Bode’s law, he obtained a first approximation to the position of the +supposed planet. He then asked Professor Challis, of Cambridge, to procure the +latest observations of Uranus from Greenwich, which Airy immediately supplied. +Then the whole work was recalculated from the beginning, with more exactness, +and assuming a smaller mean distance. +</p> + +<p> +In September, 1845, he handed to Challis the elements of the hypothetical +planet, its mass, and its apparent position for September 30th, 1845. On +September 22nd Challis wrote to Airy explaining the matter, and declaring his +belief in Adams’s capabilities. When Adams called on him Airy was away +from home, but at the end of October, 1845, he called again, and left a paper +with full particulars of his results, which had, for the most part, reduced the +discrepancies to about 1”. As a matter of fact, it has since been found +that the heliocentric place of the new planet then given was correct within +about 2°. +</p> + +<p> +Airy wrote expressing his interest, and asked for particulars about the radius +vector. Adams did not then reply, as the answer to this question could be seen +to be satisfactory by looking at the data already supplied. He was a most +unassuming man, and would not push himself forward. He may have felt, after all +the work he had done, that Airy’s very natural inquiry showed no +proportionate desire to search for the planet. Anyway, the matter lay in embryo +for nine months. +</p> + +<p> +Meanwhile, one of the ablest French astronomers, Le Verrier, experienced in +computing perturbations, was independently at work, knowing nothing about +Adams. He applied to his calculations every possible refinement, and, +considering the novelty of the problem, his calculation was one of the most +brilliant in the records of astronomy. In criticism it has been said that these +were exhibitions of skill rather than helps to a solution of the particular +problem, and that, in claiming to find the elements of the orbit within certain +limits, he was claiming what was, under the circumstances, impossible, as the +result proved. +</p> + +<p> +In June, 1846, Le Verrier announced, in the <i>Comptes Rendus de +l’Academie des Sciences</i>, that the longitude of the disturbing planet, +for January 1st, 1847, was 325, and that the probable error did not exceed +10°. +</p> + +<p> +This result agreed so well with Adams’s (within 1°) that Airy urged +Challis to apply the splendid Northumberland equatoreal, at Cambridge, to the +search. Challis, however, had already prepared an exhaustive plan of attack +which must in time settle the point. His first work was to observe, and make a +catalogue, or chart, of all stars near Adams’s position. +</p> + +<p> +On August 31st, 1846, Le Verrier published the concluding part of his labours. +</p> + +<p> +On September 18th, 1846, Le Verrier communicated his results to the Astronomers +at Berlin, and asked them to assist in searching for the planet. By good luck +Dr. Bremiker had just completed a star-chart of the very part of the heavens +including Le Verrier’s position; thus eliminating all of Challis’s +preliminary work. The letter was received in Berlin on September 23rd; and the +same evening Galle found the new planet, of the eighth magnitude, the size of +its disc agreeing with Le Verrier’s prediction, and the heliocentric +longitude agreeing within 57’. By this time Challis had recorded, without +reduction, the observations of 3,150 stars, as a commencement for his search. +On reducing these, he found a star, observed on August 12th, which was not in +the same place on July 30th. This was the planet, and he had also observed it +on August 4th. +</p> + +<p> +The feeling of wonder, admiration, and enthusiasm aroused by this intellectual +triumph was overwhelming. In the world of astronomy reminders are met every day +of the terrible limitations of human reasoning powers; and every success that +enables the mind’s eye to see a little more clearly the meaning of things +has always been heartily welcomed by those who have themselves been engaged in +like researches. But, since the publication of the <i>Principia</i>, in 1687, +there is probably no analytical success which has raised among astronomers such +a feeling of admiration and gratitude as when Adams and Le Verrier showed the +inequalities in Uranus’s motion to mean that an unknown planet was in a +certain place in the heavens, where it was found. +</p> + +<p> +At the time there was an unpleasant display of international jealousy. The +British people thought that the earlier date of Adams’s work, and of the +observation by Challis, entitled him to at least an equal share of credit with +Le Verrier. The French, on the other hand, who, on the announcement of the +discovery by Galle, glowed with pride in the new proof of the great powers of +their astronomer, Le Verrier, whose life had a long record of successes in +calculation, were incredulous on being told that it had all been already done +by a young man whom they had never heard of. +</p> + +<p> +These displays of jealousy have long since passed away, and there is now +universally an <i>entente cordiale</i> that to each of these great men belongs +equally the merit of having so thoroughly calculated this inverse problem of +perturbations as to lead to the immediate discovery of the unknown planet, +since called Neptune. +</p> + +<p> +It was soon found that the planet had been observed, and its position recorded +as a fixed star by Lalande, on May 8th and 10th, 1795. +</p> + +<p> +Mr. Lassel, in the same year, 1846, with his two-feet reflector, discovered a +satellite, with retrograde motion, which gave the mass of the planet about a +twentieth of that of Jupiter. +</p> + +<hr /> + +<p> +<b>FOOTNOTES:</b> +</p> + +<p class="footnote"> +<a name="linknote-30" id="linknote-30"></a> <a href="#linknoteref-30">[1]</a> +Bode’s law, or something like it, had already been fore-shadowed by +Kepler and others, especially Titius (see <i>Monatliche Correspondenz</i>, vol. +vii., p. 72). +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="book03"></a>BOOK III. OBSERVATION</h2> + +</div><!--end chapter--> + +<div class="chapter"> + +<h3><a name="10"></a>10. INSTRUMENTS OF PRECISION—STATE OF THE SOLAR +SYSTEM.</h3> + +<p> +Having now traced the progress of physical astronomy up to the time when very +striking proofs of the universality of the law of gravitation convinced the +most sceptical, it must still be borne in mind that, while gravitation is +certainly the principal force governing the motions of the heavenly bodies, +there may yet be a resisting medium in space, and there may be electric and +magnetic forces to deal with. There may, further, be cases where the effects of +luminous radiative repulsion become apparent, and also Crookes’ +vacuum-effects described as “radiant matter.” Nor is it quite +certain that Laplace’s proofs of the instantaneous propagation of gravity +are final. +</p> + +<p> +And in the future, as in the past, Tycho Brahe’s dictum must be +maintained, that all theory shall be preceded by accurate observations. It is +the pride of astronomers that their science stands above all others in the +accuracy of the facts observed, as well as in the rigid logic of the +mathematics used for interpreting these facts. +</p> + +<p> +It is interesting to trace historically the invention of those instruments of +precision which have led to this result, and, without entering on the details +required in a practical handbook, to note the guiding principles of +construction in different ages. +</p> + +<p> +It is very probable that the Chaldeans may have made spheres, like the +armillary sphere, for representing the poles of the heavens; and with rings to +show the ecliptic and zodiac, as well as the equinoctial and solstitial +colures; but we have no record. We only know that the tower of Belus, on an +eminence, was their observatory. We have, however, distinct records of two such +spheres used by the Chinese about 2500 B.C. Gnomons, or some kind of sundial, +were used by the Egyptians and others; and many of the ancient nations measured +the obliquity of the ecliptic by the shadows of a vertical column in summer and +winter. The natural horizon was the only instrument of precision used by those +who determined star positions by the directions of their risings and settings; +while in those days the clepsydra, or waterclock, was the best instrument for +comparing their times of rising and setting. +</p> + +<p> +About 300 B.C. an observatory fitted with circular instruments for star +positions was set up at Alexandria, the then centre of civilisation. We know +almost nothing about the instruments used by Hipparchus in preparing his star +catalogues and his lunar and solar tables; but the invention of the astrolabe +is attributed to him.<a href="#linknote-31" name="linknoteref-31" id="linknoteref-31"><sup>[1]</sup></a> +</p> + +<p> +In more modern times Nuremberg became a centre of astronomical culture. +Waltherus, of that town, made really accurate observations of star altitudes, +and of the distances between stars; and in 1484 A.D. he used a kind of clock. +Tycho Brahe tried these, but discarded them as being inaccurate. +</p> + +<p> +Tycho Brahe (1546-1601 A.D.) made great improvements in armillary spheres, +quadrants, sextants, and large celestial globes. With these he measured the +positions of stars, or the distance of a comet from several known stars. He has +left us full descriptions of them, illustrated by excellent engravings. +Previous to his time such instruments were made of wood. Tycho always used +metal. He paid the greatest attention to the stability of mounting, to the +orientation of his instruments, to the graduation of the arcs by the then new +method of transversals, and to the aperture sight used upon his pointer. There +were no telescopes in his day, and no pendulum clocks. He recognised the fact +that there must be instrumental errors. He made these as small as was possible, +measured their amount, and corrected his observations. His table of refractions +enabled him to abolish the error due to our atmosphere so far as it could +affect naked-eye observations. The azimuth circle of Tycho’s largest +quadrant had a diameter of nine feet, and the quadrant a radius of six feet. He +introduced the mural quadrant for meridian observations.<a href="#linknote-32" name="linknoteref-32" id="linknoteref-32"><sup>[2]</sup></a> +</p> + +<div class="fig" style="width:50%;"> +<a name="illus06"></a> +<img src="images/008.jpg" style="width:100%;" alt="ANCIENT CHINESE INSTRUMENTS" /> +<p class="caption">A<small>NCIENT</small> C<small>HINESE</small> +I<small>NSTRUMENTS</small>,<br/>Including quadrant, celestial globe, and two +armillae, in the Observatory at Peking. Photographed in Peking by the author in +1875, and stolen by the Germans when the Embassies were relieved by the allies +in 1900.</p> +</div> + +<p> +The French Jesuits at Peking, in the seventeenth century, helped the Chinese in +their astronomy. In 1875 the writer saw and photographed, on that part of the +wall of Peking used by the Mandarins as an observatory, the six instruments +handsomely designed by Father Verbiest, copied from the instruments of Tycho +Brahe, and embellished with Chinese dragons and emblems cast on the supports. +He also saw there two old instruments (which he was told were Arabic) of date +1279, by Ko Show-King, astronomer to Koblai Khan, the grandson of Chenghis +Khan. One of these last is nearly identical with the armillae of Tycho; and the +other with his “armillae æquatoriæ maximæ,” with which he observed +the comet of 1585, besides fixed stars and planets.<a href="#linknote-33" name="linknoteref-33" id="linknoteref-33"><sup>[3]</sup></a> +</p> + +<p> +The discovery by Galileo of the isochronism of the pendulum, followed by +Huyghens’s adaptation of that principle to clocks, has been one of the +greatest aids to accurate observation. About the same time an equally +beneficial step was the employment of the telescope as a pointer; not the +Galilean with concave eye-piece, but with a magnifying glass to examine the +focal image, at which also a fixed mark could be placed. Kepler was the first +to suggest this. Gascoigne was the first to use it. Huyghens used a metal strip +of variable width in the focus, as a micrometer to cover a planetary disc, and +so to measure the width covered by the planet. The Marquis Malvasia, in 1662, +described the network of fine silver threads at right angles, which he used in +the focus, much as we do now. +</p> + +<p> +In the hands of such a skilful man as Tycho Brahe, the old open sights, even +without clocks, served their purpose sufficiently well to enable Kepler to +discover the true theory of the solar system. But telescopic sights and clocks +were required for proving some of Newton’s theories of planetary +perturbations. Picard’s observations at Paris from 1667 onwards seem to +embody the first use of the telescope as a pointer. He was also the first to +introduce the use of Huyghens’s clocks for observing the right ascension +of stars. Olaus Romer was born at Copenhagen in 1644. In 1675, by careful study +of the times of eclipses of Jupiter’s satellites, he discovered that +light took time to traverse space. Its velocity is 186,000 miles per second. In +1681 he took up his duties as astronomer at Copenhagen, and built the first +transit circle on a window-sill of his house. The iron axis was five feet long +and one and a-half inches thick, and the telescope was fixed near one end with +a counterpoise. The telescope-tube was a double cone, to prevent flexure. Three +horizontal and three vertical wires were used in the focus. These were +illuminated by a speculum, near the object-glass, reflecting the light from a +lantern placed over the axis, the upper part of the telescope-tube being partly +cut away to admit the light. A divided circle, with pointer and reading +microscope, was provided for reading the declination. He realised the +superiority of a circle with graduations over a much larger quadrant. The +collimation error was found by reversing the instrument and using a terrestrial +mark, the azimuth error by star observations. The time was expressed in +fractions of a second. He also constructed a telescope with equatoreal +mounting, to follow a star by one axial motion. In 1728 his instruments and +observation records were destroyed by fire. +</p> + +<p> +Hevelius had introduced the vernier and tangent screw in his measurement of arc +graduations. His observatory and records were burnt to the ground in 1679. +Though an old man, he started afresh, and left behind him a catalogue of 1,500 +stars. +</p> + +<p> +Flamsteed began his duties at Greenwich Observatory, as first Astronomer Royal, +in 1676, with very poor instruments. In 1683 he put up a mural arc of +140°, and in 1689 a better one, seventy-nine inches radius. He conducted +his measurements with great skill, and introduced new methods to attain +accuracy, using certain stars for determining the errors of his instruments; +and he always reduced his observations to a form in which they could be readily +used. He introduced new methods for determining the position of the equinox and +the right ascension of a fundamental star. He produced a catalogue of 2,935 +stars. He supplied Sir Isaac Newton with results of observation required in his +theoretical calculations. He died in 1719. +</p> + +<p> +Halley succeeded Flamsteed to find that the whole place had been gutted by the +latter’s executors. In 1721 he got a transit instrument, and in 1726 a +mural quadrant by Graham. His successor in 1742, Bradley, replaced this by a +fine brass quadrant, eight feet radius, by Bird; and Bradley’s zenith +sector was purchased for the observatory. An instrument like this, specially +designed for zenith stars, is capable of greater rigidity than a more universal +instrument; and there is no trouble with refraction in the zenith. For these +reasons Bradley had set up this instrument at Kew, to attempt the proof of the +earth’s motion by observing the annual parallax of stars. He certainly +found an annual variation of zenith distance, but not at the times of year +required by the parallax. This led him to the discovery of the +“aberration” of light and of nutation. Bradley has been described +as the founder of the modern system of accurate observation. He died in 1762, +leaving behind him thirteen folio volumes of valuable but unreduced +observations. Those relating to the stars were reduced by Bessel and published +in 1818, at Königsberg, in his well-known standard work, <i>Fundamenta +Astronomiae</i>. In it are results showing the laws of refraction, with tables +of its amount, the maximum value of aberration, and other constants. +</p> + +<p> +Bradley was succeeded by Bliss, and he by Maskelyne (1765), who carried on +excellent work, and laid the foundations of the Nautical Almanac (1767). Just +before his death he induced the Government to replace Bird’s quadrant by +a fine new mural <i>circle</i>, six feet in diameter, by Troughton, the +divisions being read off by microscopes fixed on piers opposite to the divided +circle. In this instrument the micrometer screw, with a divided circle for +turning it, was applied for bringing the micrometer wire actually in line with +a division on the circle—a plan which is still always adopted. +</p> + +<p> +Pond succeeded Maskelyne in 1811, and was the first to use this instrument. +From now onwards the places of stars were referred to the pole, not to the +zenith; the zero being obtained from measures on circumpolar stars. Standard +stars were used for giving the clock error. In 1816 a new transit instrument, +by Troughton, was added, and from this date the Greenwich star places have +maintained the very highest accuracy. +</p> + +<p> +George Biddell Airy, Seventh Astronomer Royal,<a href="#linknote-34" name="linknoteref-34" id="linknoteref-34"><sup>[4]</sup></a> +commenced his Greenwich labours in 1835. His first and greatest reformation in +the work of the observatory was one he had already established at Cambridge, +and is now universally adopted. He held that an observation is not completed +until it has been reduced to a useful form; and in the case of the sun, moon, +and planets these results were, in every case, compared with the tables, and +the tabular error printed. +</p> + +<p> +Airy was firmly impressed with the object for which Charles II. had wisely +founded the observatory in connection with navigation, and for observations of +the moon. Whenever a meridian transit of the moon could be observed this was +done. But, even so, there are periods in the month when the moon is too near +the sun for a transit to be well observed. Also weather interferes with many +meridian observations. To render the lunar observations more continuous, Airy +employed Troughton’s successor, James Simms, in conjunction with the +engineers, Ransome and May, to construct an altazimuth with three-foot circles, +and a five-foot telescope, in 1847. The result was that the number of lunar +observations was immediately increased threefold, many of them being in a part +of the moon’s orbit which had previously been bare of observations. From +that date the Greenwich lunar observations have been a model and a standard for +the whole world. +</p> + +<p> +Airy also undertook to superintend the reduction of all Greenwich lunar +observations from 1750 to 1830. The value of this laborious work, which was +completed in 1848, cannot be over-estimated. +</p> + +<p> +The demands of astronomy, especially in regard to small minor planets, required +a transit instrument and mural circle with a more powerful telescope. Airy +combined the functions of both, and employed the same constructors as before to +make a <i>transit-circle</i> with a telescope of eleven and a-half feet focus +and a circle of six-feet diameter, the object-glass being eight inches in +diameter. +</p> + +<p> +Airy, like Bradley, was impressed with the advantage of employing stars in the +zenith for determining the fundamental constants of astronomy. He devised a +<i>reflex zenith tube</i>, in which the zenith point was determined by +reflection from a surface of mercury. The design was so simple, and seemed so +perfect, that great expectations were entertained. But unaccountable variations +comparable with those of the transit circle appeared, and the instrument was +put out of use until 1903, when the present Astronomer Royal noticed that the +irregularities could be allowed for, being due to that remarkable variation in +the position of the earth’s axis included in circles of about six yards +diameter at the north and south poles, discovered at the end of the nineteenth +century. The instrument is now being used for investigating these variations; +and in the year 1907 as many as 1,545 observations of stars were made with the +reflex zenith tube. +</p> + +<p> +In connection with zenith telescopes it must be stated that Respighi, at the +Capitol Observatory at Rome, made use of a deep well with a level mercury +surface at the bottom and a telescope at the top pointing downwards, which the +writer saw in 1871. The reflection of the micrometer wires and of a star very +near the zenith (but not quite in the zenith) can be observed together. His +mercury trough was a circular plane surface with a shallow edge to retain the +mercury. The surface quickly came to rest after disturbance by street traffic. +</p> + +<p> +Sir W. M. H. Christie, Eighth Astronomer Royal, took up his duties in that +capacity in 1881. Besides a larger altazimuth that he erected in 1898, he has +widened the field of operations at Greenwich by the extensive use of +photography and the establishment of large equatoreals. From the point of view +of instruments of precision, one of the most important new features is the +astrographic equatoreal, set up in 1892 and used for the Greenwich section of +the great astrographic chart just completed. Photography has come to be of use, +not only for depicting the sun and moon, comets and nebulae, but also to obtain +accurate relative positions of neighbouring stars; to pick up objects that are +invisible in any telescope; and, most of all perhaps, in fixing the positions +of faint satellites. Thus Saturn’s distant satellite, Phoebe, and the +sixth and seventh satellites of Jupiter, have been followed regularly in their +courses at Greenwich ever since their discovery with the thirty-inch reflector +(erected in 1897); and while doing so Mr. Melotte made, in 1908, the splendid +discovery on some of the photographic plates of an eighth satellite of Jupiter, +at an enormous distance from the planet. From observations in the early part of +1908, over a limited arc of its orbit, before Jupiter approached the sun, Mr. +Cowell computed a retrograde orbit and calculated the future positions of this +satellite, which enabled Mr. Melotte to find it again in the autumn—a +great triumph both of calculation and of photographic observation. This +satellite has never been seen, and has been photographed only at Greenwich, +Heidelberg, and the Lick Observatory. +</p> + +<p> +Greenwich Observatory has been here selected for tracing the progress of +accurate measurement. But there is one instrument of great value, the +heliometer, which is not used at Greenwich. This serves the purpose of a double +image micrometer, and is made by dividing the object-glass of a telescope along +a diameter. Each half is mounted so as to slide a distance of several inches +each way on an arc whose centre is the focus. The amount of the movement can be +accurately read. Thus two fields of view overlap, and the adjustment is made to +bring an image of one star over that of another star, and then to do the same +by a displacement in the opposite direction. The total movement of the +half-object glass is double the distance between the star images in the focal +plane. Such an instrument has long been established at Oxford, and German +astronomers have made great use of it. But in the hands of Sir David Gill (late +His Majesty’s Astronomer at the Cape of Good Hope), and especially in his +great researches on Solar and on Stellar parallax, it has been recognised as an +instrument of the very highest accuracy, measuring the distance between stars +correctly to less than a tenth of a second of arc. +</p> + +<p> +The superiority of the heliometer over all other devices (except photography) +for measuring small angles has been specially brought into prominence by Sir +David Gill’s researches on the distance of the sun—<i>i.e.,</i> the +scale of the solar system. A measurement of the distance of any planet fixes +the scale, and, as Venus approaches the earth most nearly of all the planets, +it used to be supposed that a Transit of Venus offered the best opportunity for +such measurement, especially as it was thought that, as Venus entered on the +solar disc, the sweep of light round the dark disc of Venus would enable a very +precise observation to be made. The Transit of Venus in 1874, in which the +present writer assisted, overthrew this delusion. +</p> + +<p> +In 1877 Sir David Gill used Lord Crawford’s heliometer at the Island of +Ascension to measure the parallax of Mars in opposition, and found the +sun’s distance 93,080,000 miles. He considered that, while the +superiority of the heliometer had been proved, the results would be still +better with the points of light shown by minor planets rather than with the +disc of Mars. +</p> + +<p> +In 1888-9, at the Cape, he observed the minor planets Iris, Victoria, and +Sappho, and secured the co-operation of four other heliometers. His final +result was 92,870,000 miles, the parallax being 8",802 (<i>Cape Obs</i>., Vol. +VI.). +</p> + +<p> +So delicate were these measures that Gill detected a minute periodic error of +theory of twenty-seven days, owing to a periodically erroneous position of the +centre of gravity of the earth and moon to which the position of the observer +was referred. This led him to correct the mass of the moon, and to fix its +ratio to the earth’s mass = 0.012240. +</p> + +<p> +Another method of getting the distance from the sun is to measure the velocity +of the earth’s orbital motion, giving the circumference traversed in a +year, and so the radius of the orbit. This has been done by comparing +observation and experiment. The aberration of light is an angle 20” 48, +giving the ratio of the earth’s velocity to the velocity of light. The +velocity of light is 186,000 miles a second; whence the distance to the sun is +92,780,000 miles. There seems, however, to be some uncertainty about the true +value of the aberration, any determination of which is subject to +irregularities due to the “seasonal errors.” The velocity of light +was experimentally found, in 1862, by Fizeau and Foucault, each using an +independent method. These methods have been developed, and new values found, by +Cornu, Michaelson, Newcomb, and the present writer. +</p> + +<p> +Quite lately Halm, at the Cape of Good Hope, measured spectroscopically the +velocity of the earth to and from a star by observations taken six months +apart. Thence he obtained an accurate value of the sun’s distance.<a href="#linknote-35" name="linknoteref-35" id="linknoteref-35"><sup>[5]</sup></a> +</p> + +<p> +But the remarkably erratic minor planet, Eros, discovered by Witte in 1898, +approaches the earth within 15,000,000 miles at rare intervals, and, with the +aid of photography, will certainly give us the best result. A large number of +observatories combined to observe the opposition of 1900. Their results are not +yet completely reduced, but the best value deduced so far for the +parallax<a href="#linknote-36" name="linknoteref-36" id="linknoteref-36"><sup>[6]</sup></a> +is 8".807 ± 0".0028.<a href="#linknote-37" name="linknoteref-37" id="linknoteref-37"><sup>[7]</sup></a> +</p> + +<hr /> + +<p> +<b>FOOTNOTES:</b> +</p> + +<p class="footnote"> +<a name="linknote-31" id="linknote-31"></a> <a href="#linknoteref-31">[1]</a> +In 1480 Martin Behaim, of Nuremberg, produced his <i>astrolabe</i> for +measuring the latitude, by observation of the sun, at sea. It consisted of a +graduated metal circle, suspended by a ring which was passed over the thumb, +and hung vertically. A pointer was fixed to a pin at the centre. This arm, +called the <i>alhidada</i>, worked round the graduated circle, and was pointed +to the sun. The altitude of the sun was thus determined, and, by help of solar +tables, the latitude could be found from observations made at apparent noon. +</p> + +<p class="footnote"> +<a name="linknote-32" id="linknote-32"></a> <a href="#linknoteref-32">[2]</a> +See illustration on p. 76. +</p> + +<p class="footnote"> +<a name="linknote-33" id="linknote-33"></a> <a href="#linknoteref-33">[3]</a> +See Dreyer’s article on these instruments in <i>Copernicus</i>, Vol. I. +They were stolen by the Germans after the relief of the Embassies, in 1900. The +best description of these instruments is probably that contained in an +interesting volume, which may be seen in the library of the R. A. S., entitled +<i>Chinese Researches</i>, by Alexander Wyllie (Shanghai, 1897). +</p> + +<p class="footnote"> +<a name="linknote-34" id="linknote-34"></a> <a href="#linknoteref-34">[4]</a> +Sir George Airy was very jealous of this honourable title. He rightly held that +there is only one Astronomer Royal at a time, as there is only one Mikado, one +Dalai Lama. He said that His Majesty’s Astronomer at the Cape of Good +Hope, His Majesty’s Astronomer for Scotland, and His Majesty’s +Astronomer for Ireland are not called Astronomers Royal. +</p> + +<p class="footnote"> +<a name="linknote-35" id="linknote-35"></a> <a href="#linknoteref-35">[5]</a> +<i>Annals of the Cape Observatory</i>, vol. x., part 3. +</p> + +<p class="footnote"> +<a name="linknote-36" id="linknote-36"></a> <a href="#linknoteref-36">[6]</a> +The parallax of the sun is the angle subtended by the earth’s radius at +the sun’s distance. +</p> + +<p class="footnote"> +<a name="linknote-37" id="linknote-37"></a> <a href="#linknoteref-37">[7]</a> +A. R. Hinks, R.A.S.; Monthly Notices, June, 1909. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h3><a name="11"></a>11. HISTORY OF THE TELESCOPE</h3> + +<p> +Accounts of wonderful optical experiments by Roger Bacon (who died in 1292), +and in the sixteenth century by Digges, Baptista Porta, and Antonio de Dominis +(Grant, <i>Hist. Ph. Ast</i>.), have led some to suppose that they invented the +telescope. The writer considers that it is more likely that these notes refer +to a kind of <i>camera obscura</i>, in which a lens throws an inverted image of +a landscape on the wall. +</p> + +<p> +The first telescopes were made in Holland, the originator being either Henry +Lipperhey,<a href="#linknote-38" name="linknoteref-38" id="linknoteref-38"><sup>[1]</sup></a> Zacharias Jansen, or James Metius, and the +date 1608 or earlier. +</p> + +<p> +In 1609 Galileo, being in Venice, heard of the invention, went home and worked +out the theory, and made a similar telescope. These telescopes were all made +with a convex object-glass and a concave eye-lens, and this type is spoken of +as the Galilean telescope. Its defects are that it has no real focus where +cross-wires can be placed, and that the field of view is very small. Kepler +suggested the convex eye-lens in 1611, and Scheiner claimed to have used one in +1617. But it was Huyghens who really introduced them. In the seventeenth +century telescopes were made of great length, going up to 300 feet. Huyghens +also invented the compound eye-piece that bears his name, made of two convex +lenses to diminish spherical aberration. +</p> + +<p> +But the defects of colour remained, although their cause was unknown until +Newton carried out his experiments on dispersion and the solar spectrum. To +overcome the spherical aberration James Gregory,<a href="#linknote-39" name="linknoteref-39" id="linknoteref-39"><sup>[2]</sup></a> of +Aberdeen and Edinburgh, in 1663, in his <i>Optica Promota</i>, proposed a +reflecting speculum of parabolic form. But it was Newton, about 1666, who first +made a reflecting telescope; and he did it with the object of avoiding colour +dispersion. +</p> + +<p> +Some time elapsed before reflectors were much used. Pound and Bradley used one +presented to the Royal Society by Hadley in 1723. Hawksbee, Bradley, and +Molyneaux made some. But James Short, of Edinburgh, made many excellent +Gregorian reflectors from 1732 till his death in 1768. +</p> + +<p> +Newton’s trouble with refractors, chromatic aberration, remained +insurmountable until John Dollond (born 1706, died 1761), after many +experiments, found out how to make an achromatic lens out of two +lenses—one of crown glass, the other of flint glass—to destroy the +colour, in a way originally suggested by Euler. He soon acquired a great +reputation for his telescopes of moderate size; but there was a difficulty in +making flint-glass lenses of large size. The first actual inventor and +constructor of an achromatic telescope was Chester Moor Hall, who was not in +trade, and did not patent it. Towards the close of the eighteenth century a +Swiss named Guinand at last succeeded in producing larger flint-glass discs +free from striae. Frauenhofer, of Munich, took him up in 1805, and soon +produced, among others, Struve’s Dorpat refractor of 9.9 inches diameter +and 13.5 feet focal length, and another, of 12 inches diameter and 18 feet +focal length, for Lamont, of Munich. +</p> + +<p> +In the nineteenth century gigantic <i>reflectors</i> have been made. +Lassel’s 2-foot reflector, made by himself, did much good work, and +discovered four new satellites. But Lord Rosse’s 6-foot reflector, 54 +feet focal length, constructed in 1845, is still the largest ever made. The +imperfections of our atmosphere are against the use of such large apertures, +unless it be on high mountains. During the last half century excellent specula +have been made of silvered glass, and Dr. Common’s 5-foot speculum +(removed, since his death, to Harvard) has done excellent work. Then there are +the 5-foot Yerkes reflector at Chicago, and the 4-foot by Grubb at Melbourne. +</p> + +<p> +Passing now from these large reflectors to refractors, further improvements +have been made in the manufacture of glass by Chance, of Birmingham, Feil and +Mantois, of Paris, and Schott, of Jena; while specialists in grinding lenses, +like Alvan Clark, of the U.S.A., and others, have produced many large +refractors. +</p> + +<p> +Cooke, of York, made an object-glass, 25-inch diameter, for Newall, of +Gateshead, which has done splendid work at Cambridge. We have the Washington +26-inch by Clark, the Vienna 27-inch by Grubb, the Nice 29½-inch by +Gautier, the Pulkowa 30-inch by Clark. Then there was the sensation of +Clark’s 36-inch for the Lick Observatory in California, and finally his +<i>tour de force</i>, the Yerkes 40-inch refractor, for Chicago. +</p> + +<p> +At Greenwich there is the 28-inch photographic refractor, and the Thompson +equatoreal by Grubb, carrying both the 26-inch photographic refractor and the +30-inch reflector. At the Cape of Good Hope we find Mr. Frank McClean’s +24-inch refractor, with an object-glass prism for spectroscopic work. +</p> + +<p> +It would be out of place to describe here the practical adjuncts of a modern +equatoreal—the adjustments for pointing it, the clock for driving it, the +position-micrometer and various eye-pieces, the photographic and spectroscopic +attachments, the revolving domes, observing seats, and rising floors and +different forms of mounting, the siderostats and coelostats, and other +convenient adjuncts, besides the registering chronograph and numerous +facilities for aiding observation. On each of these a chapter might be written; +but the most important part of the whole outfit is the man behind the +telescope, and it is with him that a history is more especially concerned. +</p> + +<h4>SPECTROSCOPE.</h4> + +<p> +Since the invention of the telescope no discovery has given so great an impetus +to astronomical physics as the spectroscope; and in giving us information about +the systems of stars and their proper motions it rivals the telescope. +</p> + +<p> +Frauenhofer, at the beginning of the nineteenth century, while applying +Dollond’s discovery to make large achromatic telescopes, studied the +dispersion of light by a prism. Admitting the light of the sun through a narrow +slit in a window-shutter, an inverted image of the slit can be thrown, by a +lens of suitable focal length, on the wall opposite. If a wedge or prism of +glass be interposed, the image is deflected to one side; but, as Newton had +shown, the images formed by the different colours of which white light is +composed are deflected to different extents—the violet most, the red +least. The number of colours forming images is so numerous as to form a +continuous spectrum on the wall with all the colours—red, orange, yellow, +green, blue, indigo, and violet. But Frauenhofer found with a narrow slit, well +focussed by the lens, that some colours were missing in the white light of the +sun, and these were shown by dark lines across the spectrum. These are the +Frauenhofer lines, some of which he named by the letters of the alphabet. The D +line is a very marked one in the yellow. These dark lines in the solar spectrum +had already been observed by Wollaston.<a href="#linknote-40" name="linknoteref-40" id="linknoteref-40"><sup>[3]</sup></a> +</p> + +<p> +On examining artificial lights it was found that incandescent solids and +liquids (including the carbon glowing in a white gas flame) give continuous +spectra; gases, except under enormous pressure, give bright lines. If sodium or +common salt be thrown on the colourless flame of a spirit lamp, it gives it a +yellow colour, and its spectrum is a bright yellow line agreeing in position +with line D of the solar spectrum. +</p> + +<p> +In 1832 Sir David Brewster found some of the solar black lines increased in +strength towards sunset, and attributed them to absorption in the earth’s +atmosphere. He suggested that the others were due to absorption in the +sun’s atmosphere. Thereupon Professor J. D. Forbes pointed out that +during a nearly total eclipse the lines ought to be strengthened in the same +way; as that part of the sun’s light, coming from its edge, passes +through a great distance in the sun’s atmosphere. He tried this with the +annular eclipse of 1836, with a negative result which has never been accounted +for, and which seemed to condemn Brewster’s view. +</p> + +<p> +In 1859 Kirchoff, on repeating Frauenhofer’s experiment, found that, if a +spirit lamp with salt in the flame were placed in the path of the light, the +black D line is intensified. He also found that, if he used a limelight instead +of the sunlight and passed it through the flame with salt, the spectrum showed +the D line black; or the vapour of sodium absorbs the same light that it +radiates. This proved to him the existence of sodium in the sun’s +atmosphere.<a href="#linknote-41" name="linknoteref-41" id="linknoteref-41"><sup>[4]</sup></a> Iron, calcium, and other elements were +soon detected in the same way. +</p> + +<p> +Extensive laboratory researches (still incomplete) have been carried out to +catalogue (according to their wave-length on the undulatory theory of light) +all the lines of each chemical element, under all conditions of temperature and +pressure. At the same time, all the lines have been catalogued in the light of +the sun and the brighter of the stars. +</p> + +<p> +Another method of obtaining spectra had long been known, by transmission +through, or reflection from, a grating of equidistant lines ruled upon glass or +metal. H. A. Rowland developed the art of constructing these gratings, which +requires great technical skill, and for this astronomers owe him a debt of +gratitude. +</p> + +<p> +In 1842 Doppler<a href="#linknote-42" name="linknoteref-42" id="linknoteref-42"><sup>[5]</sup></a> proved that the colour of a luminous +body, like the pitch or note of a sounding body, must be changed by velocity of +approach or recession. Everyone has noticed on a railway that, on meeting a +locomotive whistling, the note is lowered after the engine has passed. The +pitch of a sound or the colour of a light depends on the number of waves +striking the ear or eye in a second. This number is increased by approach and +lowered by recession. +</p> + +<p> +Thus, by comparing the spectrum of a star alongside a spectrum of hydrogen, we +may see all the lines, and be sure that there is hydrogen in the star; yet the +lines in the star-spectrum may be all slightly displaced to one side of the +lines of the comparison spectrum. If towards the violet end, it means mutual +approach of the star and earth; if to the red end, it means recession. The +displacement of lines does not tell us whether the motion is in the star, the +earth, or both. The displacement of the lines being measured, we can calculate +the rate of approach or recession in miles per second. +</p> + +<p> +In 1868 Huggins<a href="#linknote-43" name="linknoteref-43" id="linknoteref-43"><sup>[6]</sup></a> succeeded in thus measuring the +velocities of stars in the direction of the line of sight. +</p> + +<p> +In 1873 Vogel<a href="#linknote-44" name="linknoteref-44" id="linknoteref-44"><sup>[7]</sup></a> compared the spectra of the sun’s +East (approaching) limb and West (receding) limb, and the displacement of lines +endorsed the theory. This last observation was suggested by Zöllner. +</p> + +<hr /> + +<p> +<b>FOOTNOTES:</b> +</p> + +<p class="footnote"> +<a name="linknote-38" id="linknote-38"></a> <a href="#linknoteref-38">[1]</a> +In the <i>Encyclopaedia Britannica</i>, article “Telescope,” and in +Grant’s <i>Physical Astronomy</i>, good reasons are given for awarding +the honour to Lipperhey. +</p> + +<p class="footnote"> +<a name="linknote-39" id="linknote-39"></a> <a href="#linknoteref-39">[2]</a> +Will the indulgent reader excuse an anecdote which may encourage some workers +who may have found their mathematics defective through want of use? James +Gregory’s nephew David had a heap of MS. notes by Newton. These descended +to a Miss Gregory, of Edinburgh, who handed them to the present writer, when an +undergraduate at Cambridge, to examine. After perusal, he lent them to his +kindest of friends, J. C. Adams (the discoverer of Neptune), for his opinion. +Adams’s final verdict was: “I fear they are of no value. It is +pretty evident that, when he wrote these notes, <i>Newton’s mathematics +were a little rusty</i>.” +</p> + +<p class="footnote"> +<a name="linknote-40" id="linknote-40"></a> <a href="#linknoteref-40">[3]</a> +<i>R. S. Phil. Trans</i>. +</p> + +<p class="footnote"> +<a name="linknote-41" id="linknote-41"></a> <a href="#linknoteref-41">[4]</a> +The experiment had been made before by one who did not understand its meaning;. +But Sir George G. Stokes had already given verbally the true explanation of +Frauenhofer lines. +</p> + +<p class="footnote"> +<a name="linknote-42" id="linknote-42"></a> <a href="#linknoteref-42">[5]</a> +<i>Abh. d. Kön. Böhm. d. Wiss</i>., Bd. ii., 1841-42, p. 467. See also Fizeau +in the <i>Ann. de Chem. et de Phys</i>., 1870, p. 211. +</p> + +<p class="footnote"> +<a name="linknote-43" id="linknote-43"></a> <a href="#linknoteref-43">[6]</a> +<i>R. S. Phil. Trans</i>., 1868. +</p> + +<p class="footnote"> +<a name="linknote-44" id="linknote-44"></a> <a href="#linknoteref-44">[7]</a> +<i>Ast. Nach</i>., No. 1, 864. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="book04"></a>BOOK IV. THE PHYSICAL PERIOD</h2> + +<p> +We have seen how the theory of the solar system was slowly developed by the +constant efforts of the human mind to find out what are the rules of cause and +effect by which our conception of the present universe and its development +seems to be bound. In the primitive ages a mere record of events in the heavens +and on the earth gave the only hope of detecting those uniform sequences from +which to derive rules or laws of cause and effect upon which to rely. Then came +the geometrical age, in which rules were sought by which to predict the +movements of heavenly bodies. Later, when the relation of the sun to the +courses of the planets was established, the sun came to be looked upon as a +cause; and finally, early in the seventeenth century, for the first time in +history, it began to be recognised that the laws of dynamics, exactly as they +had been established for our own terrestrial world, hold good, with the same +rigid invariability, at least as far as the limits of the solar system. +</p> + +<p> +Throughout this evolution of thought and conjecture there were two types of +astronomers—those who supplied the facts, and those who supplied the +interpretation through the logic of mathematics. So Ptolemy was dependent upon +Hipparchus, Kepler on Tycho Brahe, and Newton in much of his work upon +Flamsteed. +</p> + +<p> +When Galileo directed his telescope to the heavens, when Secchi and Huggins +studied the chemistry of the stars by means of the spectroscope, and when +Warren De la Rue set up a photoheliograph at Kew, we see that a progress in the +same direction as before, in the evolution of our conception of the universe, +was being made. Without definite expression at any particular date, it came to +be an accepted fact that not only do earthly dynamics apply to the heavenly +bodies, but that the laws we find established here, in geology, in chemistry, +and in the laws of heat, may be extended with confidence to the heavenly +bodies. Hence arose the branch of astronomy called astronomical physics, a +science which claims a large portion of the work of the telescope, +spectroscope, and photography. In this new development it is more than ever +essential to follow the dictum of Tycho Brahe—not to make theories until +all the necessary facts are obtained. The great astronomers of to-day still +hold to Sir Isaac Newton’s declaration, “Hypotheses non +fingo.” Each one may have his suspicions of a theory to guide him in a +course of observation, and may call it a working hypothesis. But the cautious +astronomer does not proclaim these to the world; and the historian is certainly +not justified in including in his record those vague speculations founded on +incomplete data which may be demolished to-morrow, and which, however +attractive they may be, often do more harm than good to the progress of true +science. Meanwhile the accumulation of facts has been prodigious, and the +revelations of the telescope and spectroscope entrancing. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h3><a name="12"></a>12. THE SUN.</h3> + +<p> +One of Galileo’s most striking discoveries, when he pointed his telescope +to the heavenly bodies, was that of the irregularly shaped spots on the sun, +with the dark central <i>umbra</i> and the less dark, but more extensive, +<i>penumbra</i> surrounding it, sometimes with several umbrae in one penumbra. +He has left us many drawings of these spots, and he fixed their period of +rotation as a lunar month. +</p> + +<p> +It is not certain whether Galileo, Fabricius, or Schemer was the first to see +the spots. They all did good work. The spots were found to be ever varying in +size and shape. Sometimes, when a spot disappears at the western limb of the +sun, it is never seen again. In other cases, after a fortnight, it reappears at +the eastern limb. The faculae, or bright areas, which are seen all over the +sun’s surface, but specially in the neighbourhood of spots, and most +distinctly near the sun’s edge, were discovered by Galileo. A high +telescopic power resolves their structure into an appearance like +willow-leaves, or rice-grains, fairly uniform in size, and more marked than on +other parts of the sun’s surface. +</p> + +<div class="fig" style="width:60%;"> +<a name="illus07"></a> +<img src="images/009.jpg" style="width:100%;" alt="SOLAR SURFACE" /> +<p class="caption">S<small>OLAR</small> S<small>URFACE</small>.<br/>As +Photographed at the Royal Observatory, Greenwich, showing sun-spots with umbræ, +penumbræ, and faculæ.</p> +</div> + +<p> +Speculations as to the cause of sun-spots have never ceased from +Galileo’s time to ours. He supposed them to be clouds. Scheiner<a href="#linknote-45" name="linknoteref-45" id="linknoteref-45"><sup>[1]</sup></a> said they were the indications of tumultuous movements +occasionally agitating the ocean of liquid fire of which he supposed the sun to +be composed. +</p> + +<p> +A. Wilson, of Glasgow, in 1769,<a href="#linknote-46" name="linknoteref-46" id="linknoteref-46"><sup>[2]</sup></a> noticed a movement of +the umbra relative to the penumbra in the transit of the spot over the +sun’s surface; exactly as if the spot were a hollow, with a black base +and grey shelving sides. This was generally accepted, but later investigations +have contradicted its universality. Regarding the cause of these hollows, +Wilson said:— +</p> + +<p class="letter">Whether their first production and subsequent numberless +changes depend upon the eructation of elastic vapours from below, or upon +eddies or whirlpools commencing at the surface, or upon the dissolving of the +luminous matter in the solar atmosphere, as clouds are melted and again given +out by our air; or, if the reader pleases, upon the annihilation and +reproduction of parts of this resplendent covering, is left for theory to guess +at.<a href="#linknote-47" name="linknoteref-47" id="linknoteref-47"><sup>[3]</sup></a> +</p> + +<p> +Ever since that date theory has been guessing at it. The solar astronomer is +still applying all the instruments of modern research to find out which of +these suppositions, or what modification of any of them, is nearest the truth. +The obstacle—one that is perhaps fatal to a real theory—lies in the +impossibility of reproducing comparative experiments in our laboratories or in +our atmosphere. +</p> + +<p> +Sir William Herschel propounded an explanation of Wilson’s observation +which received much notice, but which, out of respect for his memory, is not +now described, as it violated the elementary laws of heat. +</p> + +<p> +Sir John Herschel noticed that the spots are mostly confined to two zones +extending to about 35° on each side of the equator, and that a zone of +equatoreal calms is free from spots. But it was R. C. Carrington<a href="#linknote-48" name="linknoteref-48" id="linknoteref-48"><sup>[4]</sup></a> who, by his continuous observations at Redhill, in +Surrey, established the remarkable fact that, while the rotation period in the +highest latitudes, 50°, where spots are seen, is twenty-seven-and-a-half +days, near the equator the period is only twenty-five days. His splendid volume +of observations of the sun led to much new information about the average +distribution of spots at different epochs. +</p> + +<p> +Schwabe, of Dessau, began in 1826 to study the solar surface, and, after many +years of work, arrived at a law of frequency which has been more fruitful of +results than any discovery in solar physics.<a href="#linknote-49" name="linknoteref-49" id="linknoteref-49"><sup>[5]</sup></a> In 1843 +he announced a decennial period of maxima and minima of sun-spot displays. In +1851 it was generally accepted, and, although a period of eleven years has been +found to be more exact, all later observations, besides the earlier ones which +have been hunted up for the purpose, go to establish a true periodicity in the +number of sun-spots. But quite lately Schuster<a href="#linknote-50" name="linknoteref-50" id="linknoteref-50"><sup>[6]</sup></a> has +given reasons for admitting a number of co-existent periods, of which the +eleven-year period was predominant in the nineteenth century. +</p> + +<p> +In 1851 Lament, a Scotchman at Munich, found a decennial period in the daily +range of magnetic declination. In 1852 Sir Edward Sabine announced a similar +period in the number of “magnetic storms” affecting all of the +three magnetic elements—declination, dip, and intensity. Australian and +Canadian observations both showed the decennial period in all three elements. +Wolf, of Zurich, and Gauthier, of Geneva, each independently arrived at the +same conclusion. +</p> + +<p> +It took many years before this coincidence was accepted as certainly more than +an accident by the old-fashioned astronomers, who want rigid proof for every +new theory. But the last doubts have long vanished, and a connection has been +further traced between violent outbursts of solar activity and simultaneous +magnetic storms. +</p> + +<p> +The frequency of the Aurora Borealis was found by Wolf to follow the same +period. In fact, it is closely allied in its cause to terrestrial magnetism. +Wolf also collected old observations tracing the periodicity of sun-spots back +to about 1700 A.D. +</p> + +<p> +Spoerer deduced a law of dependence of the average latitude of sun-spots on the +phase of the sun-spot period. +</p> + +<p> +All modern total solar eclipse observations seem to show that the shape of the +luminous corona surrounding the moon at the moment of totality has a special +distinct character during the time of a sun-spot maximum, and another, totally +different, during a sun-spot minimum. +</p> + +<p> +A suspicion is entertained that the total quantity of heat received by the +earth from the sun is subject to the same period. This would have far-reaching +effects on storms, harvests, vintages, floods, and droughts; but it is not safe +to draw conclusions of this kind except from a very long period of +observations. +</p> + +<p> +Solar photography has deprived astronomers of the type of Carrington of the +delight in devoting a life’s work to collecting data. It has now become +part of the routine work of an observatory. +</p> + +<p> +In 1845 Foucault and Fizeau took a daguerreotype photograph of the sun. In 1850 +Bond produced one of the moon of great beauty, Draper having made some attempts +at an even earlier date. But astronomical photography really owes its beginning +to De la Rue, who used the collodion process for the moon in 1853, and +constructed the Kew photoheliograph in 1857, from which date these instruments +have been multiplied, and have given us an accurate record of the sun’s +surface. Gelatine dry plates were first used by Huggins in 1876. +</p> + +<p> +It is noteworthy that from the outset De la Rue recognised the value of +stereoscopic vision, which is now known to be of supreme accuracy. In 1853 he +combined pairs of photographs of the moon in the same phase, but under +different conditions regarding libration, showing the moon from slightly +different points of view. These in the stereoscope exhibited all the relief +resulting from binocular vision, and looked like a solid globe. In 1860 he used +successive photographs of the total solar eclipse stereoscopically, to prove +that the red prominences belong to the sun, and not to the moon. In 1861 he +similarly combined two photographs of a sun-spot, the perspective effect +showing the umbra like a floor at the bottom of a hollow penumbra; and in one +case the faculæ were discovered to be sailing over a spot apparently at some +considerable height. These appearances may be partly due to a proper motion; +but, so far as it went, this was a beautiful confirmation of Wilson’s +discovery. Hewlett, however, in 1894, after thirty years of work, showed that +the spots are not always depressions, being very subject to disturbance. +</p> + +<p> +The Kew photographs<a href="#linknote-51" name="linknoteref-51" id="linknoteref-51"><sup>[7]</sup></a> contributed a vast amount of +information about sun-spots, and they showed that the faculæ generally follow +the spots in their rotation round the sun. +</p> + +<p> +The constitution of the sun’s photosphere, the layer which is the +principal light-source on the sun, has always been a subject of great interest; +and much was done by men with exceptionally keen eyesight, like Mr. Dawes. But +it was a difficult subject, owing to the rapidity of the changes in appearance +of the so-called rice-grains, about 1” in diameter. The rapid +transformations and circulations of these rice-grains, if thoroughly studied, +might lead to a much better knowledge of solar physics. This seemed almost +hopeless, as it was found impossible to identify any “rice-grain” +in the turmoil after a few minutes. But M. Hansky, of Pulkowa (whose recent +death is deplored), introduced successfully a scheme of photography, which +might almost be called a solar cinematograph. He took photographs of the sun at +intervals of fifteen or thirty seconds, and then enlarged selected portions of +these two hundred times, giving a picture corresponding to a solar disc of six +metres diameter. In these enlarged pictures he was able to trace the movements, +and changes of shape and brightness, of individual rice-grains. Some granules +become larger or smaller. Some seem to rise out of a mist, as it were, and to +become clearer. Others grow feebler. Some are split in two. Some are rotated +through a right angle in a minute or less, although each of the grains may be +the size of Great Britain. Generally they move together in groups of very +various velocities, up to forty kilometres a second. These movements seem to +have definite relation to any sun-spots in the neighbourhood. From the results +already obtained it seems certain that, if this method of observation be +continued, it cannot fail to supply facts of the greatest importance. +</p> + +<p> +It is quite impossible to do justice here to the work of all those who are +engaged on astronomical physics. The utmost that can be attempted is to give a +fair idea of the directions of human thought and endeavour. During the last +half-century America has made splendid progress, and an entirely new process of +studying the photosphere has been independently perfected by Professor Hale at +Chicago, and Deslandres at Paris.<a href="#linknote-52" name="linknoteref-52" id="linknoteref-52"><sup>[8]</sup></a> They have succeeded +in photographing the sun’s surface in monochromatic light, such as the +light given off as one of the bright lines of hydrogen or of calcium, by means +of the “Spectroheliograph.” The spectroscope is placed with its +slit in the focus of an equatoreal telescope, pointed to the sun, so that the +circular image of the sun falls on the slit. At the other end of the +spectroscope is the photographic plate. Just in front of this plate there is +another slit parallel to the first, in the position where the image of the +first slit formed by the K line of calcium falls. Thus is obtained a photograph +of the section of the sun, made by the first slit, only in K light. As the +image of the sun passes over the first slit the photographic plate is moved at +the same rate and in the same direction behind the second slit; and as +successive sections of the sun’s image in the equatoreal enter the +apparatus, so are these sections successively thrown in their proper place on +the photographic plate, always in K light. By using a high dispersion the +faculæ which give off K light can be correctly photographed, not only at the +sun’s edge, but all over his surface. The actual mechanical method of +carrying out the observation is not quite so simple as what is here described. +</p> + +<p> +By choosing another line of the spectrum instead of calcium K—for +example, the hydrogen line H<sub>(3)</sub>—we obtain two photographs, one +showing the appearance of the calcium floculi, and the other of the hydrogen +floculi, on the same part of the solar surface; and nothing is more astonishing +than to note the total want of resemblance in the forms shown on the two. This +mode of research promises to afford many new and useful data. +</p> + +<p> +The spectroscope has revealed the fact that, broadly speaking, the sun is +composed of the same materials as the earth. Ångstrom was the first to map +out all of the lines to be found in the solar spectrum. But Rowland, of +Baltimore, after having perfected the art of making true gratings with +equidistant lines ruled on metal for producing spectra, then proceeded to make +a map of the solar spectrum on a large scale. +</p> + +<p> +In 1866 Lockyer<a href="#linknote-53" name="linknoteref-53" id="linknoteref-53"><sup>[9]</sup></a> threw an image of the sun upon the +slit of a spectroscope, and was thus enabled to compare the spectrum of a spot +with that of the general solar surface. The observation proved the darkness of +a spot to be caused by increased absorption of light, not only in the dark +lines, which are widened, but over the entire spectrum. In 1883 Young resolved +this continuous obscurity into an infinite number of fine lines, which have all +been traced in a shadowy way on to the general solar surface. Lockyer also +detected displacements of the spectrum lines in the spots, such as would be +produced by a rapid motion in the line of sight. It has been found that both +uprushes and downrushes occur, but there is no marked predominance of either in +a sun-spot. The velocity of motion thus indicated in the line of sight +sometimes appears to amount to 320 miles a second. But it must be remembered +that pressure of a gas has some effect in displacing the spectral lines. So we +must go on, collecting data, until a time comes when the meaning of all the +facts can be made clear. +</p> + +<p> +<i>Total Solar Eclipses</i>.—During total solar eclipses the time is so +short, and the circumstances so impressive, that drawings of the appearance +could not always be trusted. The red prominences of jagged form that are seen +round the moon’s edge, and the corona with its streamers radiating or +interlacing, have much detail that can hardly be recorded in a sketch. By the +aid of photography a number of records can be taken during the progress of +totality. From a study of these the extent of the corona is demonstrated in one +case to extend to at least six diameters of the moon, though the eye has traced +it farther. This corona is still one of the wonders of astronomy, and leads to +many questions. What is its consistency, if it extends many million miles from +the sun’s surface? How is it that it opposed no resistance to the motion +of comets which have almost grazed the sun’s surface? Is this the origin +of the zodiacal light? The character of the corona in photographic records has +been shown to depend upon the phase of the sun-spot period. During the sun-spot +maximum the corona seems most developed over the spot-zones—i.e., neither +at the equator nor the poles. The four great sheaves of light give it a square +appearance, and are made up of rays or plumes, delicate like the petals of a +flower. During a minimum the nebulous ring seems to be made of tufts of fine +hairs with aigrettes or radiations from both poles, and streamers from the +equator. +</p> + +<div class="fig" style="width:55%;"> +<a name="illus08"></a> +<img src="images/010.jpg" style="width:100%;" alt="SOLAR ECLIPSE, 1882." /> +<p class="caption">S<small>OLAR</small> E<small>CLIPSE</small>, 1882.<br/>From +drawing by W. H. Wesley, Secretary R.A.S.; showing the prominences, the corona, +and an unknown comet.</p> +</div> + +<p> +On September 19th, 1868, eclipse spectroscopy began with the Indian eclipse, in +which all observers found that the red prominences showed a bright line +spectrum, indicating the presence of hydrogen and other gases. So bright was it +that Jansen exclaimed: “<i>Je verrai ces lignes-là en dehors des +éclipses</i>.” And the next day he observed the lines at the edge of +the uneclipsed sun. Huggins had suggested this observation in February, 1868, +his idea being to use prisms of such great dispersive power that the continuous +spectrum reflected by our atmosphere should be greatly weakened, while a bright +line would suffer no diminution by the high dispersion. On October 20th +Lockyer,<a href="#linknote-54" name="linknoteref-54" id="linknoteref-54"><sup>[10]</sup></a> having news of the eclipse, but not of +Jansen’s observations the day after, was able to see these lines. This +was a splendid performance, for it enabled the prominences to be observed, not +only during eclipses, but every day. Moreover, the next year Huggins was able, +by using a wide slit, to see the whole of a prominence and note its shape. +Prominences are classified, according to their form, into “flame” +and “cloud” prominences, the spectrum of the latter showing +calcium, hydrogen, and helium; that of the former including a number of metals. +</p> + +<p> +The D line of sodium is a double line, and in the same eclipse (1868) an orange +line was noticed which was afterwards found to lie close to the two components +of the D line. It did not correspond with any known terrestrial element, and +the unknown element was called “helium.” It was not until 1895 that +Sir William Ramsay found this element as a gas in the mineral cleavite. +</p> + +<p> +The spectrum of the corona is partly continuous, indicating light reflected +from the sun’s body. But it also shows a green line corresponding with no +known terrestrial element, and the name “coronium” has been given +to the substance causing it. +</p> + +<p> +A vast number of facts have been added to our knowledge about the sun by +photography and the spectroscope. Speculations and hypotheses in plenty have +been offered, but it may be long before we have a complete theory evolved to +explain all the phenomena of the storm-swept metallic atmosphere of the sun. +</p> + +<p> +The proceedings of scientific societies teem with such facts and “working +hypotheses,” and the best of them have been collected by Miss Clerke in +her <i>History of Astronomy during the Nineteenth Century</i>. As to +established facts, we learn from the spectroscopic researches (1) that the +continuous spectrum is derived from the <i>photosphere</i> or solar gaseous +material compressed almost to liquid consistency; (2) that the <i>reversing +layer</i> surrounds it and gives rise to black lines in the spectrum; that the +<i>chromosphere</i> surrounds this, is composed mainly of hydrogen, and is the +cause of the red prominences in eclipses; and that the gaseous <i>corona</i> +surrounds all of these, and extends to vast distances outside the sun’s +visible surface. +</p> + +<hr /> + +<p> +<b>FOOTNOTES:</b> +</p> + +<p class="footnote"> +<a name="linknote-45" id="linknote-45"></a> <a href="#linknoteref-45">[1]</a> +<i>Rosa Ursina</i>, by C. Scheiner, <i>fol</i>.; Bracciani, 1630. +</p> + +<p class="footnote"> +<a name="linknote-46" id="linknote-46"></a> <a href="#linknoteref-46">[2]</a> +<i>R. S. Phil. Trans</i>., 1774. +</p> + +<p class="footnote"> +<a name="linknote-47" id="linknote-47"></a> <a href="#linknoteref-47">[3]</a> +<i>Ibid</i>, 1783. +</p> + +<p class="footnote"> +<a name="linknote-48" id="linknote-48"></a> <a href="#linknoteref-48">[4]</a> +<i>Observations on the Spots on the Sun, etc.,</i> 4°; London and +Edinburgh, 1863. +</p> + +<p class="footnote"> +<a name="linknote-49" id="linknote-49"></a> <a href="#linknoteref-49">[5]</a> +<i>Periodicität der Sonnenflecken. Astron. Nach. XXI.</i>, 1844, P. 234. +</p> + +<p class="footnote"> +<a name="linknote-50" id="linknote-50"></a> <a href="#linknoteref-50">[6]</a> +<i>R.S. Phil. Trans.</i> (ser. A), 1906, p. 69-100. +</p> + +<p class="footnote"> +<a name="linknote-51" id="linknote-51"></a> <a href="#linknoteref-51">[7]</a> +“Researches on Solar Physics,” by De la Rue, Stewart and Loewy; +<i>R. S. Phil. Trans</i>., 1869, 1870. +</p> + +<p class="footnote"> +<a name="linknote-52" id="linknote-52"></a> <a href="#linknoteref-52">[8]</a> +“The Sun as Photographed on the K line”; <i>Knowledge</i>, London, +1903, p. 229. +</p> + +<p class="footnote"> +<a name="linknote-53" id="linknote-53"></a> <a href="#linknoteref-53">[9]</a> +<i>R. S. Proc.</i>, xv., 1867, p. 256. +</p> + +<p class="footnote"> +<a name="linknote-54" id="linknote-54"></a> <a href="#linknoteref-54">[10]</a> +<i>Acad. des Sc.</i>, Paris; <i>C. R.</i>, lxvii., 1868, p. 121. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h3><a name="13"></a>13. THE MOON AND PLANETS.</h3> + +<p> +<i>The Moon</i>.—Telescopic discoveries about the moon commence with +Galileo’s discovery that her surface has mountains and valleys, like the +earth. He also found that, while she always turns the same face to us, there is +periodically a slight twist to let us see a little round the eastern or western +edge. This was called <i>libration</i>, and the explanation was clear when it +was understood that in showing always the same face to us she makes one +revolution a month on her axis <i>uniformly</i>, and that her revolution round +the earth is not uniform. +</p> + +<p> +Galileo said that the mountains on the moon showed greater differences of level +than those on the earth. Shröter supported this opinion. W. Herschel +opposed it. But Beer and Mädler measured the heights of lunar mountains by +their shadows, and found four of them over 20,000 feet above the surrounding +plains. +</p> + +<p> +Langrenus<a href="#linknote-55" name="linknoteref-55" id="linknoteref-55"><sup>[1]</sup></a> was the first to do serious work on +selenography, and named the lunar features after eminent men. Riccioli also +made lunar charts. In 1692 Cassini made a chart of the full moon. Since then we +have the charts of Schröter, Beer and Mädler (1837), and of Schmidt, +of Athens (1878); and, above all, the photographic atlas by Loewy and Puiseux. +</p> + +<p> +The details of the moon’s surface require for their discussion a whole +book, like that of Neison or the one by Nasmyth and Carpenter. Here a few words +must suffice. Mountain ranges like our Andes or Himalayas are rare. Instead of +that, we see an immense number of circular cavities, with rugged edges and flat +interior, often with a cone in the centre, reminding one of instantaneous +photographs of the splash of a drop of water falling into a pool. Many of these +are fifty or sixty miles across, some more. They are generally spoken of as +resembling craters of volcanoes, active or extinct, on the earth. But some of +those who have most fully studied the shapes of craters deny altogether their +resemblance to the circular objects on the moon. These so-called craters, in +many parts, are seen to be closely grouped, especially in the snow-white parts +of the moon. But there are great smooth dark spaces, like the clear black ice +on a pond, more free from craters, to which the equally inappropriate name of +seas has been given. The most conspicuous crater, <i>Tycho</i>, is near the +south pole. At full moon there are seen to radiate from Tycho numerous streaks +of light, or “rays,” cutting through all the mountain formations, +and extending over fully half the lunar disc, like the star-shaped cracks made +on a sheet of ice by a blow. Similar cracks radiate from other large craters. +It must be mentioned that these white rays are well seen only in full light of +the sun at full moon, just as the white snow in the crevasses of a glacier is +seen bright from a distance only when the sun is high, and disappears at +sunset. Then there are deep, narrow, crooked “rills” which may have +been water-courses; also “clefts” about half a mile wide, and often +hundreds of miles long, like deep cracks in the surface going straight through +mountain and valley. +</p> + +<p> +The moon shares with the sun the advantage of being a good subject for +photography, though the planets are not. This is owing to her larger apparent +size, and the abundance of illumination. The consequence is that the finest +details of the moon, as seen in the largest telescope in the world, may be +reproduced at a cost within the reach of all. +</p> + +<p> +No certain changes have ever been observed; but several suspicions have been +expressed, especially as to the small crater <i>Linné</i>, in the <i>Mare +Serenitatis</i>. It is now generally agreed that no certainty can be expected +from drawings, and that for real evidence we must await the verdict of +photography. +</p> + +<p> +No trace of water or of an atmosphere has been found on the moon. It is +possible that the temperature is too low. In any case, no displacement of a +star by atmospheric refraction at occultation has been surely recorded. The +moon seems to be dead. +</p> + +<p> +The distance of the moon from the earth is just now the subject of +re-measurement. The base line is from Greenwich to Cape of Good Hope, and the +new feature introduced is the selection of a definite point on a crater +(Mösting A), instead of the moon’s edge, as the point whose distance +is to be measured. +</p> + +<p> +<i>The Inferior Planets</i>.—When the telescope was invented, the phases +of Venus attracted much attention; but the brightness of this planet, and her +proximity to the sun, as with Mercury also, seemed to be a bar to the discovery +of markings by which the axis and period of rotation could be fixed. Cassini +gave the rotation as twenty-three hours, by observing a bright spot on her +surface. Shröter made it 23h. 21m. 19s. This value was supported by +others. In 1890 Schiaparelli<a href="#linknote-56" name="linknoteref-56" id="linknoteref-56"><sup>[2]</sup></a> announced that Venus +rotates, like our moon, once in one of her revolutions, and always directs the +same face to the sun. This property has also been ascribed to Mercury; but in +neither case has the evidence been generally accepted. Twenty-four hours is +probably about the period of rotation for each of these planets. +</p> + +<p> +Several observers have claimed to have seen a planet within the orbit of +Mercury, either in transit over the sun’s surface or during an eclipse. +It has even been named <i>Vulcan</i>. These announcements would have received +little attention but for the fact that the motion of Mercury has irregularities +which have not been accounted for by known planets; and Le Verrier<a href="#linknote-57" name="linknoteref-57" id="linknoteref-57"><sup>[3]</sup></a> has stated that an intra-Mercurial planet or ring of +asteroids would account for the unexplained part of the motion of the line of +apses of Mercury’s orbit amounting to 38” per century. +</p> + +<p> +<i>Mars</i>.—The first study of the appearance of Mars by Miraldi led him +to believe that there were changes proceeding in the two white caps which are +seen at the planet’s poles. W. Herschel attributed these caps to ice and +snow, and the dates of his observations indicated a melting of these ice-caps +in the Martian summer. +</p> + +<p> +Schroter attributed the other markings on Mars to drifting clouds. But Beer and +Mädler, in 1830-39, identified the same dark spots as being always in the +same place, though sometimes blurred by mist in the local winter. A spot +sketched by Huyghens in 1672, one frequently seen by W. Herschel in 1783, +another by Arago in 1813, and nearly all the markings recorded by Beer and +Mädler in 1830, were seen and drawn by F. Kaiser in Leyden during +seventeen nights of the opposition of 1862 (<i>Ast. Nacht.</i>, No. 1,468), +whence he deduced the period of rotation to be 24h. 37m. 22s.,62—or +one-tenth of a second less than the period deduced by R. A. Proctor from a +drawing by Hooke in 1666. +</p> + +<p> +It must be noted that, if the periods of rotation both of Mercury and Venus be +about twenty-four hours, as seems probable, all the four planets nearest to the +sun rotate in the same period, while the great planets rotate in about ten +hours (Uranus and Neptune being still indeterminate). +</p> + +<p> +The general surface of Mars is a deep yellow; but there are dark grey or +greenish patches. Sir John Herschel was the first to attribute the ruddy colour +of Mars to its soil rather than to its atmosphere. +</p> + +<p> +The observations of that keen-sighted observer Dawes led to the first good map +of Mars, in 1869. In the 1877 opposition Schiaparelli revived interest in the +planet by the discovery of canals, uniformly about sixty miles wide, running +generally on great circles, some of them being three or four thousand miles +long. During the opposition of 1881-2 the same observer re-observed the canals, +and in twenty of them he found the canals duplicated,<a href="#linknote-58" name="linknoteref-58" id="linknoteref-58"><sup>[4]</sup></a> +the second canal being always 200 to 400 miles distant from its fellow. +</p> + +<p> +The existence of these canals has been doubted. Mr. Lowell has now devoted +years to the subject, has drawn them over and over again, and has photographed +them; and accepts the explanation that they are artificial, and that vegetation +grows on their banks. Thus is revived the old controversy between Whewell and +Brewster as to the habitability of the planets. The new arguments are not yet +generally accepted. Lowell believes he has, with the spectroscope, proved the +existence of water on Mars. +</p> + +<p> +One of the most unexpected and interesting of all telescopic discoveries took +place in the opposition of 1877, when Mars was unusually near to the earth. The +Washington Observatory had acquired the fine 26-inch refractor, and Asaph Hall +searched for satellites, concealing the planet’s disc to avoid the glare. +On August 11th he had a suspicion of a satellite. This was confirmed on the +16th, and on the following night a second one was added. They are exceedingly +faint, and can be seen only by the most powerful telescopes, and only at the +times of opposition. Their diameters are estimated at six or seven miles. It +was soon found that the first, Deimos, completes its orbit in 30h. 18m. But the +other, Phobos, at first was a puzzle, owing to its incredible velocity being +unsuspected. Later it was found that the period of revolution was only 7h. 39m. +22s. Since the Martian day is twenty-four and a half hours, this leads to +remarkable results. Obviously the easterly motion of the satellite overwhelms +the diurnal rotation of the planet, and Phobos must appear to the inhabitants, +if they exist, to rise in the west and set in the east, showing two or even +three full moons in a day, so that, sufficiently well for the ordinary purposes +of life, the hour of the day can be told by its phases. +</p> + +<p> +The discovery of these two satellites is, perhaps, the most interesting +telescopic visual discovery made with the large telescopes of the last half +century; photography having been the means of discovering all the other new +satellites except Jupiter’s fifth (in order of discovery). +</p> + +<div class="fig" style="width:60%;"> +<a name="illus09"></a> +<img src="images/011.jpg" style="width:100%;" alt="JUPITER." /> +<p class="caption">J<small>UPITER</small>.<br/>From a drawing by E. M. +Antoniadi, showing transit of a satellite’s shadow, the belts, and the +“great red spot” (<i>Monthly Notices</i>, R. A. S., vol. lix., pl. +x.).</p> +</div> + +<p> +<i>Jupiter.</i>—Galileo’s discovery of Jupiter’s satellites +was followed by the discovery of his belts. Zucchi and Torricelli seem to have +seen them. Fontana, in 1633, reported three belts. In 1648 Grimaldi saw but +two, and noticed that they lay parallel to the ecliptic. Dusky spots were also +noticed as transient. Hooke<a href="#linknote-59" name="linknoteref-59" id="linknoteref-59"><sup>[5]</sup></a> measured the motion of one +in 1664. In 1665 Cassini, with a fine telescope, 35-feet focal length, observed +many spots moving from east to west, whence he concluded that Jupiter rotates +on an axis like the earth. He watched an unusually permanent spot during +twenty-nine rotations, and fixed the period at 9h. 56m. Later he inferred that +spots near the equator rotate quicker than those in higher latitudes (the same +as Carrington found for the sun); and W. Herschel confirmed this in 1778-9. +</p> + +<p> +Jupiter’s rapid rotation ought, according to Newton’s theory, to be +accompanied by a great flattening at the poles. Cassini had noted an oval form +in 1691. This was confirmed by La Hire, Römer, and Picard. Pound measured +the ellipticity = 1/(13.25). +</p> + +<p> +W. Herschel supposed the spots to be masses of cloud in the atmosphere—an +opinion still accepted. Many of them were very permanent. Cassini’s great +spot vanished and reappeared nine times between 1665 and 1713. It was close to +the northern margin of the southern belt. Herschel supposed the belts to be the +body of the planet, and the lighter parts to be clouds confined to certain +latitudes. +</p> + +<p> +In 1665 Cassini observed transits of the four satellites, and also saw their +shadows on the planet, and worked out a lunar theory for Jupiter. Mathematical +astronomers have taken great interest in the perturbations of the satellites, +because their relative periods introduce peculiar effects. Airy, in his +delightful book, <i>Gravitation</i>, has reduced these investigations to simple +geometrical explanations. +</p> + +<p> +In 1707 and 1713 Miraldi noticed that the fourth satellite varies much in +brightness. W. Herschel found this variation to depend upon its position in its +orbit, and concluded that in the positions of feebleness it is always +presenting to us a portion of its surface, which does not well reflect the +sun’s light; proving that it always turns the same face to Jupiter, as is +the case with our moon. This fact had also been established for Saturn’s +fifth satellite, and may be true for all satellites. +</p> + +<p> +In 1826 Struve measured the diameters of the four satellites, and found them to +be 2,429, 2,180, 3,561, and 3,046 miles. +</p> + +<p> +In modern times much interest has been taken in watching a rival to +Cassini’s famous spot. The “great red spot” was first +observed by Niesten, Pritchett, and Tempel, in 1878, as a rosy cloud attached +to a whitish zone beneath the dark southern equatorial band, shaped like the +new war balloons, 30,000 miles long and 7,000 miles across. The next year it +was brick-red. A white spot beside it completed a rotation in less time by +5½ minutes than the red spot—a difference of 260 miles an hour. Thus +they came together again every six weeks, but the motions did not continue +uniform. The spot was feeble in 1882-4, brightened in 1886, and, after many +changes, is still visible. +</p> + +<p> +Galileo’s great discovery of Jupiter’s four moons was the last word +in this connection until September 9th, 1892, when Barnard, using the 36-inch +refractor of the Lick Observatory, detected a tiny spot of light closely +following the planet. This proved to be a new satellite (fifth), nearer to the +planet than any other, and revolving round it in 11h. 57m. 23s. Between its +rising and setting there must be an interval of 2½ Jovian days, and two or +three full moons. The sixth and seventh satellites were found by the +examination of photographic plates at the Lick Observatory in 1905, since which +time they have been continuously photographed, and their orbits traced, at +Greenwich. On examining these plates in 1908 Mr. Melotte detected the eighth +satellite, which seems to be revolving in a retrograde orbit three times as far +from its planet as the next one (seventh), in these two points agreeing with +the outermost of Saturn’s satellites (Phoebe). +</p> + +<p> +<i>Saturn.</i>—This planet, with its marvellous ring, was perhaps the +most wonderful object of those first examined by Galileo’s telescope. He +was followed by Dominique Cassini, who detected bands like Jupiter’s +belts. Herschel established the rotation of the planet in 1775-94. From +observations during one hundred rotations he found the period to be 10h. 16m. +0s., 44. Herschel also measured the ratio of the polar to the equatoreal +diameter as 10:11. +</p> + +<p> +The ring was a complete puzzle to Galileo, most of all when the planet reached +a position where the plane of the ring was in line with the earth, and the ring +disappeared (December 4th, 1612). It was not until 1656 that Huyghens, in his +small pamphlet <i>De Saturni Luna Observatio Nova</i>, was able to suggest in a +cypher the ring form; and in 1659, in his Systema Saturnium, he gave his +reasons and translated the cypher: “The planet is surrounded by a slender +flat ring, everywhere distinct from its surface, and inclined to the +ecliptic.” This theory explained all the phases of the ring which had +puzzled others. This ring was then, and has remained ever since, a unique +structure. We in this age have got accustomed to it. But Huyghens’s +discovery was received with amazement. +</p> + +<p> +In 1675 Cassini found the ring to be double, the concentric rings being +separated by a black band—a fact which was placed beyond dispute by +Herschel, who also found that the thickness of the ring subtends an angle less +than 0".3. Shröter estimated its thickness at 500 miles. +</p> + +<p> +Many speculations have been advanced to explain the origin and constitution of +the ring. De Sejour said<a href="#linknote-60" name="linknoteref-60" id="linknoteref-60"><sup>[6]</sup></a> that it was thrown off from +Saturn’s equator as a liquid ring, and afterwards solidified. He noticed +that the outside would have a greater velocity, and be less attracted to the +planet, than the inner parts, and that equilibrium would be impossible; so he +supposed it to have solidified into a number of concentric rings, the exterior +ones having the least velocity. +</p> + +<p> +Clerk Maxwell, in the Adams prize essay, gave a physico-mathematical +demonstration that the rings must be composed of meteoritic matter like gravel. +Even so, there must be collisions absorbing the energy of rotation, and tending +to make the rings eventually fall into the planet. The slower motion of the +external parts has been proved by the spectroscope in Keeler’s hands, +1895. +</p> + +<p> +Saturn has perhaps received more than its share of attention owing to these +rings. This led to other discoveries. Huyghens in 1655, and J. D. Cassini in +1671, discovered the sixth and eighth satellites (Titan and Japetus). Cassini +lost his satellite, and in searching for it found Rhea (the fifth) in 1672, +besides his old friend, whom he lost again. He added the third and fourth in +1684 (Tethys and Dione). The first and second (Mimas and Encelades) were added +by Herschel in 1789, and the seventh (Hyperion) simultaneously by Lassel and +Bond in 1848. The ninth (Phoebe) was found on photographs, by Pickering in +1898, with retrograde motion; and he has lately added a tenth. +</p> + +<p> +The occasional disappearance of Cassini’s Japetus was found on +investigation to be due to the same causes as that of Jupiter’s fourth +satellite, and proves that it always turns the same face to the planet. +</p> + +<p> +<i>Uranus and Neptune</i>.—The splendid discoveries of Uranus and two +satellites by Sir William Herschel in 1787, and of Neptune by Adams and Le +Verrier in 1846, have been already described. Lassel added two more satellites +to Uranus in 1851, and found Neptune’s satellite in 1846. All of the +satellites of Uranus have retrograde motion, and their orbits are inclined +about 80° to the ecliptic. +</p> + +<p> +The spectroscope has shown the existence of an absorbing atmosphere on Jupiter +and Saturn, and there are suspicions that they partake something of the +character of the sun, and emit some light besides reflecting solar light. On +both planets some absorption lines seem to agree with the aqueous vapour lines +of our own atmosphere; while one, which is a strong band in the red common to +both planets, seems to agree with a line in the spectrum of some reddish stars. +</p> + +<p> +Uranus and Neptune are difficult to observe spectroscopically, but appear to +have peculiar spectra agreeing together. Sometimes Uranus shows Frauenhofer +lines, indicating reflected solar light. But generally these are not seen, and +six broad bands of absorption appear. One is the F. of hydrogen; another is the +red-star line of Jupiter and Saturn. Neptune is a very difficult object for the +spectroscope. +</p> + +<p> +Quite lately<a href="#linknote-61" name="linknoteref-61" id="linknoteref-61"><sup>[7]</sup></a> P. Lowell has announced that V. M. +Slipher, at Flagstaff Observatory, succeeded in 1907 in rendering some plates +sensitive far into the red. A reproduction is given of photographed spectra of +the four outermost planets, showing (1) a great number of new lines and bands; +(2) intensification of hydrogen F. and C. lines; (3) a steady increase of +effects (1) and (2) as we pass from Jupiter and Saturn to Uranus, and a still +greater increase in Neptune. +</p> + +<p> +<i>Asteroids</i>.—The discovery of these new planets has been described. +At the beginning of the last century it was an immense triumph to catch a new +one. Since photography was called into the service by Wolf, they have been +caught every year in shoals. It is like the difference between sea fishing with +the line and using a steam trawler. In the 1908 almanacs nearly seven hundred +asteroids are included. The computation of their perturbations and ephemerides +by Euler’s and Lagrange’s method of variable elements became so +laborious that Encke devised a special process for these, which can be applied +to many other disturbed orbits.<a href="#linknote-62" name="linknoteref-62" id="linknoteref-62"><sup>[8]</sup></a> +</p> + +<p> +When a photograph is taken of a region of the heavens including an asteroid, +the stars are photographed as points because the telescope is made to follow +their motion; but the asteroids, by their proper motion, appear as short lines. +</p> + +<p> +The discovery of Eros and the photographic attack upon its path have been +described in their relation to finding the sun’s distance. +</p> + +<p> +A group of four asteroids has lately been found, with a mean distance and +period equal to that of Jupiter. To three of these masculine names have been +given—Hector, Patroclus, Achilles; the other has not yet been named. +</p> + +<hr /> + +<p> +<b>FOOTNOTES:</b> +</p> + +<p class="footnote"> +<a name="linknote-55" id="linknote-55"></a> <a href="#linknoteref-55">[1]</a> +Langrenus (van Langren), F. Selenographia sive lumina austriae philippica; +Bruxelles, 1645. +</p> + +<p class="footnote"> +<a name="linknote-56" id="linknote-56"></a> <a href="#linknoteref-56">[2]</a> +<i>Astr. Nach.</i>, 2,944. +</p> + +<p class="footnote"> +<a name="linknote-57" id="linknote-57"></a> <a href="#linknoteref-57">[3]</a> +<i>Acad. des Sc.</i>, Paris; <i>C.R.</i>, lxxxiii., 1876. +</p> + +<p class="footnote"> +<a name="linknote-58" id="linknote-58"></a> <a href="#linknoteref-58">[4]</a> +<i>Mem. Spettr. Ital.</i>, xi., p. 28. +</p> + +<p class="footnote"> +<a name="linknote-59" id="linknote-59"></a> <a href="#linknoteref-59">[5]</a> +<i>R. S. Phil. Trans</i>., No. 1. +</p> + +<p class="footnote"> +<a name="linknote-60" id="linknote-60"></a> <a href="#linknoteref-60">[6]</a> +Grant’s <i>Hist. Ph. Ast</i>., p. 267. +</p> + +<p class="footnote"> +<a name="linknote-61" id="linknote-61"></a> <a href="#linknoteref-61">[7]</a> +<i>Nature</i>, November 12th, 1908. +</p> + +<p class="footnote"> +<a name="linknote-62" id="linknote-62"></a> <a href="#linknoteref-62">[8]</a> +<i>Ast. Nach</i>., Nos. 791, 792, 814, translated by G. B. Airy. <i>Naut. +Alm</i>., Appendix, 1856. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h3><a name="14"></a>14. COMETS AND METEORS.</h3> + +<p> +Ever since Halley discovered that the comet of 1682 was a member of the solar +system, these wonderful objects have had a new interest for astronomers; and a +comparison of orbits has often identified the return of a comet, and led to the +detection of an elliptic orbit where the difference from a parabola was +imperceptible in the small portion of the orbit visible to us. A remarkable +case in point was the comet of 1556, of whose identity with the comet of 1264 +there could be little doubt. Hind wanted to compute the orbit more exactly than +Halley had done. He knew that observations had been made, but they were lost. +Having expressed his desire for a search, all the observations of Fabricius and +of Heller, and also a map of the comet’s path among the stars, were +eventually unearthed in the most unlikely manner, after being lost nearly three +hundred years. Hind and others were certain that this comet would return +between 1844 and 1848, but it never appeared. +</p> + +<p> +When the spectroscope was first applied to finding the composition of the +heavenly bodies, there was a great desire to find out what comets are made of. +The first opportunity came in 1864, when Donati observed the spectrum of a +comet, and saw three bright bands, thus proving that it was a gas and at least +partly self-luminous. In 1868 Huggins compared the spectrum of Winnecke’s +comet with that of a Geissler tube containing olefiant gas, and found exact +agreement. Nearly all comets have shown the same spectrum.<a href="#linknote-63" name="linknoteref-63" id="linknoteref-63"><sup>[1]</sup></a> A very few comets have given bright band spectra +differing from the normal type. Also a certain kind of continuous spectrum, as +well as reflected solar light showing Frauenhofer lines, have been seen. +</p> + +<div class="fig" style="width:60%;"> +<a name="illus10"></a> +<img src="images/012.jpg" style="width:100%;" alt="COPY OF THE DRAWING MADE BY +PAUL FABRICIUS." /> +<p class="caption">C<small>OPY OF THE</small> D<small>RAWING</small> +M<small>ADE BY</small> P<small>AUL</small> F<small>ABRICIUS</small>.<br/>To +define the path of comet 1556. After being lost for 300 years, this drawing was +recovered by the prolonged efforts of Mr. Hind and Professor Littrow in +1856.</p> +</div> + +<p> +When Wells’s comet, in 1882, approached very close indeed to the sun, the +spectrum changed to a mono-chromatic yellow colour, due to sodium. +</p> + +<p> +For a full account of the wonders of the cometary world the reader is referred +to books on descriptive astronomy, or to monographs on comets.<a href="#linknote-64" name="linknoteref-64" id="linknoteref-64"><sup>[2]</sup></a> Nor can the very uncertain speculations about the +structure of comets’ tails be given here. A new explanation has been +proposed almost every time that a great discovery has been made in the theory +of light, heat, chemistry, or electricity. +</p> + +<p> +Halley’s comet remained the only one of which a prediction of the return +had been confirmed, until the orbit of the small, ill-defined comet found by +Pons in 1819 was computed by Encke, and found to have a period of 3 1/3 years. +It was predicted to return in 1822, and was recognised by him as identical with +many previous comets. This comet, called after Encke, has showed in each of its +returns an inexplicable reduction of mean distance, which led to the assertion +of a resisting medium in space until a better explanation could be found.<a href="#linknote-65" name="linknoteref-65" id="linknoteref-65"><sup>[3]</sup></a> +</p> + +<p> +Since that date fourteen comets have been found with elliptic orbits, whose +aphelion distances are all about the same as Jupiter’s mean distance; and +six have an aphelion distance about ten per cent, greater than Neptune’s +mean distance. Other comets are similarly associated with the planets Saturn +and Uranus. +</p> + +<p> +The physical transformations of comets are among the most wonderful of +unexplained phenomena in the heavens. But, for physical astronomers, the +greatest interest attaches to the reduction of radius vector of Encke’s +comet, the splitting of Biela’s comet into two comets in 1846, and the +somewhat similar behaviour of other comets. It must be noted, however, that +comets have a sensible size, that all their parts cannot travel in exactly the +same orbit under the sun’s gravitation, and that their mass is not +sufficient to retain the parts together very forcibly; also that the inevitable +collision of particles, or else fluid friction, is absorbing energy, and so +reducing the comet’s velocity. +</p> + +<p> +In 1770 Lexell discovered a comet which, as was afterwards proved by +investigations of Lexell, Burchardt, and Laplace, had in 1767 been deflected by +Jupiter out of an orbit in which it was invisible from the earth into an orbit +with a period of 5½ years, enabling it to be seen. In 1779 it again +approached Jupiter closer than some of his satellites, and was sent off in +another orbit, never to be again recognised. +</p> + +<p> +But our interest in cometary orbits has been added to by the discovery that, +owing to the causes just cited, a comet, if it does not separate into discrete +parts like Biela’s, must in time have its parts spread out so as to cover +a sensible part of the orbit, and that, when the earth passes through such part +of a comet’s orbit, a meteor shower is the result. +</p> + +<p> +A magnificent meteor shower was seen in America on November 12th-13th, 1833, +when the paths of the meteors all seemed to radiate from a point in the +constellation Leo. A similar display had been witnessed in Mexico by Humboldt +and Bonpland on November 12th, 1799. H. A. Newton traced such records back to +October 13th, A.D. 902. The orbital motion of a cloud or stream of small +particles was indicated. The period favoured by H. A. Newton was 354½ +days; another suggestion was 375½ days, and another 33¼ years. He +noticed that the advance of the date of the shower between 902 and 1833, at the +rate of one day in seventy years, meant a progression of the node of the orbit. +Adams undertook to calculate what the amount would be on all the five +suppositions that had been made about the period. After a laborious work, he +found that none gave one day in seventy years except the 33¼-year period, +which did so exactly. H. A. Newton predicted a return of the shower on the +night of November 13th-14th, 1866. He is now dead; but many of us are alive to +recall the wonder and enthusiasm with which we saw this prediction being +fulfilled by the grandest display of meteors ever seen by anyone now alive. +</p> + +<p> +The <i>progression</i> of the nodes proved the path of the meteor stream to be +retrograde. The <i>radiant</i> had almost the exact longitude of the point +towards which the earth was moving. This proved that the meteor cluster was at +perihelion. The period being known, the eccentricity of the orbit was +obtainable, also the orbital velocity of the meteors in perihelion; and, by +comparing this with the earth’s velocity, the latitude of the radiant +enabled the inclination to be determined, while the longitude of the earth that +night was the longitude of the node. In such a way Schiaparelli was able to +find first the elements of the orbit of the August meteor shower (Perseids), +and to show its identity with the orbit of Tuttle’s comet 1862.iii. Then, +in January 1867, Le Verrier gave the elements of the November meteor shower +(Leonids); and Peters, of Altona, identified these with Oppolzer’s +elements for Tempel’s comet 1866—Schiaparelli having independently +attained both of these results. Subsequently Weiss, of Vienna, identified the +meteor shower of April 20th (Lyrids) with comet 1861. Finally, that +indefatigable worker on meteors, A. S. Herschel, added to the number, and in +1878 gave a list of seventy-six coincidences between cometary and meteoric +orbits. +</p> + +<p> +Cometary astronomy is now largely indebted to photography, not merely for +accurate delineations of shape, but actually for the discovery of most of them. +The art has also been applied to the observation of comets at distances from +their perihelia so great as to prevent their visual observation. Thus has Wolf, +of Heidelburg, found upon old plates the position of comet 1905.v., as a star +of the 15.5 magnitude, 783 days before the date of its discovery. From the +point of view of the importance of finding out the divergence of a cometary +orbit from a parabola, its period, and its aphelion distance, this increase of +range attains the very highest value. +</p> + +<p> +The present Astronomer Royal, appreciating this possibility, has been searching +by photography for Halley’s comet since November, 1907, although its +perihelion passage will not take place until April, 1910. +</p> + +<hr /> + +<p> +<b>FOOTNOTES:</b> +</p> + +<p class="footnote"> +<a name="linknote-63" id="linknote-63"></a> <a href="#linknoteref-63">[1]</a> +In 1874, when the writer was crossing the Pacific Ocean in H.M.S. +“Scout,” Coggia’s comet unexpectedly appeared, and (while +Colonel Tupman got its positions with the sextant) he tried to use the prism +out of a portable direct-vision spectroscope, without success until it was put +in front of the object-glass of a binocular, when, to his great joy, the three +band images were clearly seen. +</p> + +<p class="footnote"> +<a name="linknote-64" id="linknote-64"></a> <a href="#linknoteref-64">[2]</a> +Such as <i>The World of Comets</i>, by A. Guillemin; <i>History of Comets</i>, +by G. R. Hind, London, 1859; <i>Theatrum Cometicum</i>, by S. de Lubienietz, +1667; <i>Cometographie</i>, by Pingré, Paris, 1783; <i>Donati’s +Comet</i>, by Bond. +</p> + +<p class="footnote"> +<a name="linknote-65" id="linknote-65"></a> <a href="#linknoteref-65">[3]</a> +The investigations by Von Asten (of St. Petersburg) seem to support, and later +ones, especially those by Backlund (also of St. Petersburg), seem to discredit, +the idea of a resisting medium. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h3><a name="15"></a>15. THE FIXED STARS AND NEBULÆ.</h3> + +<p> +Passing now from our solar system, which appears to be subject to the action of +the same forces as those we experience on our globe, there remains an +innumerable host of fixed stars, nebulas, and nebulous clusters of stars. To +these the attention of astronomers has been more earnestly directed since +telescopes have been so much enlarged. Photography also has enabled a vast +amount of work to be covered in a comparatively short period, and the +spectroscope has given them the means, not only of studying the chemistry of +the heavens, but also of detecting any motion in the line of sight from less +than a mile a second and upwards in any star, however distant, provided it be +bright enough. +</p> + +<div class="fig" style="width:50%;"> +<a name="illus11"></a> +<img src="images/013.jpg" style="width:100%;" alt="SIR WILLIAM HERSCHEL, F.R.S.—1738-1822." /> +<p class="caption">S<small>IR</small> W<small>ILLIAM</small> +H<small>ERSCHEL</small>, F.R.S.—1738-1822.<br/>Painted by Lemuel F. +Abbott; National Portrait Gallery, Room XX.</p> +</div> + +<p> +In the field of telescopic discovery beyond our solar system there is no one +who has enlarged our knowledge so much as Sir William Herschel, to whom we owe +the greatest discovery in dynamical astronomy among the stars—viz., that +the law of gravitation extends to the most distant stars, and that many of them +describe elliptic orbits about each other. W. Herschel was born at Hanover in +1738, came to England in 1758 as a trained musician, and died in 1822. He +studied science when he could, and hired a telescope, until he learnt to make +his own specula and telescopes. He made 430 parabolic specula in twenty-one +years. He discovered 2,500 nebulæ and 806 double stars, counted the stars in +3,400 guage-fields, and compared the principal stars photometrically. +</p> + +<p> +Some of the things for which he is best known were results of those accidents +that happen only to the indefatigable enthusiast. Such was the discovery of +Uranus, which led to funds being provided for constructing his 40-feet +telescope, after which, in 1786, he settled at Slough. In the same way, while +trying to detect the annual parallax of the stars, he failed in that quest, but +discovered binary systems of stars revolving in ellipses round each other; just +as Bradley’s attack on stellar parallax failed, but led to the discovery +of aberration, nutation, and the true velocity of light. +</p> + +<p> +<i>Parallax</i>.—The absence of stellar parallax was the great objection +to any theory of the earth’s motion prior to Kepler’s time. It is +true that Kepler’s theory itself could have been geometrically expressed +equally well with the earth or any other point fixed. But in Kepler’s +case the obviously implied physical theory of the planetary motions, even +before Newton explained the simplicity of conception involved, made astronomers +quite ready to waive the claim for a rigid proof of the earth’s motion by +measurement of an annual parallax of stars, which they had insisted on in +respect of Copernicus’s revival of the idea of the earth’s orbital +motion. +</p> + +<p> +Still, the desire to measure this parallax was only intensified by the +practical certainty of its existence, and by repeated failures. The attempts of +Bradley failed. The attempts of Piazzi and Brinkley,<a href="#linknote-66" name="linknoteref-66" id="linknoteref-66"><sup>[1]</sup></a> +early in the nineteenth century, also failed. The first successes, afterwards +confirmed, were by Bessel and Henderson. Both used stars whose proper motion +had been found to be large, as this argued proximity. Henderson, at the Cape of +Good Hope, observed α Centauri, whose annual proper motion he found to +amount to 3".6, in 1832-3; and a few years later deduced its parallax 1".16. +His successor at the Cape, Maclear, reduced this to 0".92. +</p> + +<p> +In 1835 Struve assigned a doubtful parallax of 0".261 to Vega (α Lyræ). +But Bessel’s observations, between 1837 and 1840, of 61 Cygni, a star +with the large proper motion of over 5”, established its annual parallax +to be 0".3483; and this was confirmed by Peters, who found the value 0".349. +</p> + +<p> +Later determinations for α<sub>2</sub> Centauri, by Gill,<a href="#linknote-67" name="linknoteref-67" id="linknoteref-67"><sup>[2]</sup></a> make its parallax 0".75—This is the nearest known +fixed star; and its light takes 4 1/3 years to reach us. The lightyear is taken +as the unit of measurement in the starry heavens, as the earth’s mean +distance is “the astronomical unit” for the solar system.<a href="#linknote-68" name="linknoteref-68" id="linknoteref-68"><sup>[3]</sup></a> The proper motions and parallaxes combined tell us the +velocity of the motion of these stars across the line of sight: α Centauri +14.4 miles a second=4.2 astronomical units a year; 61 Cygni 37.9 miles a +second=11.2 astronomical units a year. These successes led to renewed zeal, and +now the distances of many stars are known more or less accurately. +</p> + +<p> +Several of the brightest stars, which might be expected to be the nearest, have +not shown a parallax amounting to a twentieth of a second of arc. Among these +are Canopus, α Orionis, α Cygni, β Centauri, and γ +Cassiopeia. Oudemans has published a list of parallaxes observed.<a href="#linknote-69" name="linknoteref-69" id="linknoteref-69"><sup>[4]</sup></a> +</p> + +<p> +<i>Proper Motion.</i>—In 1718 Halley<a href="#linknote-70" name="linknoteref-70" id="linknoteref-70"><sup>[5]</sup></a> detected +the proper motions of Arcturus and Sirius. In 1738 J. Cassinis<a href="#linknote-71" name="linknoteref-71" id="linknoteref-71"><sup>[6]</sup></a> showed that the former had moved five minutes of arc +since Tycho Brahe fixed its position. In 1792 Piazzi noted the motion of 61 +Cygni as given above. For a long time the greatest observed proper motion was +that of a small star 1830 Groombridge, nearly 7” a year; but others have +since been found reaching as much as 10”. +</p> + +<p> +Now the spectroscope enables the motion of stars to be detected at a single +observation, but only that part of the motion that is in the line of sight. For +a complete knowledge of a star’s motion the proper motion and parallax +must also be known. +</p> + +<p> +When Huggins first applied the Doppler principle to measure velocities in the +line of sight,<a href="#linknote-72" name="linknoteref-72" id="linknoteref-72"><sup>[7]</sup></a> the faintness of star spectra +diminished the accuracy; but Vögel, in 1888, overcame this to a great +extent by long exposures of photographic plates. +</p> + +<p> +It has often been noticed that stars which seem to belong to a group of nearly +uniform magnitude have the same proper motion. The spectroscope has shown that +these have also often the same velocity in the line of sight. Thus in the Great +Bear, β, γ, δ, ε, ζ, all agree as to angular proper +motion. δ was too faint for a spectroscopic measurement, but all the +others have been shown to be approaching us at a rate of twelve to twenty miles +a second. The same has been proved for proper motion, and line of sight motion, +in the case of Pleiades and other groups. +</p> + +<p> +Maskelyne measured many proper motions of stars, from which W. Herschel<a href="#linknote-73" name="linknoteref-73" id="linknoteref-73"><sup>[8]</sup></a> came to the conclusion that these apparent motions are +for the most part due to a motion of the solar system in space towards a point +in the constellation Hercules, R.A. 257°; N. Decl. 25°. This grand +discovery has been amply confirmed, and, though opinions differ as to the exact +direction, it happens that the point first indicated by Herschel, from totally +insufficient data, agrees well with modern estimates. +</p> + +<p> +Comparing the proper motions and parallaxes to get the actual velocity of each +star relative to our system, C.L. Struve found the probable velocity of the +solar system in space to be fifteen miles a second, or five astronomical units +a year. +</p> + +<p> +The work of Herschel in this matter has been checked by comparing spectroscopic +velocities in the line of sight which, so far as the sun’s motion is +concerned, would give a maximum rate of approach for stars near Hercules, a +maximum rate of recession for stars in the opposite part of the heavens, and no +effect for stars half-way between. In this way the spectroscope has confirmed +generally Herschel’s view of the direction, and makes the velocity eleven +miles a second, or nearly four astronomical units a year. +</p> + +<p> +The average proper motion of a first magnitude star has been found to be 0".25 +annually, and of a sixth magnitude star 0".04. But that all bright stars are +nearer than all small stars, or that they show greater proper motion for that +reason, is found to be far from the truth. Many statistical studies have been +made in this connection, and interesting results may be expected from this +treatment in the hands of Kapteyn of Groningen, and others.<a href="#linknote-74" name="linknoteref-74" id="linknoteref-74"><sup>[9]</sup></a> +</p> + +<p> +On analysis of the directions of proper motions of stars in all parts of the +heavens, Kapteyn has shown<a href="#linknote-75" name="linknoteref-75" id="linknoteref-75"><sup>[10]</sup></a> that these indicate, +besides the solar motion towards Hercules, two general drifts of stars in +nearly opposite directions, which can be detected in any part of the heavens. +This result has been confirmed from independent data by Eddington (<i>R.A.S., +M.N.</i>) and Dyson (<i>R.S.E. Proc.</i>). +</p> + +<p> +Photography promises to assist in the measurement of parallax and proper +motions. Herr Pulfrich, of the firm of Carl Zeiss, has vastly extended the +applications of stereoscopic vision to astronomy—a subject which De la +Rue took up in the early days of photography. He has made a stereo-comparator +of great beauty and convenience for comparing stereoscopically two star +photographs taken at different dates. Wolf of Heidelberg has used this for many +purposes. His investigations depending on the solar motion in space are +remarkable. He photographs stars in a direction at right angles to the line of +the sun’s motion. He has taken photographs of the same region fourteen +years apart, the two positions of his camera being at the two ends of a +base-line over 5,000,000,000 miles apart, or fifty-six astronomical units. On +examining these stereoscopically, some of the stars rise out of the general +plane of the stars, and seem to be much nearer. Many of the stars are thus seen +to be suspended in space at different distances corresponding exactly to their +real distances from our solar system, except when their proper motion +interferes. The effect is most striking; the accuracy of measurement exceeds +that of any other method of measuring such displacements, and it seems that +with a long interval of time the advantage of the method increases. +</p> + +<p> +<i>Double Stars.</i>—The large class of double stars has always been much +studied by amateurs, partly for their beauty and colour, and partly as a test +for telescopic definition. Among the many unexplained stellar problems there is +one noticed in double stars that is thought by some to be likely to throw light +on stellar evolution. It is this: There are many instances where one star of +the pair is comparatively faint, and the two stars are contrasted in colour; +and in every single case the general colour of the faint companion is +invariably to be classed with colours more near to the blue end of the spectrum +than that of the principal star. +</p> + +<p> +<i>Binary Stars.</i>—Sir William Herschel began his observations of +double stars in the hope of discovering an annual parallax of the stars. In +this he was following a suggestion of Galileo’s. The presumption is that, +if there be no physical connection between the stars of a pair, the largest is +the nearest, and has the greatest parallax. So, by noting the distance between +the pair at different times of the year, a delicate test of parallax is +provided, unaffected by major instrumental errors. +</p> + +<p> +Herschel did, indeed, discover changes of distance, but not of the character to +indicate parallax. Following this by further observation, he found that the +motions were not uniform nor rectilinear, and by a clear analysis of the +movements he established the remarkable and wholly unexpected fact that in all +these cases the motion is due to a revolution about their common centre of +gravity.<a href="#linknote-76" name="linknoteref-76" id="linknoteref-76"><sup>[11]</sup></a> He gave the approximate period of +revolution of some of these: Castor, 342 years; δ Serpentis, 375 years; +γ Leonis, 1,200 years; ε Bootis, 1,681 years. +</p> + +<p> +Twenty years later Sir John Herschel and Sir James South, after re-examination +of these stars, confirmed<a href="#linknote-77" name="linknoteref-77" id="linknoteref-77"><sup>[12]</sup></a> and extended the results, +one pair of Coronæ having in the interval completed more than a whole +revolution. +</p> + +<p> +It is, then, to Sir William Herschel that we owe the extension of the law of +gravitation, beyond the limits of the solar system, to the whole universe. His +observations were confirmed by F.G.W. Struve (born 1793, died 1864), who +carried on the work at Dorpat. But it was first to Savary,<a href="#linknote-78" name="linknoteref-78" id="linknoteref-78"><sup>[13]</sup></a> and later to Encke and Sir John Herschel, that we owe +the computation of the elliptic elements of these stars; also the resulting +identification of their law of force with Newton’s force of gravitation +applied to the solar system, and the force that makes an apple fall to the +ground. As Grant well says in his <i>History</i>: “This may be justly +asserted to be one of the most sublime truths which astronomical science has +hitherto disclosed to the researches of the human mind.” +</p> + +<p> +Latterly the best work on double stars has been done by S. W. Burnham,<a href="#linknote-79" name="linknoteref-79" id="linknoteref-79"><sup>[14]</sup></a> at the Lick Observatory. The shortest period he found +was eleven years (κ Pegasi). In the case of some of these binaries the +parallax has been measured, from which it appears that in four of the surest +cases the orbits are about the size of the orbit of Uranus, these being +probably among the smallest stellar orbits. +</p> + +<p> +The law of gravitation having been proved to extend to the stars, a discovery +(like that of Neptune in its origin, though unlike it in the labour and +originality involved in the calculation) that entrances the imagination became +possible, and was realised by Bessel—the discovery of an unknown body by +its gravitational disturbance on one that was visible. In 1834 and 1840 he +began to suspect a want of uniformity in the proper motion of Sirius and +Procyon respectively. In 1844, in a letter to Sir John Herschel,<a href="#linknote-80" name="linknoteref-80" id="linknoteref-80"><sup>[15]</sup></a> he attributed these irregularities in each case to the +attraction of an invisible companion, the period of revolution of Sirius being +about half a century. Later he said: “I adhere to the conviction that +Procyon and Sirius form real binary systems, consisting of a visible and an +invisible star. There is no reason to suppose luminosity an essential quality +of cosmical bodies. The visibility of countless stars is no argument against +the invisibility of countless others.” This grand conception led Peters +to compute more accurately the orbit, and to assign the place of the invisible +companion of Sirius. In 1862 Alvan G. Clark was testing a new 18-inch +object-glass (now at Chicago) upon Sirius, and, knowing nothing of these +predictions, actually found the companion in the very place assigned to it. In +1896 the companion of Procyon was discovered by Professor Schaeberle at the +Lick Observatory. +</p> + +<p> +Now, by the refined parallax determinations of Gill at the Cape, we know that +of Sirius to be 0".38. From this it has been calculated that the mass of Sirius +equals two of our suns, and its intrinsic brightness equals twenty suns; but +the companion, having a mass equal to our sun, has only a five-hundredth part +of the sun’s brightness. +</p> + +<p> +<i>Spectroscopic Binaries</i>.—On measuring the velocity of a star in the +line of sight at frequent intervals, periodic variations have been found, +leading to a belief in motion round an invisible companion. Vogel, in 1889, +discovered this in the case of Spica (α Virginis), whose period is 4d. 0h. +19m., and the diameter of whose orbit is six million miles. Great numbers of +binaries of this type have since then been discovered, all of short period. +</p> + +<p> +Also, in 1889, Pickering found that at regular intervals of fifty-two days the +lines in the spectrum of ζ of the Great Bear are duplicated, indicating a +relative velocity, equal to one hundred miles a second, of two components +revolving round each other, of which that apparently single star must be +composed. +</p> + +<p> +It would be interesting, no doubt, to follow in detail the accumulating +knowledge about the distances, proper motions, and orbits of the stars; but +this must be done elsewhere. Enough has been said to show how results are +accumulating which must in time unfold to us the various stellar systems and +their mutual relationships. +</p> + +<p> +<i>Variable Stars.</i>—It has often happened in the history of different +branches of physical science that observation and experiment were so far ahead +of theory that hopeless confusion appeared to reign; and then one chance result +has given a clue, and from that time all differences and difficulties in the +previous researches have stood forth as natural consequences, explaining one +another in a rational sequence. So we find parallax, proper motion, double +stars, binary systems, variable stars, and new stars all bound together. +</p> + +<p> +The logical and necessary explanation given of the cause of ordinary +spectroscopic binaries, and of irregular proper motions of Sirius and Procyon, +leads to the inference that if ever the plane of such a binary orbit were +edge-on to us there ought to be an eclipse of the luminous partner whenever the +non-luminous one is interposed between us. This should give rise either to +intermittence in the star’s light or else to variability. It was by +supposing the existence of a dark companion to Algol that its discoverer, +Goodricke of York,<a href="#linknote-81" name="linknoteref-81" id="linknoteref-81"><sup>[16]</sup></a> in 1783, explained variable stars +of this type. Algol (β Persei) completes the period of variable brightness +in 68.8 hours. It loses three-fifths of its light, and regains it in twelve +hours. In 1889 Vogel,<a href="#linknote-82" name="linknoteref-82" id="linknoteref-82"><sup>[17]</sup></a> with the Potsdam spectrograph, +actually found that the luminous star is receding before each eclipse, and +approaching us after each eclipse; thus entirely supporting Goodricke’s +opinion. There are many variables of the Algol type, and information is +steadily accumulating. But all variable stars do not suffer the sudden +variations of Algol. There are many types, and the explanations of others have +not proved so easy. +</p> + +<p> +The Harvard College photographs have disclosed the very great prevalence of +variability, and this is certainly one of the lines in which modern discovery +must progress. +</p> + +<p> +Roberts, in South Africa, has done splendid work on the periods of variables of +the Algol type. +</p> + +<p> +<i>New Stars</i>.—Extreme instances of variable stars are the new stars +such as those detected by Hipparchus, Tycho Brahe, and Kepler, of which many +have been found in the last half-century. One of the latest great +“Novæ” was discovered in Auriga by a Scotsman, Dr. Anderson, on +February 1st, 1892, and, with the modesty of his race, he communicated the fact +to His Majesty’s Astronomer for Scotland on an unsigned post-card.<a href="#linknote-83" name="linknoteref-83" id="linknoteref-83"><sup>[18]</sup></a> Its spectrum was observed and photographed by Huggins +and many others. It was full of bright lines of hydrogen, calcium, helium, and +others not identified. The astounding fact was that lines were shown in pairs, +bright and dark, on a faint continuous spectrum, indicating apparently that a +dark body approaching us at the rate of 550 miles a second<a href="#linknote-84" name="linknoteref-84" id="linknoteref-84"><sup>[19]</sup></a> was traversing a cold nebulous atmosphere, and was +heated to incandescence by friction, like a meteor in our atmosphere, leaving a +luminous train behind it. It almost disappeared, and on April 26th it was of +the sixteenth magnitude; but on August 17th it brightened to the tenth, showing +the principal nebular band in its spectrum, and no sign of approach or +recession. It was as if it emerged from one part of the nebula, cooled down, +and rushed through another part of the nebula, rendering the nebular gas more +luminous than itself.<a href="#linknote-85" name="linknoteref-85" id="linknoteref-85"><sup>[20]</sup></a> +</p> + +<p> +Since 1892 one Nova after another has shown a spectrum as described above, like +a meteor rushing towards us and leaving a train behind, for this seems to be +the obvious meaning of the spectra. +</p> + +<p> +The same may be said of the brilliant Nova Persei, brighter at its best than +Capella, and discovered also by Dr. Anderson on February 22nd, 1901. It +increased in brightness as it reached the densest part of the nebula, then it +varied for some weeks by a couple of magnitudes, up and down, as if passing +through separate nebular condensations. In February, 1902, it could still be +seen with an opera-glass. As with the other Novæ, when it first dashed into the +nebula it was vaporised and gave a continuous spectrum with dark lines of +hydrogen and helium. It showed no bright lines paired with the dark ones to +indicate a train left behind; but in the end its own luminosity died out, and +the nebular spectrum predominated. +</p> + +<p> +The nebular illumination as seen in photographs, taken from August to November, +seemed to spread out slowly in a gradually increasing circle at the rate of +90” in forty-eight days. Kapteyn put this down to the velocity of light, +the original outburst sending its illumination to the nebulous gas and +illuminating a spherical shell whose radius increased at the velocity of light. +This supposition seems correct, in which case it can easily be shown from the +above figures that the distance of this Nova was 300 light years. +</p> + +<p> +<i>Star Catalogues.</i>—Since the days of very accurate observations +numerous star-catalogues have been produced by individuals or by observatories. +Bradley’s monumental work may be said to head the list. Lacaille’s, +in the Southern hemisphere, was complementary. Then Piazzi, Lalande, +Groombridge, and Bessel were followed by Argelander with his 324,000 stars, +Rumker’s Paramatta catalogue of the southern hemisphere, and the frequent +catalogues of national observatories. Later the Astronomische Gesellschaft +started their great catalogue, the combined work of many observatories. Other +southern ones were Gould’s at Cordova and Stone’s at the Cape. +</p> + +<p> +After this we have a new departure. Gill at the Cape, having the comet 1882.ii. +all to himself in those latitudes, wished his friends in Europe to see it, and +employed a local photographer to strap his camera to the observatory +equatoreal, driven by clockwork, and adjusted on the comet by the eye. The +result with half-an-hour’s exposure was good, so he tried three hours. +The result was such a display of sharp star images that he resolved on the Cape +Photographic Durchmusterung, which after fourteen years, with Kapteyn’s +aid in reducing, was completed. Meanwhile the brothers Henry, of Paris, were +engaged in going over Chacornac’s zodiacal stars, and were about to +catalogue the Milky Way portion, a serious labour, when they saw Gill’s +Comet photograph and conceived the idea of doing the rest of their work by +photography. Gill had previously written to Admiral Mouchez, of the Paris +Observatory, and explained to him his project for charting the heavens +photographically, by combining the work of many observatories. This led Admiral +Mouchez to support the brothers Henry in their scheme.<a href="#linknote-86" name="linknoteref-86" id="linknoteref-86"><sup>[21]</sup></a> Gill, having got his own photographic work underway, +suggested an international astrographic chart, the materials for different +zones to be supplied by observatories of all nations, each equipped with +similar photographic telescopes. At a conference in Paris, 1887, this was +decided on, the stars on the charts going down to the fourteenth magnitude, and +the catalogues to the eleventh. +</p> + +<div class="fig" style="width:60%;"> +<a name="illus12"></a> +<img src="images/014.jpg" style="width:100%;" alt="GREAT COMET, Nov. 14TH, 1882." /> +<p class="caption">G<small>REAT</small> C<small>OMET</small>, +N<small>OV</small>. 14<small>TH</small>, 1882. (Exposure 2hrs. 20m.)<br/>By +kind permission of Sir David Gill. From this photograph originated all stellar +chart-photography.</p> +</div> + +<p> +This monumental work is nearing completion. The labour involved was immense, +and the highest skill was required for devising instruments and methods to read +off the star positions from the plates. +</p> + +<p> +Then we have the Harvard College collection of photographic plates, always +being automatically added to; and their annex at Arequipa in Peru. +</p> + +<p> +Such catalogues vary in their degree of accuracy; and fundamental catalogues of +standard stars have been compiled. These require extension, because the +differential methods of the heliometer and the camera cannot otherwise be made +absolute. +</p> + +<p> +The number of stars down to the fourteenth magnitude may be taken at about +30,000,000; and that of all the stars visible in the greatest modern telescopes +is probably about 100,000,000. +</p> + +<p> +<i>Nebulæ and Star-clusters.</i>—Our knowledge of nebulæ really dates +from the time of W. Herschel. In his great sweeps of the heavens with his giant +telescopes he opened in this direction a new branch of astronomy. At one time +he held that all nebulæ might be clusters of innumerable minute stars at a +great distance. Then he recognised the different classes of nebulæ, and became +convinced that there is a widely-diffused “shining fluid” in space, +though many so-called nebulæ could be resolved by large telescopes into stars. +He considered that the Milky Way is a great star cluster, whose form may be +conjectured from numerous star-gaugings. He supposed that the compact +“planetary nebulæ” might show a stage of evolution from the diffuse +nebulæ, and that his classifications actually indicate various stages of +development. Such speculations, like those of the ancients about the solar +system, are apt to be harmful to true progress of knowledge unless in the hands +of the ablest mathematical physicists; and Herschel violated their principles +in other directions. But here his speculations have attracted a great deal of +attention, and, with modifications, are accepted, at least as a working +hypothesis, by a fair number of people. +</p> + +<p> +When Sir John Herschel had extended his father’s researches into the +Southern Hemisphere he was also led to the belief that some nebulae were a +phosphorescent material spread through space like fog or mist. +</p> + +<p> +Then his views were changed by the revelations due to the great discoveries of +Lord Rosse with his gigantic refractor,<a href="#linknote-87" name="linknoteref-87" id="linknoteref-87"><sup>[22]</sup></a> when one +nebula after another was resolved into a cluster of minute stars. At that time +the opinion gained ground that with increase of telescopic power this would +prove to be the case with all nebulæ. +</p> + +<p> +In 1864 all doubt was dispelled by Huggins<a href="#linknote-88" name="linknoteref-88" id="linknoteref-88"><sup>[23]</sup></a> in his +first examination of the spectrum of a nebula, and the subsequent extension of +this observation to other nebulæ; thus providing a certain test which increase +in the size of telescopes could never have given. In 1864 Huggins found that +all true nebulae give a spectrum of bright lines. Three are due to hydrogen; +two (discovered by Copeland) are helium lines; others are unknown. Fifty-five +lines have been photographed in the spectrum of the Orion nebula. It seems to +be pretty certain that all true nebulae are gaseous, and show almost exactly +the same spectrum. +</p> + +<p> +Other nebulæ, and especially the white ones like that in Andromeda, which have +not yet been resolved into stars, show a continuous spectrum; others are +greenish and give no lines. +</p> + +<p> +A great deal has to be done by the chemist before the astronomer can be on sure +ground in drawing conclusions from certain portions of his spectroscopic +evidence. +</p> + +<p> +The light of the nebulas is remarkably actinic, so that photography has a +specially fine field in revealing details imperceptible in the telescope. In +1885 the brothers Henry photographed, round the star Maia in the Pleiades, a +spiral nebula 3’ long, as bright on the plate as that star itself, but +quite invisible in the telescope; and an exposure of four hours revealed other +new nebula in the same district. That painstaking and most careful observer, +Barnard, with 10¼ hours’ exposure, extended this nebulosity for +several degrees, and discovered to the north of the Pleiades a huge diffuse +nebulosity, in a region almost destitute of stars. By establishing a 10-inch +instrument at an altitude of 6,000 feet, Barnard has revealed the wide +distribution of nebular matter in the constellation Scorpio over a space of +4° or 5° square. Barnard asserts that the “nebular +hypothesis” would have been killed at its birth by a knowledge of these +photographs. Later he has used still more powerful instruments, and extended +his discoveries. +</p> + +<p> +The association of stars with planetary nebulæ, and the distribution of nebulæ +in the heavens, especially in relation to the Milky Way, are striking facts, +which will certainly bear fruit when the time arrives for discarding vague +speculations, and learning to read the true physical structure and history of +the starry universe. +</p> + +<p> +<i>Stellar Spectra.</i>—When the spectroscope was first available for +stellar research, the leaders in this branch of astronomy were Huggins and +Father Secchi,<a href="#linknote-89" name="linknoteref-89" id="linknoteref-89"><sup>[24]</sup></a> of Rome. The former began by devoting +years of work principally to the most accurate study of a few stars. The latter +devoted the years from 1863 to 1867 to a general survey of the whole heavens, +including 4,000 stars. He divided these into four principal classes, which have +been of the greatest service. Half of his stars belonged to the first class, +including Sirius, Vega, Regulus, Altair. The characteristic feature of their +spectra is the strength and breadth of the hydrogen lines and the extreme +faintness of the metallic lines. This class of star is white to the eye, and +rich in ultra violet light. +</p> + +<p> +The second class includes about three-eighths of his stars, including Capella, +Pollux, and Arcturus. These stars give a spectrum like that of our sun, and +appear yellowish to the eye. +</p> + +<p> +The third class includes α Herculis, α Orionis (Betelgeux), Mira +Ceti, and about 500 red and variable stars. The spectrum has fluted bands +shaded from blue to red, and sharply defined at the more refrangible edge. +</p> + +<p> +The fourth class is a small one, containing no stars over fifth magnitude, of +which 152 Schjellerup, in Canes Venatici, is a good example. This spectrum also +has bands, but these are shaded on the violet side and sharp on the red side. +They are due to carbon in some form. These stars are ruby red in the telescope. +</p> + +<p> +It would appear, then, that all stars are suns with continuous spectra, and the +classes are differentiated by the character of the absorbent vapours of their +atmospheres. +</p> + +<p> +It is very likely that, after the chemists have taught us how to interpret all +the varieties of spectrum, it will be possible to ascribe the different +spectrum-classes to different stages in the life-history of every star. Already +there are plenty of people ready to lay down arbitrary assumptions about the +lessons to be drawn from stellar spectra. Some say that they know with +certainty that each star begins by being a nebula, and is condensed and heated +by condensation until it begins to shine as a star; that it attains a climax of +temperature, then cools down, and eventually becomes extinct. They go so far as +to declare that they know what class of spectrum belongs to each stage of a +star’s life, and how to distinguish between one that is increasing and +another that is decreasing in temperature. +</p> + +<p> +The more cautious astronomers believe that chemistry is not sufficiently +advanced to justify all of these deductions; that, until chemists have settled +the lately raised question of the transmutation of elements, no theory can be +sure. It is also held that until they have explained, without room for doubt, +the reasons for the presence of some lines, and the absence of others, of any +element in a stellar spectrum; why the arc-spectrum of each element differs +from its spark spectrum; what are all the various changes produced in the +spectrum of a gas by all possible concomitant variations of pressure and +temperature; also the meanings of all the flutings in the spectra of metalloids +and compounds; and other equally pertinent matters—until that time +arrives the part to be played by the astronomer is one of observation. By all +means, they say, make use of “working hypotheses” to add an +interest to years of laborious research, and to serve as a guide to the +direction of further labours; but be sure not to fall into the error of calling +any mere hypothesis a theory. +</p> + +<p> +<i>Nebular Hypothesis.</i>—The Nebular Hypothesis, which was first, as it +were, tentatively put forward by Laplace as a note in his <i>Système du +Monde</i>, supposes the solar system to have been a flat, disk-shaped nebula at +a high temperature in rapid rotation. In cooling it condensed, leaving +revolving rings at different distances from the centre. These themselves were +supposed to condense into the nucleus for a rotating planet, which might, in +contracting, again throw off rings to form satellites. The speculation can be +put in a really attractive form, but is in direct opposition to many of the +actual facts; and so long as it is not favoured by those who wish to maintain +the position of astronomy as the most exact of the sciences—exact in its +facts, exact in its logic—this speculation must be recorded by the +historian, only as he records the guesses of the ancient Greeks--as an +interesting phase in the history of human thought. +</p> + +<p> +Other hypotheses, having the same end in view, are the meteoritic hypothesis of +Lockyer and the planetesimal hypothesis that has been largely developed in the +United States. These can best be read in the original papers to various +journals, references to which may be found in the footnotes of Miss +Clerke’s <i>History of Astronomy during the Nineteenth Century</i>. The +same can be said of Bredichin’s hypothesis of comets’ tails, +Arrhenius’s book on the applications of the theory of light repulsion, +the speculations on radium, the origin of the sun’s heat and the age of +the earth, the electron hypothesis of terrestrial magnetism, and a host of +similar speculations, all combining to throw an interesting light on the +evolution of a modern train of thought that seems to delight in conjecture, +while rebelling against that strict mathematical logic which has crowned +astronomy as the queen of the sciences. +</p> + +<hr /> + +<p> +<b>FOOTNOTES:</b> +</p> + +<p class="footnote"> +<a name="linknote-66" id="linknote-66"></a> <a href="#linknoteref-66">[1]</a> +<i>R. S. Phil Trans</i>., 1810 and 1817-24. +</p> + +<p class="footnote"> +<a name="linknote-67" id="linknote-67"></a> <a href="#linknoteref-67">[2]</a> +One of the most valuable contributions to our knowledge of stellar parallaxes +is the result of Gill’s work (<i>Cape Results</i>, vol. iii., part ii., +1900). +</p> + +<p class="footnote"> +<a name="linknote-68" id="linknote-68"></a> <a href="#linknoteref-68">[3]</a> +Taking the velocity of light at 186,000 miles a second, and the earth’s +mean distance at 93,000,000 miles, 1 light-year=5,865,696,000,000 miles or +63,072 astronomical units; 1 astronomical unit a year=2.94 miles a second; and +the earth’s orbital velocity=18.5 miles a second. +</p> + +<p class="footnote"> +<a name="linknote-69" id="linknote-69"></a> <a href="#linknoteref-69">[4]</a> +Ast. Nacht., 1889. +</p> + +<p class="footnote"> +<a name="linknote-70" id="linknote-70"></a> <a href="#linknoteref-70">[5]</a> +R. S. Phil. Trans., 1718. +</p> + +<p class="footnote"> +<a name="linknote-71" id="linknote-71"></a> <a href="#linknoteref-71">[6]</a> +Mem. Acad. des Sciences, 1738, p. 337. +</p> + +<p class="footnote"> +<a name="linknote-72" id="linknote-72"></a> <a href="#linknoteref-72">[7]</a> +R. S Phil. Trans., 1868. +</p> + +<p class="footnote"> +<a name="linknote-73" id="linknote-73"></a> <a href="#linknoteref-73">[8]</a> +<i>R.S. Phil Trans.</i>, 1783. +</p> + +<p class="footnote"> +<a name="linknote-74" id="linknote-74"></a> <a href="#linknoteref-74">[9]</a> +See Kapteyn’s address to the Royal Institution, 1908. Also Gill’s +presidential address to the British Association, 1907. +</p> + +<p class="footnote"> +<a name="linknote-75" id="linknote-75"></a> <a href="#linknoteref-75">[10]</a> +<i>Brit. Assoc. Rep.</i>, 1905. +</p> + +<p class="footnote"> +<a name="linknote-76" id="linknote-76"></a> <a href="#linknoteref-76">[11]</a> +R. S. Phil. Trans., 1803, 1804. +</p> + +<p class="footnote"> +<a name="linknote-77" id="linknote-77"></a> <a href="#linknoteref-77">[12]</a> +Ibid, 1824. +</p> + +<p class="footnote"> +<a name="linknote-78" id="linknote-78"></a> <a href="#linknoteref-78">[13]</a> +Connaisance des Temps, 1830. +</p> + +<p class="footnote"> +<a name="linknote-79" id="linknote-79"></a> <a href="#linknoteref-79">[14]</a> +<i>R. A. S. Mem.</i>, vol. xlvii., p. 178; <i>Ast. Nach.</i>, No. 3,142; +Catalogue published by Lick Observatory, 1901. +</p> + +<p class="footnote"> +<a name="linknote-80" id="linknote-80"></a> <a href="#linknoteref-80">[15]</a> +<i>R. A. S., M. N.</i>, vol. vi. +</p> + +<p class="footnote"> +<a name="linknote-81" id="linknote-81"></a> <a href="#linknoteref-81">[16]</a> +<i>R. S. Phil. Trans.</i>, vol. lxxiii., p. 484. +</p> + +<p class="footnote"> +<a name="linknote-82" id="linknote-82"></a> <a href="#linknoteref-82">[17]</a> +<i>Astr. Nach.</i>, No. 2,947. +</p> + +<p class="footnote"> +<a name="linknote-83" id="linknote-83"></a> <a href="#linknoteref-83">[18]</a> +<i>R. S. E. Trans</i>., vol. xxvii. In 1901 Dr. Anderson discovered Nova +Persei. +</p> + +<p class="footnote"> +<a name="linknote-84" id="linknote-84"></a> <a href="#linknoteref-84">[19]</a> +<i>Astr. Nach</i>., No. 3,079. +</p> + +<p class="footnote"> +<a name="linknote-85" id="linknote-85"></a> <a href="#linknoteref-85">[20]</a> +For a different explanation see Sir W. Huggins’s lecture, Royal +Institution, May 13th, 1892. +</p> + +<p class="footnote"> +<a name="linknote-86" id="linknote-86"></a> <a href="#linknoteref-86">[21]</a> +For the early history of the proposals for photographic cataloguing of stars, +see the <i>Cape Photographic Durchmusterung</i>, 3 vols. (<i>Ann. of the Cape +Observatory</i>, vols. in., iv., and v., Introduction.) +</p> + +<p class="footnote"> +<a name="linknote-87" id="linknote-87"></a> <a href="#linknoteref-87">[22]</a> +<i>R. S. Phil. Trans.</i>, 1850, p. 499 <i>et seq.</i> +</p> + +<p class="footnote"> +<a name="linknote-88" id="linknote-88"></a> <a href="#linknoteref-88">[23]</a> +<i>Ibid</i>, vol. cliv., p. 437. +</p> + +<p class="footnote"> +<a name="linknote-89" id="linknote-89"></a> <a href="#linknoteref-89">[24]</a> +<i>Brit. Assoc. Rep.</i>, 1868, p. 165. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="16"></a>ILLUSTRATIONS</h2> + +<table summary="" > + +<tr> +<td> <a href="#illus01">S<small>IR</small> I<small>SAAC</small> N<small>EWTON</small></a><br/> +(From the bust by Roubiliac In Trinity College, Cambridge.)</td> +</tr> + +<tr> +<td> <a href="#illus02">C<small>HALDÆAN</small> B<small>AKED</small> B<small>RICK +OR</small> T<small>ABLET</small></a><br/> +Obverse and reverse sides, containing record of solar eclipse, 1062 B.C., used +lately by Cowell for rendering the lunar theory more accurate than was possible +by finest modern observations. (British Museum collection, No. 35908.)</td> +</tr> + +<tr> +<td> <a href="#illus03">“Q<small>UADRANS</small> M<small>URALIS SIVE</small> +T<small>ICHONICUS</small>.”</a><br/> With portrait of Tycho Brahe, +instruments, etc., painted on the wall; showing assistants using the sight, +watching the clock, and recording. (From the author’s copy of the +<i>Astronomiæ Instauratæ Mechanica</i>.)</td> +</tr> + +<tr> +<td> <a href="#illus04">P<small>ORTRAIT OF</small> J<small>OHANNES</small> +K<small>EPLER</small>.</a><br/> By F. Wanderer, from Reitlinger’s +“Johannes Kepler” (Original in Strassburg).</td> +</tr> + +<tr> +<td> <a href="#illus05">D<small>EATH</small>-M<small>ASK OF</small> +S<small>IR</small> I<small>SAAC</small> N<small>EWTON</small>.</a><br/> +Photographed specially for this work from the original, by kind permission of +the Royal Society, London.</td> +</tr> + +<tr> +<td> <a href="#illus06">A<small>NCIENT</small> C<small>HINESE</small> +I<small>NSTRUMENTS</small>,</a><br/>Including quadrant, celestial globe, and two +armillae, in the Observatory at Peking. Photographed in Peking by the author in +1875, and stolen by the Germans when the Embassies were relieved by the allies +in 1900.</td> +</tr> + +<tr> +<td> <a href="#illus07">S<small>OLAR</small> S<small>URFACE</small>.</a><br/>As +Photographed at the Royal Observatory, Greenwich, showing sun spots with umbræ, +penumbræ, and faculæ.</td> +</tr> + +<tr> +<td> <a href="#illus08">S<small>OLAR</small> E<small>CLIPSE</small>, 1882.</a><br/> +From drawing by W. H. Wesley, Secretary R.A.S.; showing the prominences, the +corona, and an unknown comet.</td> +</tr> + +<tr> +<td> <a href="#illus09">J<small>UPITER</small>.</a><br/>From a drawing by E. M. +Antoniadi, showing transit of a satellite’s shadow, the belts, and the +“great red spot” (<i>Monthly Notices</i>, R. A. S., vol. lix., pl. +x.).</td> +</tr> + +<tr> +<td> <a href="#illus10">C<small>OPY OF THE</small> D<small>RAWING</small> +M<small>ADE BY</small> P<small>AUL</small> F<small>ABRICIUS</small>.</a><br/>To +define the path of comet 1556. After being lost for 300 years, this drawing was +recovered by the prolonged efforts of Mr. Hind and Professor Littrow in +1856.</td> +</tr> + +<tr> +<td> <a href="#illus11">S<small>IR</small> W<small>ILLIAM</small> +H<small>ERSCHEL</small>, F.R.S.—1738-1822.</a><br/>Painted by Lemuel F. +Abbott; National Portrait Gallery, Room XX.</td> +</tr> + +<tr> +<td> <a href="#illus12">G<small>REAT</small> C<small>OMET</small>, +N<small>OV</small>. 14<small>TH</small>, 1882. (Exposure 2hrs. 20m.)</a><br/>By +kind permission of Sir David Gill. From this photograph originated all stellar +chart-photography.</td> +</tr> + +</table> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="index"></a>INDEX</h2> + +<p class="noindent"> +Abul Wefa, 24<br/> +Acceleration of moon’s mean motion, 60<br/> +Achromatic lens invented, 88<br/> +Adams, J. C., 61, 65, 68, 69, 70, 87, 118, 124<br/> +Airy, G. B., 13, 30, 37, 65, 69, 70, 80, 81, 114, +119<br/> +Albetegnius, 24<br/> +Alphonso, 24<br/> +Altazimuth, 81<br/> +Anaxagoras, 14, 16<br/> +Anaximander, 14<br/> +Anaximenes, 14<br/> +Anderson, T. D., 137, 138<br/> +Ångstrom, A. J., 102<br/> +Antoniadi, 113<br/> +Apian, P., 63<br/> +Apollonius, 22, 23<br/> +Arago, 111<br/> +Argelander, F. W. A., 139<br/> +Aristarchus, 18, 29<br/> +Aristillus, 17, 19<br/> +Aristotle, 16, 30, 47<br/> +Arrhenius, 146<br/> +Arzachel, 24<br/> +Asshurbanapal, 12<br/> +Asteroids, discovery of, 67, 119<br/> +Astrology, ancient and modern, 1-7, 38<br/> +<br/> +Backlund, 122<br/> +Bacon, R., 86<br/> +Bailly, 8, 65<br/> +Barnard, E. E., 115, 143<br/> +Beer and Mädler, 107, 110, 111<br/> +Behaim, 74<br/> +Bessel, F.W., 65, 79, 128, 134, 139<br/> +Biela, 123<br/> +Binet, 65<br/> +Biot, 10<br/> +Bird, 79, 80<br/> +Bliss, 80<br/> +Bode, 66, 69<br/> +Bond, G. P., 99, 117, 122<br/> +Bouvard, A., 65, 68<br/> +Bradley, J., 79, 80, 81, 87, 127, 128, 139<br/> +Bredechin, 146<br/> +Bremiker, 71<br/> +Brewster, D., 52, 91, 112<br/> +Brinkley, 128<br/> +Bruno, G., 49<br/> +Burchardt, 65, 123<br/> +Burnham, S. W., 134<br/> +<br/> +Callippus, 15, 16, 31<br/> +Carrington, R. C., 97, 99, 114<br/> +Cassini, G. D., 107, 114, 115, 116, 117, 118<br/> +Cassini, J., 109, 129<br/> +Chacornac, 139<br/> +Chaldæan astronomy, 11-13<br/> +Challis, J., 69, 70, 71, 72<br/> +Chance, 88<br/> +Charles, II., 50, 81<br/> +Chinese astronomy, 8-11<br/> +Christie, W. M. H. (Ast. Roy.), 64, 82, 125<br/> +Chueni, 9<br/> +Clairaut, A. C., 56, 63, 65<br/> +Clark, A. G., 89, 135<br/> +Clerke, Miss, 106, 146<br/> +Comets, 120<br/> +Common, A. A., 88<br/> +Cooke, 89<br/> +Copeland, R., 142<br/> +Copernicus, N., 14, 24-31, 37, 38, 41, 42, 49, 128<br/> +Cornu, 85<br/> +Cowell, P. H., 3, 5, 64, 83<br/> +Crawford, Earl of, 84<br/> +Cromellin, A. C., 5, 64<br/> +<br/> +D’Alembert, 65<br/> +Damoiseau, 65<br/> +D’Arrest, H. L., 34<br/> +Dawes, W. R., 100, 111<br/> +Delambre, J. B. J., 8, 27, 51, 65, 68<br/> +De la Rue, W., 2, 94, 99, 100, 131<br/> +Delaunay, 65<br/> +Democritus, 16<br/> +Descartes, 51<br/> +De Sejour, 117<br/> +Deslandres, II., 101<br/> +Desvignolles, 9<br/> +De Zach, 67<br/> +Digges, L., 86<br/> +Dollond, J., 87, 90<br/> +Dominis, A. di., 86<br/> +Donati, 120<br/> +Doppler, 92, 129<br/> +Draper, 99<br/> +Dreyer, J. L. E., 29,77<br/> +Dunthorne, 60<br/> +Dyson, 131<br/> +<br/> +Eclipses, total solar, 103<br/> +Ecphantes, 16<br/> +Eddington, 131<br/> +Ellipse, 41<br/> +Empedocles, 16<br/> +Encke, J. F., 119, 122, 123, 133<br/> +Epicycles, 22<br/> +Eratosthenes, 18<br/> +Euclid, 17<br/> +Eudoxus, 15, 31<br/> +Euler, L., 60, 61, 62, 65, 88, 119<br/> +<br/> +Fabricius, D.,95, 120, 121<br/> +Feil and Mantois, 88<br/> +Fizeau, H. L., 85, 92, 99<br/> +Flamsteed, J., 50, 58, 68, 78, 79, 93<br/> +Fohi, 8<br/> +Forbes, J. D., 52, 91<br/> +Foucault, L., 85, 99<br/> +Frauenhofer, J., 88, 90, 91<br/> +<br/> +Galilei, G., 38, 46-49, 77, 93, 94, 95, 96, 107, 113, +115, 116, 133<br/> +Galle, 71, 72<br/> +Gascoigne, W., 45, 77<br/> +Gauss, C. F., 65, 67<br/> +Gauthier, 98<br/> +Gautier, 89<br/> +Gilbert, 44<br/> +Gill, D., 84, 85, 128, 135, 139, 140<br/> +Goodricke, J., 136<br/> +Gould, B. A., 139<br/> +Grant, R., 27, 47, 51, 86, 134<br/> +Graham, 79<br/> +Greek astronomy, 8-11<br/> +Gregory, J. and D., 87<br/> +Grimaldi, 113<br/> +Groombridge, S., 139<br/> +Grubb, 88, 89<br/> +Guillemin, 122<br/> +Guinand, 88<br/> +<br/> +Hale, G. E., 101<br/> +Hall, A., 112<br/> +Hall, C. M., 88<br/> +Halley, E., 19, 51, 58, 60, 61, 62, 63, 64, 79, 120, +122, 125, 129<br/> +Halley’s comet, 62-64<br/> +Halm, 85<br/> +Hansen, P. A., 3, 65<br/> +Hansky, A. P., 100<br/> +Harding, C. L., 67<br/> +Heliometer, 83<br/> +Heller, 120<br/> +Helmholtz, H. L. F., 35<br/> +Henderson, T., 128<br/> +Henry, P. and P., 139, 140, 143<br/> +Heraclides, 16<br/> +Heraclitus, 14<br/> +Herodotus, 13<br/> +Herschel, W., 65, 68, 97, 107, 110, 114, 115, 116, +117, 118, 126, 127,<br/> +130, 131, 132, 141, 142<br/> +Herschel, J., 97, 111, 133, 134, 142<br/> +Herschel, A. S., 125<br/> +Hevelius, J., 178<br/> +Hind, J. R., 5, 64, 120, 121, 122<br/> +Hipparchus, 3, 18, 19, 20, 22, 23, 24, 26, 36, 55, +60, 74, 93, 137<br/> +Hooke, R., 51, 111, 114<br/> +Horrocks, J., 50, 56<br/> +Howlett, 100<br/> +Huggins, W., 92, 93, 99, 106, 120, 129, 137, 138, +142, 144<br/> +Humboldt and Bonpland, 124<br/> +Huyghens, C., 47, 77, 87, 110, 116, 117<br/> +<br/> +Ivory, 65<br/> +<br/> +Jansen, P. J. C., 105, 106<br/> +Jansen, Z., 86<br/> +<br/> +Kaiser, F., 111<br/> +Kapteyn, J. C., 131, 138, 139<br/> +Keeler, 117<br/> +Kepler, J., 17, 23, 26, 29, 30, 36, 37, 38-46, 48, +49, 50, 52, 53, 63,<br/> +66, 77, 87, 93, 127, 137<br/> +Kepler’s laws, 42<br/> +Kirchoff, G.R., 91<br/> +Kirsch, 9<br/> +Knobel, E.B., 12, 13<br/> +Ko-Show-King, 76<br/> +<br/> +Lacaile, N.L., 139<br/> +Lagrange, J.L., 61, 62, 65, 119<br/> +La Hire, 114<br/> +Lalande, J.J.L., 60, 63, 65, 66, 72, 139<br/> +Lamont, J., 98<br/> +Langrenus, 107<br/> +Laplace, P.S. de, 50, 58, 61, 62, 65,66, 123, 146<br/> +Lassel, 72, 88, 117, 118<br/> +Law of universal gravitation, 53<br/> +Legendre, 65<br/> +Leonardo da Vinci, 46<br/> +Lewis, G.C., 17<br/> +Le Verrier, U.J.J., 65, 68, 70, 71,72, 110, 118, 125<br/> +Lexell, 66, 123<br/> +Light year, 128<br/> +Lipperhey, H., 86<br/> +Littrow, 121<br/> +Lockyer, J.N., 103, 105, 146<br/> +Logarithms invented, 50<br/> +Loewy, 2, 100<br/> +Long inequality of Jupiter and Saturn, 50, 62<br/> +Lowell, P., 111, 112, 118<br/> +Lubienietz, S. de, 122<br/> +Luther, M., 38<br/> +Lunar theory, 37, 50, 56, 64<br/> +<br/> +Maclaurin, 65<br/> +Maclear, T., 128<br/> +Malvasia, 77<br/> +Martin, 9<br/> +Maxwell, J. Clerk, 117<br/> +Maskelyne, N., 80, 130<br/> +McLean, F., 89<br/> +Medici, Cosmo di, 48<br/> +Melancthon, 38<br/> +Melotte, 83, 116<br/> +Meteors, 123<br/> +Meton, 15<br/> +Meyer, 57, 65<br/> +Michaelson, 85<br/> +Miraldi, 110, 114<br/> +Molyneux, 87<br/> +Moon, physical observations, 107<br/> +Mouchez, 139<br/> +Moyriac de Mailla, 8<br/> +<br/> +Napier, Lord, 50<br/> +Nasmyth and Carpenter, 108<br/> +Nebulae, 141, 146<br/> +Neison, E., 108<br/> +Neptune, discovery of, 68-72<br/> +Newall, 89<br/> +Newcomb, 85<br/> +Newton, H.A., 124<br/> +Newton, I., 5, 19, 43, 49, 51-60, 62, 64, 68, 77, +79, 87, 90, 93, 94,<br/> +114, 127, 133<br/> +Nicetas, 16, 25<br/> +Niesten, 115<br/> +Nunez, P., 35<br/> +<br/> +Olbers, H.W.M., 67<br/> +Omar, 11, 24<br/> +Oppolzer, 13, 125<br/> +Oudemans, 129<br/> +<br/> +Palitsch, G., 64<br/> +Parallax, solar, 85, 86<br/> +Parmenides, 14<br/> +Paul III., 30<br/> +Paul V., 48<br/> +Pemberton, 51<br/> +Peters, C.A.F., 125, 128, 135<br/> +Photography, 99<br/> +Piazzi, G., 67, 128, 129, 139<br/> +Picard, 54, 77, 114<br/> +Pickering, E.C., 118, 135<br/> +Pingré, 13, 122<br/> +Plana, 65<br/> +Planets and satellites, physical observations, 109-119<br/> +Plato, 17, 23, 26, 40<br/> +Poisson, 65<br/> +Pond, J., 80<br/> +Pons, 122<br/> +Porta, B., 86<br/> +Pound, 87, 114<br/> +Pontecoulant, 64<br/> +Precession of the equinoxes, 19-21, 55, 57<br/> +Proctor, R.A., 111<br/> +Pritchett, 115<br/> +Ptolemy, 11, 13, 21, 22, 23, 24, 93<br/> +Puiseux and Loewy, 108<br/> +Pulfrich, 131<br/> +Purbach, G., 24<br/> +Pythagoras, 14, 17, 25, 29<br/> +<br/> +Ramsay, W., 106<br/> +Ransome and May, 81<br/> +Reflecting telescopes invented, 87<br/> +Regiomontanus (Müller), 24<br/> +Respighi, 82<br/> +Retrograde motion of planets, 22<br/> +Riccioli, 107<br/> +Roberts, 137<br/> +Römer, O.,78, 114<br/> +Rosse, Earl of, 88, 142<br/> +Rowland, H. A., 92, 102<br/> +Rudolph H.,37, 39<br/> +Rumker, C., 139<br/> +<br/> +Sabine, E., 98<br/> +Savary, 133<br/> +Schaeberle, J. M., 135<br/> +Schiaparelli, G. V., 110, 111, 124, 125<br/> +Scheiner, C., 87, 95, 96<br/> +Schmidt, 108<br/> +Schott, 88<br/> +Schröter, J. H., 107, 110, 111, 124, 125<br/> +Schuster, 98<br/> +Schwabe, G. H., 97<br/> +Secchi, A., 93, 144<br/> +Short, 87<br/> +Simms, J., 81<br/> +Slipher, V. M., 119<br/> +Socrates, 17<br/> +Solon, 15<br/> +Souciet, 8<br/> +South, J., 133<br/> +Spectroscope, 89-92<br/> +Spectroheliograph, 101<br/> +Spoerer, G. F. W., 98<br/> +Spots on the sun, 84;<br/> +periodicity of, 97<br/> +Stars, Parallax, 127;<br/> +proper motion, 129;<br/> +double, 132;<br/> +binaries, 132, 135;<br/> +new, 19, 36, 137;<br/> +catalogues of, 19, 36, 139;<br/> +spectra of, 143<br/> +Stewart, B., 2, 100<br/> +Stokes, G. G., 91<br/> +Stone, E. J., 139<br/> +Struve, C. L., 130<br/> +Struve, F. G. W,, 88, 115, 128, 133<br/> +<br/> +Telescopes invented, 47, 86;<br/> +large, 88<br/> +Temple, 115, 125<br/> +Thales, 13, 16<br/> +Theon, 60<br/> +Transit circle of Römer, 78<br/> +Timocharis, 17, 19<br/> +Titius, 66<br/> +Torricelli, 113<br/> +Troughton, E., 80<br/> +Tupman, G. L., 120<br/> +Tuttle, 125<br/> +Tycho Brahe, 23, 25, 30, 33-38, 39, 40, 44, 50, 75, 77, 93, 94, 129, 137<br/> +<br/> +Ulugh Begh, 24<br/> +Uranus, discovery of, 65<br/> +<br/> +Velocity of light, 86, 128;<br/> +of earth in orbit, 128<br/> +Verbiest, 75<br/> +Vogel, H. C., 92, 129, 135, 136<br/> +Von Asten, 122<br/> +<br/> +Walmsley, 65<br/> +Walterus, B., 24, 74<br/> +Weiss, E., 125<br/> +Wells, 122<br/> +Wesley, 104<br/> +Whewell, 112<br/> +Williams, 10<br/> +Wilson, A., 96, 100<br/> +Winnecke, 120<br/> +Witte, 86<br/> +Wollaston, 90<br/> +Wolf, M., 119, 125, 132<br/> +Wolf, R., 98<br/> +Wren, C., 51<br/> +Wyllie, A., 77<br/> +<br/> +Yao, 9<br/> +Young, C. A., 103<br/> +Yu-Chi, 8<br/> +<br/> +Zenith telescopes, 79, 82<br/> +Zöllner, 92<br/> +Zucchi, 113 +</p> + +</div><!--end chapter--> + +<pre> + + + + + +End of the Project Gutenberg EBook of History of Astronomy, by George Forbes + +*** END OF THIS PROJECT GUTENBERG EBOOK HISTORY OF ASTRONOMY *** + +***** This file should be named 8172-h.htm or 8172-h.zip ***** +This and all associated files of various formats will be found in: + http://www.gutenberg.org/8/1/7/8172/ + +Produced by Jonathan Ingram, Dave Maddock, Charles Franks +and the Online Distributed Proofreading Team. + +Updated editions will replace the previous one--the old editions will +be renamed. + +Creating the works from print editions not protected by U.S. copyright +law 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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