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DESCRIPTIVE ASTRONOMY.

HISTOET.

Abtronomy is the most ancient of all sciences. The study of the stars is doubtless as old as man himself, and hence many of its discoveries date back of authentic records, amid the dim mysteries of tradition. In tracing its history, we shall speak only of those prominent facts which will best enable us to understand its progress and glorious achievements.

The Chinese.—This people boast much of their astronomical discoveries. Indeed their emperor claims a celestial ancestry, and styles himself "Son of the Sun." They possess an account of a conjunction of four planets and the moon, which must have occurred a century before the Flood. They have also the first record of an eclipse of the sun, which took place about two hundred and twenty years* after the Deluge. It is reported that one of their kings, two thousand years before Christ, put to death the principal officers of state because they had failed to calculate an approaching eclipse.

* October 13, 2127 B. C.

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, two centuries before Christ, he found in that city a record.of their observations reaching back 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 Grectans.—In the seventh century B. 0., TkcHes, 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 of 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 planets are placed at intervals corresponding to the scale in music, 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. c, 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 flourished 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, and 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,Venus is fastened—not to the bar directly, but to a sort of

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