Astron´omy

(from Gr. astron , a heavenly body, and nemein , to cla ify or arrange) is that science which investigates the motions, distances, magnitudes, and various phenomena of the heavenly bodies. The science may be divided into several branches. Descriptive astronomy denotes merely a presentation of astronomical facts in a systematic but popular form; practical astronomy treats of the instruments used in observing the celestial bodies, the methods 289 of their employment, and the manner of deducing results from the observations; investigation of the causes of the motions of these bodies was formerly termed physical astronomy , but now generally dynamical or gravitational astronomy ; physical astronomy or astro-physics is the comparatively modern branch which deals with their physical conditions, radiation, temperature, and chemical constitution. Recent years have added two new fields of investigation which are full of promise for the advancement of astronomical science. The first of these— celestial photography —has furnished us with invaluable light-pictures of the sun, moon, and other bodies, and has recorded the existence of myriads of stars invisible even to the best telescopes; while the second, spectrum analysis , now employed by many scientists, reveals to us a knowledge of the physical constituents of the universe, telling us for instance that in the sun (or his atmosphere) there exist many of the elements familiar to us on the earth. It is also applied to the determination of the velocities with which stars are approaching, or receding from, our system; and to the measurement of movements taking place within the solar atmospheric envelopes. From analysis of some of the unresolved nebulæ the inference is drawn that they are not star-swarms but simply incandescent gas; whence a second inference results favourable to the hypothesis of the gradual condensation of nebulæ, and the succe ive evolutions of suns and systems. The most remote period to which we can go back in tracing the history of astronomy refers us to a time about 2500 B.C. , when the Chinese are said to have recorded the simultaneous conjunction of Saturn, Jupiter, Mars, Mercury, and the moon. This remarkable phenomenon is found, by calculating backward, to have taken place 2460 B.C. Astronomy has also an undoubtedly high antiquity in India. The mean annual motion of Jupiter and Saturn was observed as early as 3062 years B.C. ; tables of the sun, moon, and planets were formed, and eclipses calculated. In the time of Alexander the Great, the Chaldeans or Babylonians had carried on astronomical observations for 1900 years. They regarded comets as bodies travelling in extended orbits, and predicted their return; and there is reason to believe that they had correct ideas regarding the solar system. The priests of Egypt gave astronomy a religious character; but their knowledge of the science is testified to only by their ancient zodiacs and the position of their pyramids with relation to the cardinal points. It was among the Greeks that astronomy took a more scientific form. Thales of Miletus (born 639 B.C. ) predicted a solar eclipse, and his succe ors held opinions which are in many respects wonderfully in accordance with modern ideas. Pythagoras (500 B.C. ) and his followers formed theories of the planetary system. They taught the sphericity and revolution of the earth, but placed an imaginary 'Central Fire', not the sun itself, at the centre of the system. Great progre was made in astronomy under the Ptolemies, and we find Timochares and Aristyllus employed about 300 B.C. in making useful planetary observations. But Aristarchus of Samos (born 267 B.C. ) is said, on the authority of Arch i me des, to have far surpa ed them, by teaching the double motion of the earth around its axis and around the sun. A hundred years later Hipparch us determined more exactly the length of the solar year, and the eccentricity of the ecliptic, discovered the prece ion of the equinoxes, and even undertook a catalogue of the stars. It was in the second century after Christ that Claudius Ptolemy, a famous mathematician of Pelusium in Egypt, propounded the system that bears his name, viz., that the earth was the centre of the universe, and that the sun, moon, and planets revolved around it in the following order: nearest to the earth was the sphere of the moon; then followed the spheres of Mercury, Venus, the Sun, Mars, Jupiter, and Saturn; then came the sphere of the fixed stars; these were succeeded by two crystalline spheres and an outer sphere named the primum mobile or first movable, which last was again circumscribed by the cœlum empyreum , of a cubic shape, wherein happy souls found their abode. The Arabs began to make scientific astronomical observations about the middle of the eighth century, and for 400 years they prosecuted the science with a iduity. Ibn-Yunis ( A.D. 1000) made important observations of the perturbations and eccentricities of Jupiter and Saturn. In the sixteenth century Nicol a us Copernicus, born in 1473, introduced the system that bears his name, and which recognized the sun's central place in the solar system, and that all the other bodies, the earth included, revolve around it. This arrangement of the universe (see Copernicus ) came at length to be generally received on account of the simplicity it substituted for the complexities and difficulties of the theory of Ptolemy. The observations and calculations of Tycho Brahe, a Danish astronomer, born in 1546, continued over many years, were of the highest value, and secured for him the title of regenerator of practical astronomy. His a istant and pupil, Kepler, born in 1571, was enabled, principally from the data provided by his master's labours, to arrive at those laws which have made his name famous: 1. That the planets move, not in circular, but in elliptical orbits, of which the sun occupies a focus. 2. That the radius vector, or imaginary straight line joining the sun and any planet, moves over equal 290 spaces in equal times. 3. That the squares of the times of the revolutions of the planets are as the cubes of their mean distances from the sun. Galileo, who died in 1642, advanced the science by his observations and by the new revelations he made through his telescopes, which established the truth of the Copernican theory. Newton, born in 1642, carried physical astronomy suddenly to comparative perfection. Accepting Kepler's laws as a statement of the facts of planetary motion, he deduced from them his theory of gravitation. The science was enriched towards the close of the eighteenth century by the discovery by Herschel of the planet Uranus and its satellites, the resolution of the Milky Way into myriads of stars, and the investigation of nebulæ and of double and triple stars. The splendid analytical researches of Lalande, Lagrange, Delambre, and Laplace mark the same period. The nineteenth century opened with the discovery of the first four minor planets; and the existence of another planet (Neptune), more distant from the sun than Uranus, was, in 1845, independently predicted by Leverrier and Adams. Of late years the sun has attracted a number of observers, the spectroscope and photography having been especially fruitful in this field of investigation. By various methods the sun's mean distance has been ascertained within very small limits of error, and found to be nearly 93,000,000 miles. Many additions have been made to the known secondary planets or satellites, including some with retrograde motions. A vast number of asteroids has been discovered, and the width of the zone occupied by them found to be much more extensive. Much succe has been achieved in ascertaining the parallax of fixed stars. The objects with which astronomy has chiefly to deal are the earth, the sun, the moon, the planets, the fixed stars, comets, nebulæ, and meteors. The stellar universe is composed of an unknown host of stars, many millions in number. Those visible to the naked eye were in ancient times grouped into the constellations still recognized. The nebulæ are cloud-like patches of light scattered all over the heavens. Some of them have been resolved into star-clusters, but many of them are ma es of incandescent gas. Of the so-called fixed stars, many form binary or multiple systems, the members revolving in orbits under each other's attractions, while other more scattered groups are moving clusters, travelling in parallel paths through space like flocks of birds. Variable stars and extinct or dark stars are also known. The fixed stars preserve, at least to unaided vision, an unalterable relation to each other, because of their vast distance from the earth. Their apparent movement from east to west is the result of the earth's revolution on its axis in twenty-four hours from west to east. The planets have not only an apparent, but also a real and proper motion, since, like our earth, they revolve around the sun in their several orbits and periods. The nearest of these bodies to the sun is Mercury . Venus , the second planet from the sun, is to us the brightest and most beautiful of all the planets. The Earth is the first planet accompanied by a satellite or moon. Mars , the next planet, has two satellites, discovered in 1877. Its surface has a variegated character, and the existence of land, water, snow, and ice has been inferred. The Asteroids , of which over 1000 are known, form a broad zone of small bodies, at distances from a little beyond the earth's to that of Jupiter. Jupiter , the largest planet, has at least nine satellites, of which the two outermost have retrograde motion. Its surface is diversified by spots, markings, and bands parallel to its equator. Saturn , with its nine or more satellites and broad thin rings in its equatorial plane, is, perhaps, the most striking telescopic object in the heavens. Urănus —discovered by Herschel in 1781—is accompanied by four satellites. Neptune , the farthest removed from the sun, has one satellite, the motion of which is retrograde. Besides the planets, quite a number of comets are known to be members of the solar system. The physical constitution of these bodies is still one of the enigmas of astronomy. The observation of meteors has recently attracted much attention. They are seen in largest numbers in the autumn months. Meteor streams are supposed to represent the results of the disintegration of comets. Among the more modern astronomers we may mention: Gustav Kirchhoff, G. B. Donati, Christian Doppler, H. C. Vogel, Sir William Huggins, Simon Newcomb, and Sir David Gill. See Earth , Sun , Moon , Planet , Comet , Stars , Asteroids , Celestial Photography , Spectrography , .— BIBLIOGRAPHY : Sir J. N. Lockyer, Dawn of Astronomy ; Sir G. C. Lewis, Historical Survey of the Astronomy of the Ancients ; Sir. F. W. Dyson, Astronomy ; Sir R. Ball, Atlas, and Popular Guide to the Heavens ; G. P. Servi , Astronomy with an Opera-gla ; The Pleasures of the Telescope ; A. M. Clerke, History of Astronomy during the 19th Century , H. Macpherson, Romance of Modern Astronomy ; C. A. Young, General Astronomy ; G. F. Chambers, Handbook of Astronomy (3 vols.); E. W. Maunder, Astronomy of the Bible ; A. C. D. Crommelin, The Star World ; Agnes Giberne, Sun, Moon, and Stars (popular).