ASTRONOMY, the science that treats of all the heavenly bodies, including the earth, as related to them. It is the old est of the sciences, and the mother of those generally called exact mathema tics, geodesy and physics.
Astronomy may to-day be broadly di vided into two branches, mathematical and physical, and these are almost sy nonymous with two terms recently in troduced, the old and the new astron omy, as defined by the statement that the old tells us where the heavenly bodies are, the new, what they are. The characteristic feature of the instruments and methods of the new versus the old astronomy is that the new deals with some special form of radiant energy, measuring or analyzing the vibrations transmitted throughout all space by means of the elastic medium called ether.
Under the two broad divisions stated above, mathematical astronomy would include the following divisions, which are not, however, mutually exclusive: Spherical astronomy, which treats of angles and directions on the celestial sphere; practical astronomy, treating of the instruments, methods of observa tion, and of calculation employed to get at the facts and data of astronomy; theoretical astronomy, which deals with the orbits, tables and ephemerides of the sun, moon, planets, and comets, includ ing the effect of their mutual attrac tions, and gravitational or mechanical astronomy, which treats of the forces (principally gravitation) at work in space and the motions resulting there from. This last was formerly called physical astronomy, but the name has been monopolized by the new astronomy within the last few decades, and must now be reserved for it. This second branch, likewise called astronomical physics and astrophysics, attempts to answer the question of what the heav enly bodies are, the nature and con stitution of their interiors, surfaces, atmosphere, their temperatures and radi ations, and the effect of these radiations upon other bodies, and all allied ques tions arising out of these. Its princi
pal instrument, the spectroscope, has likewise furnished data otherwise un attainable in the field of mathematical astronomy, viz., the determination of the motion to or from us of the heavenly bodies by displacement of the lines of their spectra due to this motion.
Chinese, Hindus, Chal deans, Egyptians, and Greeks investi gated the heavens long before the Chris tian era. In China, astronomy was intimately associated with state poli tics; the Indians, Chaldeans, and Egyp tians made it a matter of religion. The Greek historians attribute the earliest knowledge of astronomical science to the Chaldeans and Egyptians. They say that the former discovered the Saros or cycle of 223 lunations, nearly equal to 18 years, by which they predicted the return of previously observed eclipses and made use of other empirical cycles or periods.
Thales (640 B. C.), the founder of the Ionic school, laid the foundation of Greek astronomy. The successors of Thales held opinions which, in many re spects, are wonderfully in accordance with modern ideas. Anaximander, it is said, held that the earth moved about its own axis, and that the moon's light was reflected from the sun. To him is also attributed the belief in the plural ity of worlds.
Pythagoras (500 B. C.) promulgated the true theory that the sun is the center of the planetary world, and that the earth revolves round it. But the views of Pythagoras met with little or no support from his successors until the time of Copernicus. Between Pythag oras and the advent of the Alex andrian school, nearly two centuries later, among the most prominent names in astronomical annals is that of Meton. who introduced the Metonic Cycle, con sisting of 125 months of 30 days each, and of 100 of 29 days, making a period of 6,940 days, nearly equal to 19 solar years.
To the Alexandrian school, owing its existence to the Ptolemies, we are in debted for the first systematic observa tions in astronomy.