Eudoxus, though a Pythagorean in sentiment, was the intimate friend of Plato. A desire to study astrono my carried him into Egypt, Asia, and Italy, where he collected all the observations that had been made on the rising and setting of the stars. Nectanebus, king of Egypt, introduced him to the priests of Heliopolis, who instructed him in his favourite science; and, when he returned to Greece, he published the solar period of four years, which supposes the annual revolution of the sun to be performed in 3651. days. He made the revo lution of the moon 29d 12h 43' 30, and suggested the lunisolar period of 160 years. In order to represent the apparent motions of the planets, Eudoxus supposed that each planet had a part of the heavens to itself, composed of a number of concentric spheres, whose combined motion formed the apparent motion of the pla net. The sun, for example, had three spheres: one of which, moving from east to west in twenty-four hours, produced his diurnal motion; another of his spheres revolved round the pole of the ecliptic in 3651 days, and thus occasioned his annual motion; while the third sphere turned upon an axis perpendicular to a circle in clined to the ecliptic, in order to account for his change of declination. The moon, and all the other planets, had three spheres, to explain their daily periods, and their motion in longitude and latitude ; and a fourth sphere was added to the planets, in order to account for their stations and retrogradations. This hypothesis, absurd as it is, is not devoid of ingenuity, and is the first attempt that was made to explain the planetary motions. As new motions and inequalities were dis covered, the number of spheres was increased, till the heavens were filled with no fewer than fifty-five of these cumbrous appendages. Eudoxus is said to have disco vered the inclination of the moon's orbit, and the retro grade motion of her nodes; and to have estimated the sun's diameter to be nine times greater than that of the moon. In his two works, called the Mirror and the Phe nomena, which are now lost, he described the constella tions, and explained the times of the rising and setting of the stars.
Astronomy did not derive much advantage from the genius of Aristotle. He embraced the opinion of Eudoxus respecting the solid orbits of the planets. He rejected the Pythagorean system, and maintained, in opposition to the philosopher of Samos, that the comets were merely exhalations which were raised into the higher regions. He observed a comet, whose tail occupied nearly a third of the heavens, and he ranked the milky way among these wandering stars. Aristotle observed an occultation of Mars by the moon, which, according to Kepler, happened in the year 357 ; and he had also the good fortune to witness an occultation of one of the stars of Gemini by the planet Jupiter.
The history of astronomy was written about this time by Eudemus and Theophrastus, but both their works have been lost. Helicon of Cizicene predicted an eclipse, which happened at the time he announced. Cleanthes, the Stoic, maintained, with great justice, that the sun described a spiral in his motion to the north and south of the equator. Theophrastus considered the two celes tial hemispheres as badly joined together, and supposed that the milky way was occasioned by the light that pas sed through the opening. Opuntius, the disciple of Plato, composed several works on the distances of the sun and moon, and on the magnitudes and eclipses of these luminaries. About the same time Calippus, per ceiving the errors of the Metonic cycle, proposed the Calippic period of seventy-six years, containing four Metonic, cycles, and 940 lunations ; which, being more exact than the period of Mcton, was adopted in the year S30 A. C., in the seventh year of the sixth Metonic cycle. Besides improving the calendar, Calippus collected many observations on the rising of the stars ; and, for the purposes of agriculture, he added the meteorological predictions which they seemed to authorise.
Pytheas, one of the last of the Greck astronomers, flourished at Marseilles in the time of Alexander, and travelled into Iceland, where, at the time of the summer solstice, he saw the sun touching the northern point of the horizon. By means of a gnomon, he observed, at the summer solstice, at Marseilles, that the length of the shadow was to the height of the gnomon as 120 to 41# ; an observation which makes the obliquity of the eccliptic 23' 50'. Pytheas observed also, that in his time there were no stars near the pole ; and he is said to have discovered the dependence of the tides upon the position of the moon.
However numerous and important are the discoveries which we have already recorded, astronomy has not yet assumed the form of a science. Important but uncon nected observations, sagacious. conjectures unsupported by argument, and determinations approximating to ac curacy, are the only trophies of which the science can yet boast. The form and position of the planetary orbits were completely unknown : The sun, the moon, the planets, and the fixed stars, were all crowded round the earth at distances of which no conception was formed ; and their apparent motion in the heavens was completely misunderstood. No sooner, however, was the Alexan drian school established, than a vigorous impulse was given to the human mind. Astronomy assumed a new form. The stars retreated into the bosom of illimita ble space, and the planetary system was left at an im measurable distance from the sidereal firmament ; the paths of the planets were carefully traced, and their relative distances rudely determined ; the solar and lunar irregularities were better understood ; the posi tions and progressive motion of the fixed stars were as certained ; and, by a series of connected observations, made with angular instruments, and reduced by trigono metrical calculations, a system of astronomy was Created, which paved the way for those sublime views of crea tion, which the science of the heavens was destined to unfold.
When the immense territories which Alexander had acquired were divided among his generals, Egypt fell to the share of Ptolemy Soter, whose learning and munificence attracted crowds of philosophers to Alexan dria. Ptolemy Philadelphus, who inherited the virtues along with the throne of his father, held out new en couragements to learning, and in a short time all the wisdom of Greece was concentrated in the capital of Egypt. A superb and spacious building, composed of galleries and magnificent apartments, was erected for the accommodation of the philosophers. The splendid library of Demetrius Phalereus was added to this noble establishment, and every book and instrument was pro cured, which could contribute to the advancement of literature and science. Forgetting the cares and the vices of a prince, Ptolemy encouraged the philosophers of the Alexandrian school by his presence and conversa tion, and inspired them with that emulation and ambition of excellence, which call into action every exertion of industry, and every spring of genius. The possession of a crown is a poor inheritance, unless when it is sub servient to the advancement of the human species The splendour of a throne, and the bloody atchievements by which it is frequently supported, may captivate vulgar admiration ; hut the philosopher is affected only with the wisdom and virtue of him who fills it. The name of Ptolemy Philadelphus will be associated with the his tory of human knowledge, and will be cherished in the recollection of future ages, when time has effaced from the page of history the triumphs and the very names of tyrants.