Page:The New International Encyclopædia 1st ed. v. 02.djvu/203

ASTROLOGY. country of the earth, while fecundity and prosperity were to reign among its people.

Consult: Sibley, A New and Complete Illustration of the Celestial Science of Astrology (London, 1789); Butler, Solar Biology (5th ed., Applegate, Cal., 1892); Bennett, Astrology (New York, 1894); Maury, La magie el l'astrologie à l'antiquité et au moyen âge (4th ed., Paris, 1877); Meyer, Handbuch der Astrologie (Berlin, 1891).

AS'TRONOM'ICAL AND AS'TROPHYS'ICAL SOCI'ETY OF AMER'ICA. A national organization with membership limited to persons possessing technical attainments in astronomical and astrophysical science. The Society was founded in 1899.

ASTRON'OMY (Gk. aarpov, astron, star + v6/ioi, nomos, law). The oldest of the sciences. The early history of astronomy is perhaps more important than that of any other science; in- deed, it may be said that a study of the state of scientific culture among the early peoples amounts to little more than an examination of their notions on astronomy. The science had its beginning with the Chaldeans and Chinese, working, of course, independently of each other. The former, according to Greek historians, were able to predict eclipses with considerable ac- curacy. The latter, in addition, were acquainted with certain elementary forms of the calendar, and have left authentic observations of eclipses, comets, etc., extending back at least a thousand years before our era.

Among the Greeks we find various names conspicuous at a very early date. Thus Thales (c. 640-546 B.C.), Anaximander, Anaxagoras, Pythagoras (c. 580-497 B.C.), Meton, and Eudoxus, are all mentioned by later writers as distinguished astronomers. They doubtless occupied themselves principally with eclipse and calendar investigations, and with imaginary systems of the universe based upon abstract speculation rather than accurate observation. The Alexandrian Greeks carried astronomy much further. Aristarehus (Third Century B.C.), and Eratosthenes made numerous rather difficult observations, and Hipparchus (Second Century B.C.) even constructed a fundamental catalogue of stars. Ptolemy (Second Century A.D.) wrote the Almagest, an important general treatise on astronomy, and was the inventor of the Epicyclic System of the Universe, which makes the sun and planets move in circles whose centres are themselves in motion upon other circles, the earth being considered at rest. See ; ; ;.

The Arabians also did much for astronomy. The most important names are those of Al Battani (c. 900 A.D.) and Ulugh Beg, who lived about five centuries later. The latter constructed the second known catalogue of stars.

Modern European astronomy begins with Purbach and Regiomontanus in tho Fifteenth Century. Copernicus (1473-1543) stands out conspicuously as the author of a system of the universe nearly the same as that now accepted. (See .) Copernicus was followed by Tycho Brahe of Denmark (1546-1601), who left a most important collection of solar and planetary observations as well as a catalogue of 777 stars far superior in precision to those of Hipparchus and Ulugh Beg. It was upon a discussion of Tycho's observations that his famous pupil Kepler (1571-1630) built his well-known laws of planetary motion, which are now accepted, and which will be stated below. Galileo (1564-1642), one of the first constructors of the telescope, made with it unprecedented observations. To his gaze were first disclosed the moons of the planet Jupiter, and his clear mental vision saw in that planetary system a true miniature of our solar system itself — an ocular demonstration of the Copernican plan of the universe. To Galileo we owe also the discovery of the pendulum, perhaps the most important astronomical instrument. This was adapted to the astronomical clock by Huygens (1629-95), whereby observation in the modern sense was first rendered possible. For greater detail, see separate articles on each of the names here mentioned.

We come now to the great name of Newton (1642-1727). With him we may say that astronomy really begins; so that we can now substitute an account of the present state of the science for the historical outline thus far pursued. Newton's conception of the universe makes all phenomena of motion subject to a single law — the law of gravitation. (See ; ; ; ; .) According to this, every material body attracts or draws every other material body. In every case the strength of the attraction increases as the mass of the attracting body, and diminishes as the square of the distance from the body attracted. It is possible by mathematical reasoning to prove from Newton's Law just how a system of planetary bodies would move under the influence of the attraction of some larger central sun. We thus find mathematically that such motion must follow these three laws: (1) Each planet must revolve in an elliptic orbit having the sun in its focus. (2) The straight line joining the sun and planet must pass over equal areas in equal times. (3) The square of the time of revolution of each planet must be proportional to the cube of its mean distance from the sun. Now, these are precisely the laws found by Kepler from the actual observations of Tycho Brahe. Newton's law of gravitation is therefore seen to lead by deduction to the general facts actually observed by astronomers, and hence the mechanical construction of the solar system in which we live may be regarded as fully explained by Newton. Later investigators have elaborated his theory and carried the mathematical explanation of visible phenomena down to the minutest pre-