diff options
| author | Roger Frank <rfrank@pglaf.org> | 2025-10-15 05:31:04 -0700 |
|---|---|---|
| committer | Roger Frank <rfrank@pglaf.org> | 2025-10-15 05:31:04 -0700 |
| commit | 31e5feedbf4259abb69349231b68f31a743107fd (patch) | |
| tree | 228482d6442b9d7c657f52afafd55387e9abbe19 /old/8hsrs10h.htm | |
Diffstat (limited to 'old/8hsrs10h.htm')
| -rw-r--r-- | old/8hsrs10h.htm | 6444 |
1 files changed, 6444 insertions, 0 deletions
diff --git a/old/8hsrs10h.htm b/old/8hsrs10h.htm new file mode 100644 index 0000000..d954fd7 --- /dev/null +++ b/old/8hsrs10h.htm @@ -0,0 +1,6444 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> +<html> +<head> +<title>History of Astronomy, by George Forbes</title> +<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> +<style type="text/css"> +<!-- +body {margin:10%; text-align:justify} +--> +</style> +</head> +<body> + + +<pre> + +The Project Gutenberg EBook of History of Astronomy, by George Forbes + +Copyright laws are changing all over the world. Be sure to check the +copyright laws for your country before downloading or redistributing +this or any other Project Gutenberg eBook. + +This header should be the first thing seen when viewing this Project +Gutenberg file. Please do not remove it. Do not change or edit the +header without written permission. + +Please read the "legal small print," and other information about the +eBook and Project Gutenberg at the bottom of this file. Included is +important information about your specific rights and restrictions in +how the file may be used. You can also find out about how to make a +donation to Project Gutenberg, and how to get involved. + + +**Welcome To The World of Free Plain Vanilla Electronic Texts** + +**eBooks Readable By Both Humans and By Computers, Since 1971** + +*****These eBooks Were Prepared By Thousands of Volunteers!***** + + +Title: History of Astronomy + +Author: George Forbes + +Release Date: May, 2005 [EBook #8172] +[Yes, we are more than one year ahead of schedule] +[This file was first posted on June 25, 2003] + +Edition: 10 + +Language: English + +Character set encoding: ASCII + +*** START OF THE PROJECT GUTENBERG EBOOK HISTORY OF ASTRONOMY *** + + + + +Produced by Jonathan Ingram, Dave Maddock, Charles Franks +and the Online Distributed Proofreading Team. + + + + + +</pre> + + +<p align="center"><img src="001.jpg" alt="[Illustration: SIR ISAAC NEWTON (From the bust by Roubiliac In Trinity College, Cambridge.)]" /></p> + +<div align="center"> +<h1>HISTORY OF ASTRONOMY</h1> + +<h2>BY</h2> + +<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> +</div> + +<p><br /><br /></p> + +<h1>CONTENTS</h1> + +<blockquote> +<p><a href="#preface">PREFACE</a></p> +</blockquote> + +<h2>BOOK I. THE GEOMETRICAL PERIOD</h2> + +<blockquote> +<p><a href="#1">1. PRIMITIVE ASTRONOMY AND ASTROLOGY</a></p> + +<p><a href="#2">2. ANCIENT ASTRONOMY—CHINESE +AND CHALDÆANS</a></p> + +<p><a href="#3">3. ANCIENT GREEK ASTRONOMY</a></p> + +<p><a href="#4">4. THE REIGN OF EPICYCLES—FROM PTOLEMY TO COPERNICUS</a></p> +</blockquote> + +<h2>BOOK II. THE DYNAMICAL PERIOD</h2> + +<blockquote> +<p><a href="#5">5. DISCOVERY OF THE TRUE SOLAR SYSTEM—TYCHO BRAHE—KEPLER</a></p> + +<p><a href="#6">6. GALILEO AND THE TELESCOPE—NOTIONS OF GRAVITY BY HORROCKS, ETC.</a></p> + +<p><a href="#7">7. SIR ISAAC NEWTON—LAW OF UNIVERSAL GRAVITATION</a></p> + +<p><a href="#8">8. NEWTON’S SUCCESSORS—HALLEY, EULER, LAGRANGE, LAPLACE, ETC.</a></p> + +<p><a href="#9">9. DISCOVERY OF NEW PLANETS—HERSCHEL, PIAZZI, ADAMS, AND +LE VERRIER</a></p> +</blockquote> + +<h2>BOOK III. OBSERVATION</h2> + +<blockquote> + +<p><a href="#10">10. INSTRUMENTS OF PRECISION—SIZE +OF THE SOLAR SYSTEM</a></p> + +<p><a href="#11">11. HISTORY OF THE TELESCOPE—SPECTROSCOPE</a></p> +</blockquote> + +<h2>BOOK IV. THE PHYSICAL PERIOD</h2> + +<blockquote> +<p><a href="#12">12. THE SUN</a></p> + +<p><a href="#13">13. THE MOON AND PLANETS</a></p> + +<p><a href="#14">14. COMETS AND METEORS</a></p> + +<p><a href="#15">15. THE STARS AND NEBULÆ</a></p> + +<p><a href="#index">INDEX</a></p> +</blockquote> + +<hr width="75%" size="1" align="center" /> + +<p><br /><br /></p> + +<a name="preface"></a> +<h1>PREFACE</h1> + +<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>G. F.<br /> +<i>August 1st, 1909.</i></p> + +<p><br /><br /></p> + +<h1>BOOK I. THE GEOMETRICAL PERIOD</h1> + +<p><br /><br /></p> + +<a name="1"></a> +<h2>1. PRIMITIVE ASTRONOMY AND ASTROLOGY.</h2> + +<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="#fn1_1">[1]</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="#fn1_2">[2]</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="#fn1_3">[3]</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="#fn1_4">[4]</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> + +<p><br /><br /></p> + +<p><b>FOOTNOTES:</b></p> + +<p><a name="fn1_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><a name="fn1_2">[2]</a> <i>R. A. S. Monthly Notices</i>, Sup.; 1905.</p> + +<p align="center"><img src="002.jpg" alt="[Illustration: CHALDÆAN BAKED BRICK OR TABLET, <i>Obverse and reverse sides</i>, +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> + +<p><a name="fn1_3">[3]</a> <i>R. A. S. Monthly Notices</i>, vol. x., +p. 65.</p> + +<p><a name="fn1_4">[4]</a> R. S. E. Proc., vol. x., 1880.</p> + +<p><br /><br /></p> + +<a name="2"></a> +<h2>2. ANCIENT ASTRONOMY—THE CHINESE AND CHALDÆANS.</h2> + +<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="#fn2_1">[1]</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="#fn2_2">[2]</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> + +<p><br /><br /></p> + +<p><b>FOOTNOTES:</b></p> + +<p><a name="fn2_1">[1]</a> These ancient dates are uncertain.</p> + +<p><a name="fn2_2">[2]</a> <i>R. A. S. Monthly Notices</i>, vol. lxviii., +No. 5, March, 1908.</p> + +<p><br /><br /></p> + +<a name="3"></a> +<h2>3. ANCIENT GREEK ASTRONOMY.</h2> + +<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="#fn3_1">[1]</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="#fn3_2">[2]</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="#fn3_3">[3]</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="#fn3_4">[4]</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> + +<p><br /><br /></p> + +<p><b>FOOTNOTES:</b></p> + +<p><a name="fn3_1">[1]</a> <i>Phil. Mag</i>., vol. xxiv., pp. 481-4.</p> + +<p><a name="fn3_2">[2]</a></p> + +<p>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.</p> + +<p>“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><a name="fn3_3">[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><a name="fn3_4">[4]</a> See p. 6 for definition.</p> + +<p><br /><br /></p> + +<a name="4"></a> +<h2>4. THE REIGN OF EPICYCLES—FROM PTOLEMY +TO COPERNICUS.</h2> + +<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="#fn4_1">[1]</a> and +its ratio to that of the epicycle,<a href="#fn4_2">[2]</a> the distance +of the excentric<a href="#fn4_3">[3]</a> from the centre of the deferent, +and the position of the line of apses,<a href="#fn4_4">[4]</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="#fn4_5">[5]</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="#fn4_6">[6]</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="#fn4_7">[7]</a> in drawing attention +to the strange conception,</p> + +<blockquote>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.</blockquote> + +<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="#fn4_8">[8]</a> He says:—</p> + +<blockquote>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.</blockquote> + +<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="#fn4_9">[9]</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> + +<p align="center"><img src="003.jpg" alt="[Illustration: “QUADRANS MURALIS SIVE TICHONICUS.” + With portrait of Tycho Brahe, instruments, <i>etc</i>., +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> + +<p><br /><br /></p> + +<p><b>FOOTNOTES:</b></p> + +<p><a name="fn4_1">[1]</a> For definition see p. 22.</p> + +<p><a name="fn4_2">[2]</a> <i>Ibid</i>.</p> + +<p><a name="fn4_3">[3]</a> For definition see p. 18.</p> + +<p><a name="fn4_4">[4]</a> For definition see p. 18.</p> + +<p><a name="fn4_5">[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><a name="fn4_6">[6]</a> Kepler tells us that Tycho Brahe was pleased with +this device, and adapted it to his own system.</p> + +<p><a name="fn4_7">[7]</a> <i>Hist. Ast.</i>, vol. i., p. 354.</p> + +<p><a name="fn4_8">[8]</a> <i>Hist. of Phys. Ast.</i>, p. vii.</p> + +<p><a name="fn4_9">[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> + +<p><br /><br /></p> + +<h1>BOOK II. THE DYNAMICAL PERIOD</h1> + +<p><br /><br /></p> + +<a name="5"></a> +<h2>5. DISCOVERY OF THE TRUE SOLAR SYSTEM—TYCHO BRAHE—KEPLER.</h2> + +<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="#fn5_1">[1]</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> + +<blockquote>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.</blockquote> + +<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> + +<img src="004.jpg" alt="[Illustration: PORTRAIT OF JOHANNES KEPLER. + By F. Wanderer, from Reitlinger’s “Johannes +Kepler” (original in Strassburg).]" align="right" /> + +<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="#fn5_2">[2]</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="#fn5_3">[3]</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> + +<img src="006.png" alt="" align="right" /> + +<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> + +<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> + +<p><br /><br /></p> + +<p><b>FOOTNOTES:</b></p> + +<p><a name="fn5_1">[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> + +<img src="005.png" alt="" align="right" /> + +<p><a name="fn5_2">[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> + +<p><a name="fn5_3">[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> + +<p><br /><br /></p> + +<a name="6"></a> +<h2>6. GALILEO AND THE TELESCOPE—NOTIONS OF GRAVITY BY HORROCKS, ETC.</h2> + +<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> + +<p><br /><br /></p> + +<a name="7"></a> +<h2>7. SIR ISAAC NEWTON—LAW OF UNIVERSAL +GRAVITATION.</h2> + +<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="#fn7_1">[1]</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="#fn7_2">[2]</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="#fn7_3">[3]</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> + +<p align="center"><img src="007.jpg" alt="[Illustration: DEATH MASK OF SIR ISAAC NEWTON. +Photographed specially for this work from the original, +by kind permission of the Royal Society, London.]" /></p> + +<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> + +<p><br /><br /></p> + +<p><b>FOOTNOTES:</b></p> + +<p><a name="fn7_1">[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><a name="fn7_2">[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.</p> + +<p>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.:—</p> + +<p>Prop.: That gravitation occurs in all bodies, +and that it is proportional to the quantity of matter +in each.</p> + +<p>Cor. I.: The total attraction of gravitation +on a planet arises, and is composed, out of the attraction +on the separate parts.</p> + +<p>Cor. II.: The attraction on separate equal +particles of a body is reciprocally as the square +of the distance from the particles.</p> + +<p><a name="fn7_3">[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> + +<p><br /><br /></p> + +<a name="8"></a> +<h2>8. NEWTON’S SUCCESSORS—HALLEY, +EULER, LAGRANGE, LAPLACE, ETC.</h2> + +<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="#fn8_1">[1]</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="#fn8_2">[2]</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="#fn8_3">[3]</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="#fn8_4">[4]</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> + +<p><br /><br /></p> + +<p><b>FOOTNOTES:</b></p> + +<p><a name="fn8_1">[1]</a> Born 1736; died 1813.</p> + +<p><a name="fn8_2">[2]</a> Born 1749; died 1827.</p> + +<p><a name="fn8_3">[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><a name="fn8_4">[4]</a> <i>R. A. S. Monthly Notices</i>, 1907-8.</p> + +<p><br /><br /></p> + +<a name="9"></a> +<h2>9. DISCOVERY OF NEW PLANETS—HERSCHEL, +PIAZZI, ADAMS, AND LE VERRIER.</h2> + +<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="#fn9_1">[1]</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> + +<p><br /><br /></p> + +<p><b>FOOTNOTES:</b></p> + +<p><a name="fn9_1">[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> + +<p><br /><br /></p> + +<h1>BOOK III. OBSERVATION</h1> + +<p><br /><br /></p> + +<a name="10"></a> +<h2>10. INSTRUMENTS OF PRECISION—STATE +OF THE SOLAR SYSTEM.</h2> + +<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="#fn10_1">[1]</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="#fn10_2">[2]</a></p> + +<p align="center"><img src="008.jpg" alt="[Illustration: ANCIENT CHINESE INSTRUMENTS, 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> + +<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="#fn10_3">[3]</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="#fn10_4">[4]</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="#fn10_5">[5]</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="#fn10_6">[6]</a> is 8".807 ± 0".0028.<a href="#fn10_7">[7]</a></p> + +<p><br /><br /></p> + +<p><b>FOOTNOTES:</b></p> + +<p><a name="fn10_1">[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><a name="fn10_2">[2]</a> See illustration on p. 76.</p> + +<p><a name="fn10_3">[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><a name="fn10_4">[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><a name="fn10_5">[5]</a> <i>Annals of the Cape Observatory</i>, vol. x., +part 3.</p> + +<p><a name="fn10_6">[6]</a> The parallax of the sun is the angle subtended +by the earth’s radius at the sun’s distance.</p> + +<p><a name="fn10_7">[7]</a> A. R. Hinks, R.A.S.; <i>Monthly Notices</i>, June, +1909.</p> + +<p><br /><br /></p> + +<a name="11"></a> +<h2>11. HISTORY OF THE TELESCOPE</h2> + +<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="#fn11_1">[1]</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="#fn11_2">[2]</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> + +<p><b>SPECTROSCOPE.</b></p> + +<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="#fn11_3">[3]</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="#fn11_4">[4]</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="#fn11_5">[5]</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="#fn11_6">[6]</a> succeeded in thus measuring the +velocities of stars in the direction of the line of +sight.</p> + +<p>In 1873 Vogel<a href="#fn11_7">[7]</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> + +<p><br /><br /></p> + +<p><b>FOOTNOTES:</b></p> + +<p><a name="fn11_1">[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><a name="fn11_2">[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><a name="fn11_3">[3]</a> <i>R. S. Phil. Trans</i>.</p> + +<p><a name="fn11_4">[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><a name="fn11_5">[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><a name="fn11_6">[6]</a> <i>R. S. Phil. Trans</i>., 1868.</p> + +<p><a name="fn11_7">[7]</a> <i>Ast. Nach</i>., No. 1, 864.</p> + +<p><br /><br /></p> + +<h1>BOOK IV. THE PHYSICAL PERIOD</h1> + +<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> + +<p><br /><br /></p> + +<a name="12"></a> +<h2>12. THE SUN.</h2> + +<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 align="center"><img src="009.jpg" alt="[Illustration: SOLAR SURFACE, As Photographed +at the Royal Observatory, Greenwich, showing sun-spots +with umbrae, penumbrae, and faculae.]" /></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> + +<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="#fn12_1">[1]</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="#fn12_2">[2]</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> + +<blockquote>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="#fn12_3">[3]</a></blockquote> + +<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="#fn12_4">[4]</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="#fn12_5">[5]</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="#fn12_6">[6]</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="#fn12_7">[7]</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="#fn12_8">[8]</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="#fn12_9">[9]</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> + +<p align="center"><img src="010.jpg" alt="[Illustration: SOLAR ECLIPSE, 1882. From +drawing by W. H. Wesley, Secretary R.A.S.; showing +the prominences, the corona, and an unknown comet.]" /></p> + +<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="#fn12_10">[10]</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> + +<p><br /><br /></p> + +<p><b>FOOTNOTES:</b></p> + +<p><a name="fn12_1">[1]</a> <i>Rosa Ursina</i>, by C. Scheiner, <i>fol</i>.; +Bracciani, 1630.</p> + +<p><a name="fn12_2">[2]</a> <i>R. S. Phil. Trans</i>., 1774.</p> + +<p><a name="fn12_3">[3]</a> <i>Ibid</i>, 1783.</p> + +<p><a name="fn12_4">[4]</a> <i>Observations on the Spots on the Sun, etc.,</i> +4°; London and Edinburgh, 1863.</p> + +<p><a name="fn12_5">[5]</a> <i>Periodicität der Sonnenflecken. Astron. +Nach. XXI.</i>, 1844, P. 234.</p> + +<p><a name="fn12_6">[6]</a> <i>R.S. Phil. Trans.</i> (ser. +A), 1906, p. 69-100.</p> + +<p><a name="fn12_7">[7]</a> “Researches on Solar Physics,” by +De la Rue, Stewart and Loewy; <i>R. S. Phil. +Trans</i>., 1869, 1870.</p> + +<p><a name="fn12_8">[8]</a> “The Sun as Photographed on the K line”; +<i>Knowledge</i>, London, 1903, p. 229.</p> + +<p><a name="fn12_9">[9]</a> <i>R. S. Proc.</i>, xv., 1867, p. 256.</p> + +<p><a name="fn12_10">[10]</a> <i>Acad. des Sc.</i>, Paris; <i>C. R.</i>, +lxvii., 1868, p. 121.</p> + +<p><br /><br /></p> + +<a name="13"></a> +<h2>13. THE MOON AND PLANETS.</h2> + +<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="#fn13_1">[1]</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="#fn13_2">[2]</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="#fn13_3">[3]</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="#fn13_4">[4]</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> + +<p align="center"><img src="011.jpg" alt="[Illustration: JUPITER. 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> + +<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="#fn13_5">[5]</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="#fn13_6">[6]</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="#fn13_7">[7]</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="#fn13_8">[8]</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> + +<p><br /><br /></p> + +<p><b>FOOTNOTES:</b></p> + +<p><a name="fn13_1">[1]</a> Langrenus (van Langren), F. Selenographia sive +lumina austriae philippica; Bruxelles, 1645.</p> + +<p><a name="fn13_2">[2]</a> <i>Astr. Nach.</i>, 2,944.</p> + +<p><a name="fn13_3">[3]</a> <i>Acad. des Sc.</i>, Paris; <i>C.R.</i>, lxxxiii., +1876.</p> + +<p><a name="fn13_4">[4]</a> <i>Mem. Spettr. Ital.</i>, xi., p. 28.</p> + +<p><a name="fn13_5">[5]</a> <i>R. S. Phil. Trans</i>., No. 1.</p> + +<p><a name="fn13_6">[6]</a> Grant’s <i>Hist. Ph. Ast</i>., +p. 267.</p> + +<p><a name="fn13_7">[7]</a> <i>Nature</i>, November 12th, 1908.</p> + +<p><a name="fn13_8">[8]</a> <i>Ast. Nach</i>., Nos. 791, 792, 814, translated +by G. B. Airy. <i>Naut. Alm</i>., Appendix, 1856.</p> + +<p><br /><br /></p> + +<a name="14"></a> +<h2>14. COMETS AND METEORS.</h2> + +<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="#fn14_1">[1]</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> + +<p align="center"><img src="012.jpg" alt="[Illustration: COPY OF THE DRAWING MADE BY PAUL +FABRICIUS. 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> + +<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="#fn14_2">[2]</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="#fn14_3">[3]</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> + +<p><br /><br /></p> + +<p><b>FOOTNOTES:</b></p> + +<p><a name="fn14_1">[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><a name="fn14_2">[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><a name="fn14_3">[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> + +<p><br /><br /></p> + +<a name="15"></a> +<h2>15. THE FIXED STARS AND NEBULÆ.</h2> + +<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> + +<img src="013.jpg" alt="[Illustration: SIR WILLIAM HERSCHEL, F.R.S.—1738-1822. + Painted by Lemuel F. Abbott; National Portrait Gallery, +Room XX.]" align="right" /> + +<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="#fn15_1">[1]</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="#fn15_2">[2]</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="#fn15_3">[3]</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="#fn15_4">[4]</a></p> + +<p><i>Proper Motion.</i>—In 1718 Halley<a href="#fn15_5">[5]</a> +detected the proper motions of Arcturus and Sirius. +In 1738 J. Cassinis<a href="#fn15_6">[6]</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="#fn15_7">[7]</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="#fn15_8">[8]</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="#fn15_9">[9]</a></p> + +<p>On analysis of the directions of proper motions of +stars in all parts of the heavens, Kapteyn has shown<a href="#fn15_10">[10]</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="#fn15_11">[11]</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="#fn15_12">[12]</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="#fn15_13">[13]</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="#fn15_14">[14]</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="#fn15_15">[15]</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="#fn15_16">[16]</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="#fn15_17">[17]</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="#fn15_18">[18]</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="#fn15_19">[19]</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="#fn15_20">[20]</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="#fn15_21">[21]</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> + +<p align="center"><img src="014.jpg" alt="[Illustration: GREAT COMET, Nov. 14TH, 1882. +(Exposure 2hrs. 20m.) By kind permission of Sir David +Gill. From this photograph originated all stellar +chart-photography.]" /></p> + +<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="#fn15_22">[22]</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="#fn15_23">[23]</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="#fn15_24">[24]</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> + +<p><br /><br /></p> + +<p><b>FOOTNOTES:</b></p> + +<p><a name="fn15_1">[1]</a> <i>R. S. Phil Trans</i>., 1810 and 1817-24.</p> + +<p><a name="fn15_2">[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><a name="fn15_3">[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><a name="fn15_4">[4]</a> Ast. Nacht., 1889.</p> + +<p><a name="fn15_5">[5]</a> R. S. Phil. Trans., 1718.</p> + +<p><a name="fn15_6">[6]</a> Mem. Acad. des Sciences, 1738, p. 337.</p> + +<p><a name="fn15_7">[7]</a> R. S Phil. Trans., 1868.</p> + +<p><a name="fn15_8">[8]</a> <i>R.S. Phil Trans.</i>, 1783.</p> + +<p><a name="fn15_9">[9]</a> See Kapteyn’s address to the Royal Institution, +1908. Also Gill’s presidential address +to the British Association, 1907.</p> + +<p><a name="fn15_10">[10]</a> <i>Brit. Assoc. Rep.</i>, 1905.</p> + +<p><a name="fn15_11">[11]</a> R. S. Phil. Trans., 1803, 1804.</p> + +<p><a name="fn15_12">[12]</a> Ibid, 1824.</p> + +<p><a name="fn15_13">[13]</a> Connaisance des Temps, 1830.</p> + +<p><a name="fn15_14">[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><a name="fn15_15">[15]</a> <i>R. A. S., M. N.</i>, vol. vi.</p> + +<p><a name="fn15_16">[16]</a> <i>R. S. Phil. Trans.</i>, vol. lxxiii., +p. 484.</p> + +<p><a name="fn15_17">[17]</a> <i>Astr. Nach.</i>, No. 2,947.</p> + +<p><a name="fn15_18">[18]</a> <i>R. S. E. Trans</i>., vol. xxvii. +In 1901 Dr. Anderson discovered Nova Persei.</p> + +<p><a name="fn15_19">[19]</a> <i>Astr. Nach</i>., No. 3,079.</p> + +<p><a name="fn15_20">[20]</a> For a different explanation see Sir W. Huggins’s +lecture, Royal Institution, May 13th, 1892.</p> + +<p><a name="fn15_21">[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><a name="fn15_22">[22]</a> <i>R. S. Phil. Trans.</i>, 1850, p. +499 <i>et seq.</i></p> + +<p><a name="fn15_23">[23]</a> <i>Ibid</i>, vol. cliv., p. 437.</p> + +<p><a name="fn15_24">[24]</a> <i>Brit. Assoc. Rep.</i>, 1868, p. +165.</p> + +<p><br /><br /></p> + +<h1>INDEX</h1> + +<p>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</p> + +<p>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</p> + +<p>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</p> + +<p>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</p> + +<p>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</p> + +<p>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</p> + +<p>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</p> + +<p>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</p> + +<p>Ivory, 65</p> + +<p>Jansen, P. J. C., 105, 106<br /> +Jansen, Z., 86</p> + +<p>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</p> + +<p>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</p> + +<p>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</p> + +<p>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</p> + +<p>Olbers, H.W.M., 67<br /> +Omar, 11, 24<br /> +Oppolzer, 13, 125<br /> +Oudemans, 129</p> + +<p>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</p> + +<p>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</p> + +<p>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</p> + +<p>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</p> + +<p>Ulugh Begh, 24<br /> +Uranus, discovery of, 65</p> + +<p>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</p> + +<p>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</p> + +<p>Yao, 9<br /> +Young, C. A., 103<br /> +Yu-Chi, 8</p> + +<p>Zenith telescopes, 79, 82<br /> +Zöllner, 92<br /> +Zucchi, 113 </p> + + + + + + + + +<pre> + + + + + +End of the Project Gutenberg EBook of History of Astronomy, by George Forbes + +*** END OF THE PROJECT GUTENBERG EBOOK HISTORY OF ASTRONOMY *** + +This file should be named 8hsrs10h.htm or 8hsrs10h.zip +Corrected EDITIONS of our eBooks get a new NUMBER, 8hsrs11h.htm +VERSIONS based on separate sources get new LETTER, 8hsrs10ah.htm + +Produced by Jonathan Ingram, Dave Maddock, Charles Franks +and the Online Distributed Proofreading Team. + +Project Gutenberg eBooks are often created from several printed +editions, all of which are confirmed as Public Domain in the US +unless a copyright notice is included. Thus, we usually do not +keep eBooks in compliance with any particular paper edition. + +We are now trying to release all our eBooks one year in advance +of the official release dates, leaving time for better editing. +Please be encouraged to tell us about any error or corrections, +even years after the official publication date. + +Please note neither this listing nor its contents are final til +midnight of the last day of the month of any such announcement. +The official release date of all Project Gutenberg eBooks is at +Midnight, Central Time, of the last day of the stated month. A +preliminary version may often be posted for suggestion, comment +and editing by those who wish to do so. + +Most people start at our Web sites at: +http://gutenberg.net or +http://promo.net/pg + +These Web sites include award-winning information about Project +Gutenberg, including how to donate, how to help produce our new +eBooks, and how to subscribe to our email newsletter (free!). + + +Those of you who want to download any eBook before announcement +can get to them as follows, and just download by date. This is +also a good way to get them instantly upon announcement, as the +indexes our cataloguers produce obviously take a while after an +announcement goes out in the Project Gutenberg Newsletter. + +http://www.ibiblio.org/gutenberg/etext03 or +ftp://ftp.ibiblio.org/pub/docs/books/gutenberg/etext03 + +Or /etext02, 01, 00, 99, 98, 97, 96, 95, 94, 93, 92, 92, 91 or 90 + +Just search by the first five letters of the filename you want, +as it appears in our Newsletters. + + +Information about Project Gutenberg (one page) + +We produce about two million dollars for each hour we work. The +time it takes us, a rather conservative estimate, is fifty hours +to get any eBook selected, entered, proofread, edited, copyright +searched and analyzed, the copyright letters written, etc. Our +projected audience is one hundred million readers. If the value +per text is nominally estimated at one dollar then we produce $2 +million dollars per hour in 2002 as we release over 100 new text +files per month: 1240 more eBooks in 2001 for a total of 4000+ +We are already on our way to trying for 2000 more eBooks in 2002 +If they reach just 1-2% of the world's population then the total +will reach over half a trillion eBooks given away by year's end. + +The Goal of Project Gutenberg is to Give Away 1 Trillion eBooks! +This is ten thousand titles each to one hundred million readers, +which is only about 4% of the present number of computer users. + +Here is the briefest record of our progress (* means estimated): + +eBooks Year Month + + 1 1971 July + 10 1991 January + 100 1994 January + 1000 1997 August + 1500 1998 October + 2000 1999 December + 2500 2000 December + 3000 2001 November + 4000 2001 October/November + 6000 2002 December* + 9000 2003 November* +10000 2004 January* + + +The Project Gutenberg Literary Archive Foundation has been created +to secure a future for Project Gutenberg into the next millennium. + +We need your donations more than ever! + +As of February, 2002, contributions are being solicited from people +and organizations in: Alabama, Alaska, Arkansas, Connecticut, +Delaware, District of Columbia, Florida, Georgia, Hawaii, Illinois, +Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Massachusetts, +Michigan, Mississippi, Missouri, Montana, Nebraska, Nevada, New +Hampshire, New Jersey, New Mexico, New York, North Carolina, Ohio, +Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South +Dakota, Tennessee, Texas, Utah, Vermont, Virginia, Washington, West +Virginia, Wisconsin, and Wyoming. + +We have filed in all 50 states now, but these are the only ones +that have responded. + +As the requirements for other states are met, additions to this list +will be made and fund raising will begin in the additional states. +Please feel free to ask to check the status of your state. + +In answer to various questions we have received on this: + +We are constantly working on finishing the paperwork to legally +request donations in all 50 states. If your state is not listed and +you would like to know if we have added it since the list you have, +just ask. + +While we cannot solicit donations from people in states where we are +not yet registered, we know of no prohibition against accepting +donations from donors in these states who approach us with an offer to +donate. + +International donations are accepted, but we don't know ANYTHING about +how to make them tax-deductible, or even if they CAN be made +deductible, and don't have the staff to handle it even if there are +ways. + +Donations by check or money order may be sent to: + +Project Gutenberg Literary Archive Foundation +PMB 113 +1739 University Ave. +Oxford, MS 38655-4109 + +Contact us if you want to arrange for a wire transfer or payment +method other than by check or money order. + +The Project Gutenberg Literary Archive Foundation has been approved by +the US Internal Revenue Service as a 501(c)(3) organization with EIN +[Employee Identification Number] 64-622154. Donations are +tax-deductible to the maximum extent permitted by law. As fund-raising +requirements for other states are met, additions to this list will be +made and fund-raising will begin in the additional states. + +We need your donations more than ever! + +You can get up to date donation information online at: + +http://www.gutenberg.net/donation.html + + +*** + +If you can't reach Project Gutenberg, +you can always email directly to: + +Michael S. Hart hart@pobox.com + +Prof. Hart will answer or forward your message. + +We would prefer to send you information by email. + + +**The Legal Small Print** + + +(Three Pages) + +***START**THE SMALL PRINT!**FOR PUBLIC DOMAIN EBOOKS**START*** +Why is this "Small Print!" statement here? You know: lawyers. +They tell us you might sue us if there is something wrong with +your copy of this eBook, even if you got it for free from +someone other than us, and even if what's wrong is not our +fault. So, among other things, this "Small Print!" statement +disclaims most of our liability to you. It also tells you how +you may distribute copies of this eBook if you want to. + +*BEFORE!* YOU USE OR READ THIS EBOOK +By using or reading any part of this PROJECT GUTENBERG-tm +eBook, you indicate that you understand, agree to and accept +this "Small Print!" statement. If you do not, you can receive +a refund of the money (if any) you paid for this eBook by +sending a request within 30 days of receiving it to the person +you got it from. If you received this eBook on a physical +medium (such as a disk), you must return it with your request. + +ABOUT PROJECT GUTENBERG-TM EBOOKS +This PROJECT GUTENBERG-tm eBook, like most PROJECT GUTENBERG-tm eBooks, +is a "public domain" work distributed by Professor Michael S. Hart +through the Project Gutenberg Association (the "Project"). +Among other things, this means that no one owns a United States copyright +on or for this work, so the Project (and you!) can copy and +distribute it in the United States without permission and +without paying copyright royalties. Special rules, set forth +below, apply if you wish to copy and distribute this eBook +under the "PROJECT GUTENBERG" trademark. + +Please do not use the "PROJECT GUTENBERG" trademark to market +any commercial products without permission. + +To create these eBooks, the Project expends considerable +efforts to identify, transcribe and proofread public domain +works. Despite these efforts, the Project's eBooks and any +medium they may be on may contain "Defects". Among other +things, Defects may take the form of incomplete, inaccurate or +corrupt data, transcription errors, a copyright or other +intellectual property infringement, a defective or damaged +disk or other eBook medium, a computer virus, or computer +codes that damage or cannot be read by your equipment. + +LIMITED WARRANTY; DISCLAIMER OF DAMAGES +But for the "Right of Replacement or Refund" described below, +[1] Michael Hart and the Foundation (and any other party you may +receive this eBook from as a PROJECT GUTENBERG-tm eBook) disclaims +all liability to you for damages, costs and expenses, including +legal fees, and [2] YOU HAVE NO REMEDIES FOR NEGLIGENCE OR +UNDER STRICT LIABILITY, OR FOR BREACH OF WARRANTY OR CONTRACT, +INCLUDING BUT NOT LIMITED TO INDIRECT, CONSEQUENTIAL, PUNITIVE +OR INCIDENTAL DAMAGES, EVEN IF YOU GIVE NOTICE OF THE +POSSIBILITY OF SUCH DAMAGES. + +If you discover a Defect in this eBook within 90 days of +receiving it, you can receive a refund of the money (if any) +you paid for it by sending an explanatory note within that +time to the person you received it from. If you received it +on a physical medium, you must return it with your note, and +such person may choose to alternatively give you a replacement +copy. If you received it electronically, such person may +choose to alternatively give you a second opportunity to +receive it electronically. + +THIS EBOOK IS OTHERWISE PROVIDED TO YOU "AS-IS". NO OTHER +WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, ARE MADE TO YOU AS +TO THE EBOOK OR ANY MEDIUM IT MAY BE ON, INCLUDING BUT NOT +LIMITED TO WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A +PARTICULAR PURPOSE. + +Some states do not allow disclaimers of implied warranties or +the exclusion or limitation of consequential damages, so the +above disclaimers and exclusions may not apply to you, and you +may have other legal rights. + +INDEMNITY +You will indemnify and hold Michael Hart, the Foundation, +and its trustees and agents, and any volunteers associated +with the production and distribution of Project Gutenberg-tm +texts harmless, from all liability, cost and expense, including +legal fees, that arise directly or indirectly from any of the +following that you do or cause: [1] distribution of this eBook, +[2] alteration, modification, or addition to the eBook, +or [3] any Defect. + +DISTRIBUTION UNDER "PROJECT GUTENBERG-tm" +You may distribute copies of this eBook electronically, or by +disk, book or any other medium if you either delete this +"Small Print!" and all other references to Project Gutenberg, +or: + +[1] Only give exact copies of it. Among other things, this + requires that you do not remove, alter or modify the + eBook or this "small print!" statement. You may however, + if you wish, distribute this eBook in machine readable + binary, compressed, mark-up, or proprietary form, + including any form resulting from conversion by word + processing or hypertext software, but only so long as + *EITHER*: + + [*] The eBook, when displayed, is clearly readable, and + does *not* contain characters other than those + intended by the author of the work, although tilde + (~), asterisk (*) and underline (_) characters may + be used to convey punctuation intended by the + author, and additional characters may be used to + indicate hypertext links; OR + + [*] The eBook may be readily converted by the reader at + no expense into plain ASCII, EBCDIC or equivalent + form by the program that displays the eBook (as is + the case, for instance, with most word processors); + OR + + [*] You provide, or agree to also provide on request at + no additional cost, fee or expense, a copy of the + eBook in its original plain ASCII form (or in EBCDIC + or other equivalent proprietary form). + +[2] Honor the eBook refund and replacement provisions of this + "Small Print!" statement. + +[3] Pay a trademark license fee to the Foundation of 20% of the + gross profits you derive calculated using the method you + already use to calculate your applicable taxes. If you + don't derive profits, no royalty is due. Royalties are + payable to "Project Gutenberg Literary Archive Foundation" + the 60 days following each date you prepare (or were + legally required to prepare) your annual (or equivalent + periodic) tax return. Please contact us beforehand to + let us know your plans and to work out the details. + +WHAT IF YOU *WANT* TO SEND MONEY EVEN IF YOU DON'T HAVE TO? +Project Gutenberg is dedicated to increasing the number of +public domain and licensed works that can be freely distributed +in machine readable form. + +The Project gratefully accepts contributions of money, time, +public domain materials, or royalty free copyright licenses. +Money should be paid to the: +"Project Gutenberg Literary Archive Foundation." + +If you are interested in contributing scanning equipment or +software or other items, please contact Michael Hart at: +hart@pobox.com + +[Portions of this eBook's header and trailer may be reprinted only +when distributed free of all fees. Copyright (C) 2001, 2002 by +Michael S. Hart. Project Gutenberg is a TradeMark and may not be +used in any sales of Project Gutenberg eBooks or other materials be +they hardware or software or any other related product without +express permission.] + +*END THE SMALL PRINT! FOR PUBLIC DOMAIN EBOOKS*Ver.02/11/02*END* + + + +</pre> + +</body> +</html> |