Astronomy (Gr. a star, and law), the science which deals with the movements, distribution, and physical characteristics of the heavenly bodies. That astronomy is the most ancient of all the sciences, save agriculture, can scarcely be questioned. In the earliest ages men must have required measures of time, and such measures could only be obtained from the study of the motions and appearances of the celestial bodies. The origin of astronomy has been referred to several nations. The evidence in favor of the Chaldeans seems on the whole the strongest. We find in Ptolemy's Almagest the records of observations of considerable accuracy made at Babylon at a very early epoch. Some of the observations which were transmitted to Aristotle by Callisthenes were made about 2250 years B. C. The Chaldean investigations of the motions of the moon were in many respects remarkable. In particular their invention of the saros indicates not merely very accurate observation and a careful discussion of the results, but considerable ingenuity. They were also acquainted with the art of dialling; they had discovered the precession of the equinoxes, and had determined the length of the tropical year to within less than half a minute of its true value.
There are even reasons for believing that they were acquainted with the true system of the universe; and we learn from l3iodorus Siculus and Apollonius Myndius that the Chaldean astronomers regarded comets as bodies travelling in extended orbits, and even in some instances predicted the return of these objects. Indian astronomy does not appear to have been by any means so accurate as that taught by the Chaldeans. The Indian system seems indeed to have belonged to a more northerly latitude than Benares, the chief seat of Hindoo learning. Accordingly M. Bailly was led to ascribe the origin of the system to a nation which had inhabited higher latitudes; and he even went so far as to invent a nation for the occasion, the Atlantides, and to ascribe to that apocryphal nation a wholly incredible degree of learning. It may be inferred that the want of agreement between celestial phenomena in India and the Indian system of astronomy, instead of justifying M. Bailly's argument, shows rather that the Indian astronomers were but imperfectly acquainted with the phenomena of the heavens.
Nor is it easy to ac-cept the opinion of Prof. Smyth, astronomer royal for Scotland, that the ancient Egyptians, the architects of the great pyramid, were acquainted with all the facts which he conceives to have been symbolized in that remarkable edifice. That the pyramid was erected for astronomical purposes may be admitted; and we may accept Prof. Smyth's conclusion that the building of the pyramid corresponded to the time when the star a Draconis at its upper transit was visible (as well by day as by night) through the long inclined passage which forms one of the characteristic features of the pyramid. This would set the epoch about the year 2170 B. C. And it is a remarkable fact that, as Prof. Smyth points out, the Pleiades were at that time in a most peculiar position, well worthy of being monumentally commemorated; "for they were actually at the commencing point of all right ascensions, or at the very beginning of running that great round of stellar chronological mensuration which takes 25,808 years to return into itself again, and has been called elsewhere, for reasons derived from far other studies than anything hitherto connected with the great pyramid, the 'great year of the Pleiades.'" But although we may thus set the astronomical system of the early Egyptians in a far antiquity, it seems unsafe to follow Smyth in believing that the builders of the great pyramid were acquainted with the sun's distance, with the true length of the precessional period, and with other astronomical elements the discovery of which has rewarded the exact methods and the profound mathematical researches of modern times. - As to Chinese astronomy, we have abundant evidence to show that it was inexact, though undoubtedly very ancient.
Its antiquity may be inferred from the circumstance that the emperor Chwen-hio adopted as an epoch a conjunction of the planets Mercury, Mars, Jupiter, and Saturn, which has been shown by M. Bailly to have occurred no less than 2449 years B. 0. In a remarkable work on the subject of Chinese astronomy, recently published by Mr. Williams, assistant secretary of the astronomical society of England, we are told that the instruments at present used by Chinese astronomers, as well as their principal methods of calculation, were introduced by Jesuit missionaries. Yet the ancient Chinese must have possessed some familiarity with the celestial motions. They could calculate eclipses; for we learn that "in the reign of the emperor Chow-kang, the chief astronomers Ho and Hi were condemned to death for failing to announce a solar eclipse which took place 2169 B. C.;" a clear proof that the prediction of eclipses was a part of the duty of the imperial astronomers. The Chinese were also acquainted with the Metonic and Callippic cycles. - The earliest Greek school of astronomy was that founded by Thales of Miletus (600 B. C.) and termed the Ionian school.
Thales appears to have been acquainted with the motions of the sun and moon, with the explanation of seasonal changes, and with the length of the year. It has been said that he taught mariners to regard the Lesser Bear rather than the Greater as the polar constellation; but Manilius ascribes the selection of the Lesser Bear as the cynosure to the Phoenicians. To Pythagoras, who also belonged to the Ionian school, a knowledge of the true theory of the earth has been ascribed, though on insufficient grounds. According to the statement of his pupil Philolaus, he taught that " the earth and planets move in oblique circles (or ellipses) about fire, as the sun and moon do " - a statement which certainly does not as it stands indicate exact knowledge respecting the constitution of the solar system. Nicetas of Syracuse is said in like manner to have taught that the diurnal motions of the celestial bodies are caused by the rotation of the earth upon her axis. "Theophrastus," says Cicero, "narrates that Nicetas of Syracuse held that the sun, moon, and stars are at rest, and the earth alone moves, turning about its axis, by which the same phenomena are produced as if the contrary were the case." Eudoxus of Cnidus first endeavored to explain the looped paths of the planets, solving the problem by the invention of the theory of concentric spheres. - But it was by the Alexandrian school, founded under the Ptolemies, that exact and systematic observation of the celestial bodies was first undertaken.
Hipparchus of Nicaea (160 B. C.) surpassed all the astronomers of antiquity in skill and acumen. He made the first catalogue of the stars, and was the first to calculate the motions of the sun and moon. He also made a series of observations of the planets, and represented their motions by the famous theory of epicycles - a theory which, though unsound, was in so far in advance of previous ideas, that it was intended to be brought into comparison with the real motions of the celestial bodies. Hipparchus also invented plane and spherical trigonometry. Ptolemy is another distinguished member of the Alexandrian school. Some of the theories and observations which have been ascribed to him were indeed due to the labors of Hipparchus. Thus the Ptolemaic system of astronomy was wholly based on the theories of his predecessor; and the star places indicated in his works seem to have been simply deduced from Hipparchus's catalogue of 1,081 stars by introducing a correction for precession. Yet Ptolemy's labors were unquestionably important. He detected the inequality in the moon's motions called the evection, and was the first to recognize the effect of refraction in altering the apparent places of the heavenly bodies.
His work, the Almagest (or the Syn-taxis), contains nearly all that we know of the astronomy of the ancients. The school of Alexandria ceased to exist when Egypt was invaded and conquered by the Mohammedans, and the celebrated Alexandrian library destroyed, in the 7th century. The Arabians, however, formed no contemptible astronomers. They even surpassed the Greeks in the department of practical astronomy; and they handed down to the Europeans the system which they had derived from their predecessors. - In the 13th century European astronomy may be said to have had its origin or revival, though nearly two centuries elapsed before any important advance was effected. Toward the close of the 15th century the labors of Purbach and Regio-montanus prepared the way for the work of Copernicus, the founder of the true system of astronomy; while Waltherus revived the art of astronomical observation, and thus indirectly supplied the means of establishing the theories of Copernicus, Kepler, and Newton. Copernicus (born in 1473) found that by placing the sun instead of the earth at the centre of the scheme, there resulted a simple and rational explanation of all the chief motions of the planets. He was not able to show, however, that the epicycles of Hipparchus and Ptolemy could be wholly removed.
Accordingly, many astronomers, who might have been attracted to the Copernican system if it could have been presented as it is known in our day, were found in the ranks of its opponents. Among these was Tycho Brahe, the Dane, who pointed out that the apparent fixity of the stars is opposed to the Copernican theory, unless the distances of all the stars be assumed to exceed enormously the distance of the earth from the sun. He therefore adopted a modification of a system once held by the Egyptians, regarding the earth as the centre around which the sun revolves, while the planets revolve around the sun as a subordinate centre. Although this was a retrogression, astronomy owes a debt of gratitude to Tycho Brahe for the observations by which he endeavored to put the Copernican theory to the test. His observations of Mars, in particular, enabled Kepler to remove for ever from astronomy the cycles and epicycles, centrics and eccentrics of the old systems. Endeavoring to explain the motions of Mars on the Copernican theory, Kepler found himself baffled so long as he adhered to circular and uniform motions so combined as to produce epicyclic paths.
He was thus led to try whether the ellipse would better explain the movements of Mars. After long and patient study he was able in 1609 to establish his first two laws, and nine years later his third law. The three laws are as follows: 1. Every planet describes an ellipse about the sun, this orb occupying one focus of each such ellipse. 2. If a line be supposed continually drawn from the sun to any given planet, this line will sweep over equal areas in equal times. 3. The squares of the periodic times of the planets are proportional to the cubes of their mean distances. In the mean time the telescope had been invented, and when less than one year had passed after the publication of the first two laws of Kepler, Galileo had made a series of observations tending to illustrate if not even to demonstrate the truth of the Copernican system. In particular his discovery of the satellites of Jupiter, and the recognition of the motions of these orbs around their primary, was felt even by the enemies of the new theory to be strikingly in its favor. Here was a system in which the motions of the earth and planets around the sun seemed pictured in miniature. The discovery of the phases of Venus was also regarded as a serious blow to the Ptolemaic system.
The invention of the telescope supplied also the means of determining the places and therefore the motions of the celestial bodies with a degree of accuracy which had hitherto been unattainable. He-velius indeed endeavored to make a stand against the innovation, adhering until the end of his career to the methods used by the ancients. But gradually the telescope prevailed, and the way was thus prepared for the researches of Newton, whose discovery of the law of gravitation would never have been admitted but for the evidence in its favor attained by means of telescopic observations. In particular, the measurement of the earth's dimensions with the requisite accuracy could not have been accomplished without telescopic observations of star places; and Newton would have been unable to show that the moon is retained in her orbit by the same force which draws objects to the earth's surface, had not accurate measurements of the earth been obtained by Picard. We know in fact that Newton was led by erroneous ideas of the earth's dimensions to abandon the theory of gravitation for nearly 20 years. Returning to his researches in 1080, when news of Picard's results had reached him, Newton was able to establish the theory of gravitation on a firm and stable basis.
He showed that the moon is drawn to the earth by terrestrial gravity, diminished at the moon's distance in the same degree that the square of that distance exceeds the distance of points on the earth's surface from the earth's centre. He proved that when the force of attraction diminishes according to the law of the inverse square, the attracted body will obey all the laws of Kepler in its motions around the attracting orb. Then he extended his inquiries to the mutual perturbations of bodies so moving. Taking the moon as an instance of the effects of perturbation, he showed how several peculiarities in her motions which had hitherto seemed inexplicable are caused by the sun's perturbing action on the moon, that is, by. the excess or defect of his action on the moon in different parts of her orbit, as compared with his action on the earth. Pursuing his researches, he showed how the precession of the equinoxes can be accounted for by the law of gravitation; he formed and discussed two theories of the tides; he solved the problem presented by the oblateness of the earth's figure. Half a century passed before any attempts were made to extend the reasoning of the Principia, or to develop the views of its author.
During this half century British mathematicians were chiefly engaged in defending, continental mathematicians in attacking, the principle of universal gravitation. But in 1745 Euler and Clairaut began to apply the new methods of mathematical analysis to the problems discussed by Newton. Clairaut succeeded in explaining the lunar evection, which had foiled Newton; and this success encouraged continental astronomers to devote their powers to the investigation of the problems presented by the celestial motions. They mastered one after another the difficulties of the lunar and planetary perturbations. The analytical researches of Lagrange and Laplace, and in particular the discovery (independently made by both) of the great laws on which the stability of the planetary system depends, are only inferior to the discovery of the law of gravitation itself in interest and importance. It would be difficult to say which of these two geometers displayed the greater powers of analytical research. If the genius of Lagrange was the more profound, yet Laplace's labors led to more important practical results, and in discovering the real interpretation of the "long inequality " of Jupiter and Saturn he mastered a problem which had foiled his great rival.
Yet another noble achievement of Laplace's must be mentioned - his interpretation of the secular acceleration of the moon's mean motion. In recent times it has been shown indeed by Adams that Laplace's investigation of the subject was imperfect; yet undoubtedly he placed his finger on the true cause of that part of the acceleration which is due to the ordinary forms of perturbation, nor has the cause of the re-maming part of the moon's acceleration been hitherto ascertained. Finally, we may regard the publication of his Mecanique celeste as forming a veritable epoch in the history of physical astronomy. Passing over many important contributions to the theory of gravitation, we may point to the achievement of Adams and Lever-rier in the discovery of the planet Neptune as perhaps the most conclusive of the evidences yet adduced in support of Newton's theory. A planet hitherto unseen was made known to us, not as in the case of Uranus by a happy chance, but by a study of the deviations of a known planet from the path calculated for it by mathematicians. It may be added that the discovery of Neptune led to the recognition of the mastery which American astronomers and mathematicians had obtained over the more recondite departments of analysis.
It has been remarked by Prof. Grant of Glasgow that "the results which have been deduced from Bond's observations of the satellite of Neptune, and the mathematical researches of Walker and Peirce, unquestionably exhibit a degree of consistency with the actual pbserva-tions of Uranus and Neptune which has not been paralleled by any similar efforts in Europe; while at the same time they tend to throw much interesting light on the theory of both planets.1' Among the more recent contributions to the mathematics of astronomy must be mentioned Adams's discussion of the moon's secular acceleration and the researches to which that discussion led, Delaunay's extension of the lunar theory, and the inquiries of Prof. Newcomb into the same subject. - While mathematical astronomy had been thus advancing, observational astronomy made similar progress. The discovery of Saturn's ring and largest satellite by Huyghens was soon followed by the discovery of four other satellites. Later Sir W. Herschel discovered two other Saturnian satellites, while in comparatively recent times Bond in America and Las-sell in England discovered an eighth.
Uranus was added to the planetary system by Sir W. Herschel in 1781, and at sundry times four Ura-nian satellites have since been discovered, while four others are by some supposed to have been seen by Sir W. Herschel. Neptune and his satellite constitute two other known members of the planetary scheme. But to these must be added 130 small planets (see Asteroids) which travel between the paths of Mars and Jupiter; while the observations and researches of Bond and Peirce in America and Maxwell in England tend to show that the rings of Saturn are composed of multitudinous small satellites. Apart from these discoveries, the complexity of the scheme ruled over by the sun has been indicated by the discovery of the fact that multitudes of meteoric svs-terns exist within the confines of the solar domain, and that the component members of these systems must be counted by millions. The recent observations of Profs. Newton and Kirkwood in the United States, Prof. Alexander Herschel and Mr. Glaisher in England, Quetelet in Belgium, Schmidt in Athens, Heis in Germany, and Secchi in Rome, have added largely to our knowledge respecting meteors; while the mathematical researches of Schiapa-relli, Adams, Leverrier, and others, have revealed the interesting fact that these bodies are intimately associated with comets. - The telescopic study of the starry depths, though it has been prosecuted laboriously by the Iler-schels, Struve, Argelander, Madler, and others, must be regarded as still (owing to the vastness of the domain to be explored) in its infancy.
The elder Herschel first conceived the daring idea of gauging the celestial depths; but as a matter of fact the regions surveyed by the two Herschels amount to but a minute portion of the heavens. On the other hand, though Argelander's survey extended over a complete hemisphere, yet the telescopic power employed was but small. Dr. Gould, an American astronomer, is extending Argelander's system of survey to the southern heavens; and the result cannot fail to be of the utmost interest and value. We owe to the Herschels nearly all our present knowledge of the strange objects called nebulae or star cloudlets. Of these only 16 were known in Halley's time, and barely 200 when Sir W. Herschel began his telescopic labors. He and his son added between them nearly 5,000 nebulae to the list of known objects of this class. At present some 5,700 nebula) are known in all. - The theoretical considerations by which the Herschels have endeavored to interpret the scheme of the universe are too important to pass unnoticed in this brief sketch of the history of astronomy.
They have presented the galaxy to our contemplation as a scheme of suns, many equalling and many surpassing our own sun in magnitude and splendor, while they have taught that many of the star cloudlets are schemes of suns resembling the galaxy in extent and constitution. If some, as Whewell, Herbert Spencer, and others, do not regard these views as demonstrated or. even demonstrable, yet we cannot but contemplate with admiration the activity of mind which enabled the Herschels, after completing unrivalled series of observational researches, to propound theories so magnificent respecting the myriads of orbs which they had examined. - The spectroscopic analysis of the sun and other celestial bodies, in the hands of Kirchhoff, Huggins, Young, Secchi, Zollner, Lockyer, and Respighi, has revealed many facts of importance. It has been shown that in the sun many of our famil-iar elements exist in the form of vapor. In the planetary atmospheres known vapors, and especially the vapor of water, have been detected. The stars have been proved to be Buns, many closely resembling our sun in elementary constitution, others formed very differently, but all incandescent orbs as he is, and surrounded by the glowing vapors of many elementary substances.
The application of the analysis to nebulae has led to the surprising discovery that while many of these objects shine with a light resembling that of our own sun, so that they may be considered to be formed by the aggregation together of many stars, others consist almost wholly of glowing gas, nitrogen and hydrogen forming their chief constituent elements. The observations of recent solar eclipses have been rewarded by many interesting discoveries respecting the physical constitution of the sun, the colored prominences surrounding him, and the corona which lies beyond the prominences. In these discoveries, Huggins, Young, Janssen, Lockyer, Respighi, and Secchi have borne the principal part. The progress of practical astronomy, and particularly the application of the telescope to the determination of the exact position of the celestial bodies, has proceeded pari passu with the progress of mathematical analysis and direct telescopic observation. The invention of the equatorial, the transit instrument, the mural circle, and other instruments of exact observation, belongs to the comparatively early history of modern astronomy.
In the present day these instruments are constructed with a degree of perfection, and with a multiplicity of contrivances for improving their performance or extending their application, which are truly surprising. Nor have the achievements of instrumental astronomy fallen short of the promise afforded by the qualities of the instruments. It would be sufficient to point out that the telescope has revealed the greater number of those minute inequalities of planetary motion which have afforded the material for the analytical researches above referred to; but we may add that we owe to the telescope the recognition of the aberration of light, the discovery of the proper motions of the stars, the determination of the sun's distance, and the partial solution of the most difficult problem yet attacked by astronomers, the determination of the distances of the stars. Lastly, the spectroscope promises to play an important part in instrumental researches, since already it has been applied to the determination of the velocity with which stars are approaching us or receding from us, and to the measurement of movements taking place within the solar atmospheric envelopes. - For a popular view of astronomy, Sir John Her-schel's "Outlines" may be recommended; and full details respecting practical astronomy will be found in the treatise on that subject by Prof. Loomis of New York, justly described by Prof. Nichol as " the best work of the kind in the English language." A thorough knowledge of physical astronomy would require an acquaintance with such works as Laplace's Mecanique celeste, translated by Bowditch, Gauss's Theoria Motus Corporum Coelestium, translated by Admiral C. H. Davis, U. S. N. (Boston, 1858), Delambre's Astronomie, or Peirce's "Analytical Mechanics" and "Celestial Mechanics." For the history of astronomy, see Whewell's "History of the Inductive Sciences," Grant's "History of Physical Astronomy," Jahn's Geschichte der Astronomie, and Delambre's Histoire de Vastronomie. For full information concerning the modern history of astronomy, Zach's Monatliche Correspondenz, Lindenau's Zeit-schrift, Schumacher's Astronomische Nachrich-ten, continued by Dr. Peterson, and Gould's "Astronomical Journal" (Boston) must be consulted; also, the French Connaissances des temps, which contain Leverrier's discussions that led to the discovery of Neptune, the Berlin Jahrbuch, the Milan Ejfemeridi, and the American "Ephemeris and Nautical Almanac."