Astronomy

the science of the heavenly bodies. Recent writers divide the subject into two branches. The old or mathematical astronomy treats of the number, size, position, and paths of the heavenly bodies. The new or physical astronomy seeks to find out what they are made of, and in what condition they are; whether hot or cold, solid or gaseous, and whether the elements of which they are made are the same as those which make up the earth.

One important astronomical work going on at the present time is that of photographing the heavens. Various astronomical observatories are accumulating thousands and thousands of photographs to serve as the material for future study.

The Greeks, particularly Thales and Pythagoras, 500 B. C., had very correct notions about the sun as a center of the planetary world, and the earth's revolution about it. They credited the Chaldeans and Babylonians with no little astronomical knowledge. Curiously enough, the earliest treatise on astronomy is a sort of text inscribed on seventy tablets of clay. These tablets were found in a library in the ruins of Babylon, and are now preserved in the British Museum.

The various learned men connected with the Library of Alexandria appear to have rejected the Pythagorean system in favor of that put forth by Ptolemy, 150 B. C. According to the Ptolemaic system the earth is the center of the universe, around which the entire heavens, containing the sun, the moon, and the stars, revolve every twenty-four hours. This system was recognized by the Christian church and held its position for fourteen centuries. To question it was supposed to be derogatory to the authority of the Scriptures and of the church. Some account of the overthrow of the Ptolemaic ideas may be found in the articles under COPERNICUS, GALILEO, NEWTON, TYCHO BRAHE, and PTOLEMY. See also SUN, STARS, METEORS, ASTEROIDS, MOON, and articles on the various planets.