Babylonian astronomy. Grecian philosophers. Pythagoras. accounts of the appearances of remarkable comets, as well as data concerning eclipses. We must look to the plains of Babylonia for the most valuable early observations. The mild climate and open sky of Central Asia favored the development of the science of the stars. We are not surprised, then, to find that the Chaldeans were acute and patient observers through many generations, and accumulated a very respectable store of observational knowledge. Their greatest achievement lay in the line of observations of eclipses of the sun and moon. By careful study of the times at which eclipses had happened, they discovered that those phenomena repeated themselves in cycles of about eighteen years. Thus they were enabled to foretell eclipses with considerable accuracy. But of the real causes of those interesting phenomena they were ignorant. To the ancient Greeks modern astronomy owes a great debt. So sublime and mysterious are the heavenly bodies, and so intricate their motions, that the speculative minds of the early Grecian philosophers were irresistibly attracted to a study of them. Though many of their theories were groundless, and many of their statements obscure and mingled with metaphysics in a most curious fashion, yet gems of truth are to be found here and there, which well repay the labor spent in searching them out. Though Plato suggested that the world was a cube, which seemed to him the most perfect of solids, Eudoxus, Archimedes, and Aristotle made it a sphere. Nicetas is said to have ascribed the apparent daily revolution of the celestial sphere to the revolution of the earth upon its axis. To Pythagoras is attributed the beautiful but utterly erroneous doctrine of the crystalline spheres. In the outermost of these were set the fixed stars, which had, long before his time, been grouped in constellations, and associated with mythological characters. Each planet too had its sphere. To him also is ascribed the theory that the sun is the center about which the earth and the other planets move; this would nowadays be called a "class-room theory," because it was not promulgated except in a private way among his students. Philolaus, a follower of Pythagoras and a contemporary of Socrates, taught the doctrine openly. He placed the earth universe, and did not The celestial bodies But the overwhelming influence of Aristotle soon Aristotle. erased it from the Greek mind. immovable in the center of the allow it to rotate upon its axis. were permitted to revolve around the earth in decorous fashion. So powerful was the influence of this intellectual giant upon the minds of thinking men for centuries afterward, that the earth was not finally and forever displaced from the erroneous position which he assigned to it till the days of Copernicus. To the second century before Christ belongs Hippar- Hipparchus. chus, justly called the father of astronomy, who rescued Greek astronomy in large measure from the bog of speculation into which earlier philosophers had plunged it, and made it a science of observation as well as of theory. He was a genius of the highest order, being at once an accurate observer of the celestial bodies, a profound mathematician, and a brilliant theorist. He devised the system of locating places on the earth by means of their latitude and longitude. In order to facilitate his computations he invented that branch of mathematics now called trigonometry. The first catalogue of the fixed stars is due to his labors. The apparent motions of the Ptolemy. The shape of the earth. The earth's place. sun and moon he explained by an ingenious theory, which he tested by observation and computation. In determining the length of the year he made an error of only four minutes. After Hipparchus the most distinguished astronomer of antiquity was Ptolemy, who lived at Alexandria in the second century of our era, and wrote the "Almagest,” which has come down to us entire, and in which is preserved nearly all our knowledge of Greek astronomy. As the Ptolemaic system was the orthodox astronomy of the next fourteen centuries, we notice a few of its chief principles. The earth, said Ptolemy, must be round. For if one go southward new stars appear above the southern horizon, and stars in the north seem nearer the horizon than before. Besides this, the heavenly bodies do not rise at the same moment for two observers, one of whom is east of the other. Furthermore, when a sailor approaches the coast, the bases of the headlands are at first hidden from view by reason of the curvature of the sea. The earth must also be in the center of the celestial sphere, for if it were nearer to the eastern portion of the heavens than to the western the stars in the east would seem to move with greater rapidity than those in the west. Since the stars sweep across the sky each day at a perfectly regular rate, the earth must be equally distant from all of them, and thus in the center of the uni verse. What is the shape of the curve in which every heavenly body moves? Ptolemy replies that it is a circle, the most perfect of all curves. Now an objector might say that this would do for the fixed stars, the sun, and the moon, which move with exceeding regularity, but how could it explain the apparent motion of Saturn, Here or of Jupiter, both of which move irregularly? epicycles. Ptolemy had recourse to the device of the epicycle, in- Cycles and troduced by Hipparchus. The word epicycle is derived from two Greek The words, meaning FIG. 2.-CYCLE AND EPICYCLE. Ptolemy found, by comparing his observations with those of Hipparchus, that he could not explain the motions of the sun, moon, and planets with sufficient accuracy by so simple a device. But by adding additional epicycles, and by placing the earth at a short distance from the center of the large circle in the diagram, he could explain the irregularities which perplexed him. Arabian After Ptolemy's death the study of astronomy gradually declined, and suffered a decided set-back in the astronomy. burning of the great library at Alexandria, in the middle of the seventh century. To the Arabians, who now made Bagdad the literary center of the civilized world, we must look for the next advances. They were assiduous observers, and thus furnished a groundwork of fact of Western Europe. upon which later generations might build theories, and by which those theories might be tested. At last the intellectual aspirations of the peoples of The awakening Western Europe were awakened, after a slumber of centuries. The lamp of learning, which was burning in the Moorish universities of Spain, shed its beneficent rays among more northern nations. The Arabic version of Ptolemy's "Almagest" was translated into the Latin language in the thirteenth century, under the patronage of Frederick II., emperor of the Holy Roman Empire. In the same century Alphonso X., king of Leon and Castile, who was surnamed The Wise" and also "The Astronomer," published the celebrated Alphonsine tables, which were prepared with immense labor by the best mathematicians of the Moorish universities. Observations were at this time so much more accurate and numerous than in the days of Ptolemy that many epicycles had to be added to the original system, in order to make theory correspond with observation. The entire heavens were said to be Alphonso's remark. Copernicus. "Scribbled o'er With cycle upon epicycle, orb on orb." So complicated had the celestial machinery become that Alphonso is said to have told a notable gathering of bishops that if the Almighty had done him the honor to consult him concerning the mechanism of the universe, he could have offered some good advice. This irreverent remark may have been inspired by the depleted condition of the royal purse after the publication of the tables. Three centuries had yet to roll away before deliverance from the thraldom of Ptolemy came. On February 12, 1473, Nicholas Copernicus was born at Thorn in |