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The Chaldeans.—The Chaldean shepherds, watching their flocks by night under the open sky, could not fail to become familiar with many of the movements of the heavenly bodies. When Alexander took Babylon, in the year 331 B. C, he found in that city a record of their observations reaching oack about nineteen centuries, or nearly to the confusion of tongues at the Tower of Babel. The Chaldeans divided the day into twelve hours, invented the sun-dial, and also discovered the "Saros" or "Chaldean Period," which is the length of time in which the eclipses of the sun and moon repeat themselves in the same order.
The Grecians.—In the seventh century B. c., Thales, noted for his electrical discoveries, acquired much renown, and established the first school of Astronomy in Greece. He taught that the earth is round, and that the moon receives her light from the sun. He introduced the division of the earth's surface into zones, and the theory of the obliquity df the ecliptic. He also predicted an eclipse of the sun which is memorable in ancient history as having terminated a war between the Medes and Lydians. These nations were engaged in a fierce battle, but the awe produced by the darkening of the sun was so great, that both sides threw down their arms and made peace. Thales had two pupils, Anaximander and Anaxagoras. The first of these taught that the stars are suns, and that the planets are inhabited. He erected the first sun-dial, at Sparta. The second maintained that there is but one God, that the sun is solid, and as large as the country of Greece, and attempted to explain eclipses and other celestial phenomena by natural causes. For his audacity and impiety, as his countryman considered it, he and his family were doomed to perpetual banishment.
Pythagoras founded the second celebrated astronomical school, at Crotona, at which were educated hundreds of enthusiastic pupils. He knew the causes of eclipses, and calculated them by means of the Saros. He was most emphatically a dreamer. He conceived a system of the universe, in many respects correct; yet he advanced no proof, and made few converts to his views, and they were soon wellnigh forgotten. He held that the sun is the centre of the solar system, and that the planets revolve about it in circular orbits; that the earth revolves daily on its axis, and yearly around the sun; that Venus is both morning and evening star; that the planets are inhabited—and he even attempted to calculate the size of some of the animals in the moon; that the planed are placed at intervals corresponding to the scale in mus-ic, and that they move in harmony, making the "music of the spheres," but that this celestial concert is heard only by the gods—the ears of man being too gross for such divine melody.
Eudoxus, who lived in the fourth century B. O., invented the theory of the Crystalline Spheres. He held that the heavenly bodies are set, like gems, in hollow, transparent, crystal globes, which are so pure that they do not obstruct our view, while they all revolve around the earth. The planets are placed in one globe, but have a power of moving themselves, under the guidance—as Aristotle taught —of a tutelary genius, who resides in each, and rules over it as the mind rules over the body.
Hipparchus, who nourished in the second century B. c., has been called the "Newton of Antiquity." He was the most celebrated of the Greek astronomers. He calculated the length of the year to within six minutes, discovered the precession of the equinoxes, and made the first catalogue of the stars— 1081 in number.
The Egyptians.—Egypt, as well as Chaldea, was noted for its knowledge of the sciences long before they were cultivated in Greece. It was the practice of the Greek philosophers, before aspiring to the rank of teacher, to travel for years through these countries, and gather wisdom at its fountain-head. Pythagoras spent thirty years in this manner. Two hundred years after Pythagoras, the celebrated school of Alexandria was established. Here were concentrated in vast libraries and princely halls nearly all the wisdom and learning of the world. Here flourished all the sciences and arts, under the patronage of munificent kings. At this school Ptolemy, a Grecian, wrote his great work, the "Almagest," which for fourteen centuries was the textbook of astronomers. In this work was given what is known as the "Ptolemaic System." It was founded largely upon the materials gathered by previous astronomers, such as Hipparchus, whom we have already mentioned, and Eratosthenes, who computed the size of the earth by the means even now# considered the best—the measurement of an arc of the meridian.
Ptolemaic Theory.—The movements of the planets were to the ancients extremely complex. Venus, for instance, was sometimes seen as "evening star" in the west, and then again as "morning star" in the east. Sometimes she seemed to be moving in the same direction as the sun, then going apparently behind the sun, appeared to pass on again in a course directly opposite. At one time she would recede from the sun more and more slowly and coyly, until she would appear to be entirely stationary; then she would retrace her steps, ^nd seem to meet the sun. All these facts were attempted to be accounted for by an incongruous system of "cycles and epicycles," as it is called. The advocates of this theory assumed that every planet revolves in a circle, and that the earth is the fixed centre around which the sun and the heavenly bodies move. They then conceived that a bar, or something equivalent, is connected at one end with the earth; that at some part of this bar the sun is attached; while between that and the earth,Venns is fastened—not to the bar directly, but to a sort of crank; and farther on, Mercury is hitched on in the same way. In the cut, let A be the earth, S the sun, ABDF the bar (real or imaginary), BC the short bar or crank to which Venus is tied, DE another bar for Mercury, F G another bar, with still another short crank, at the end of which, H, Mars is attached.
Thus they had a complete system. They did not exactly understand the nature of these bars— whether they were real or only imaginary—but they did comprehend their action, as they thought; and so they supposed the bar revolved, carrying the sun and planets along in a large circle about the earth; while all the short cranks kept flying around, thus sweeping each planet through a smaller circle. • By this theory, we can see that the planets would sometimes go in front of the sun and sometimes behind; and their places were so accurately predicted, that the error could not be detected by the rude instruments then in use. As soon as a new motion of one of the heavenly bodies was discovered, a new crank, and of course a new circle, was