ASTRONOMY (Gk. aarpoy, nstron, star vOyoc, 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 farmer, 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 con spicuous at a very early date. Thus Thales (e.640-546 n.c.), Anaximander, Anaxagoras, Py thagoras (e.580-497 n.c.), Neton, and Endoxus, are all mentioned by later writers as distin guished 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 Alexan drian Greeks carried astronomy much further. Aristarchus (Third Century me.). and Eratos thenes made numerous rather difficult observa tions, and Plipparchus (Second Century n.c.) even constructed a fundamental catalogue of stars. Ptolemy (Second Century a.n.) wrote the Almagrat, 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 PTOLEMY : Pr:fix:At:kw SYSTEM EPICYCLE; ECCENTRIC.
The Arabians also did much for astronomy. The most important names are those of Al Bat tani Ic.900 A.D.) and Ulugh Beg, who lived about five centuries later. The latter construct ed the second known catalogue of stars.
Modern European astronomy begins with Par bach and Regiomontamis in the Fifteenth Cen tury. Copernicus (1473-1543) stands out con spicuously as the author of a system of the universe nearly the same as that now accepted. (See COPERNICAN SYSTEM.) Copernicus was fol lowed by Tycho Brahe of Denmark (1546-1601), who left a most important collection of solar and planetary observations as well as a cata logue of 777 stars far superior in precision to those of Hipparehus and Ulugh Beg. It was upon a discussion of Tycho's observations that his famous pupil Kepler (1571-1(i30) 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 ob servations. To his gaze were first disclosed the moons of the planet ,lupiter. 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 we also the dis
covery of the pendulum, perhaps the most im portant a_stronomical 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 as tronomy really begins; so that we can now substi tute an account of the present state of the science for the historical outline thus far pursued. New ton's conception of the universe makes all phe nomena of motion subject to a single law—the law of gravitation. (See GRAVITATION FORCE FALLING BODIES PROJECTILES NEw'rox.) Ac cording 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 at tracted. It is possible by mathematical reason ing to prove from Newton's Law just how a sys tem 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 join ing 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 Kep ler 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 mechani cal construction of the solar system in which we live may be regarded as fully explained by New ton. Later investigators have elaborated his theory and carried the mathematical explanation of visible phenomena down to the minutest pre cision attainable with the most powerful modern observational machinery. The work of Newton may be said to have been crowned by that of Laplace (17-191827), who analytically demons trated the stability of the solar system, and whose Meeanique celeste was a worthy successor to Newton's Principle. 1-laying thus disposed of the mechanism of our solar system, the ques tion suggests itself as to whether this same law of Newton holds sway even to the most distant confines of the visible universe. Does it control the motions of stars whose vast distance allows us barely to glimpse them in our largest tele scopes? The answer in the present state of as tronomical knowledge is not quite so certain for the distant stellar systems as it is within our immediate neighborhood in space. Yet we may say that observations so far made have brought to light no phenomena positively contradicting the universality of Newton's law.