What he chieny deserves to be commemorated for in the history of astronomy, is his philosophical doctrine regarding the motion of the earth. He taught publicly that the earth is placed at the centre of the universe; but among his chosen disciples he propagated the doctrine that the sun occupies the centre of the planetary world, and that the earth is a planet revolving round the sun. This system, which, still retains his name, being called the old or Pythagorean system of the universe, is that which was revived by Copernicus. It is, however, only just to the latter to observe, that there is a vast difference between the bare statement of the possibility of a fact, and the demonstration of its existence by irrefragable arguments. Pythagoras having remarked the relation which subsists between the tone of a musical chord and the rapidity of its vibration, was led by analogy to extend the same relation to the planets, and to suppose that they emit sounds pro portional to their respective distances, and form a celestial concert too melodious to affect the gross organs of mankind. Another fancy into which he was led by his passion for analogies, was the application of the five geometrical solids to the elements of the world. The cube symbolically represented the earth; the pyramid, fire; the octahedron, air; the icosahedron, or twenty-sided figure, water; and the dodecahedron, or figure with twelve faces, the exterior sphere of the universe. Pythagoras left no writings ; and it is doubtful whether he really entertained many of the opinions and reveries which have usually been ascribed to him. Philolaus of Crotona, a disciple of Pythagoras, embraced the doctrine of his master with regard to the revolution of the earth about the sun. He supposed the sun to be a disk of glass which reflects the light of the universe. He made the lunar month consist of 29 J days, the lunar year of 354 days, and the solar year of 365 J days. Nicetas of Syracuse seems to have been the first who openly taught the Pythagorean system of the universe. Cicero, on the authority of Theophrastus, the ancient historian of astro nomy, gives him the credit of maintaining that the apparent motion of the stars arises from the diurnal motion of the earth about its axis;[1] but this rational doctrine seems to have been first broached by Heraclides of Pontus, and
Ecphantus, a disciple of Pythagoras.
The introduction of the Metonic cycle forms an era in the history of the early astronomy of Greece. The Chal deans, as we have already stated, established several lunisolar periods ; and the difficulty of reconciling the motions of the sun and moon, or of assigning a period at the end of which these two luminaries again occupy the same positions relatively to the stars, had long embarrassed those who had the care of regulating the festivals. Meton and Euctemon had the merit of first obviating this difficulty, at least for a time ; for the motions of the sun and moon being incommensurable, no period can be assigned which will bring them back to precisely the same situations. These two astronomers formed a cycle of nineteen lunar years, twelve of which contained each 12 lunations, and the seven others each 13, which they intercalated among the former. It had long been known that the synodic month consisted of 29i days nearly; and in order to avoid the fraction, it had been usual to make the twelve synodic months, which compose the solar year, to consist of 29 and 30 days alternately ; the former being called deficient and the latter full months. Meton made his period consist of 125 full and 110 deficient months, which gives 6940 days for the 235 lunations, and is nearly equal to 19 solar years. This cycle commenced on the 16th of July in the year 433 B.C. It was received with acclamation by the people as sembled at the Olympic games, and adopted in all the cities and colonies of Greece. It was also engraved in golden letters on tables of brass, whence it received the appellation of the golden number, and has been the basis of the calen dars of all the nations of modern Europe. It is still in ecclesiastical use, with such modifications as time has rendered necessary.
Eudoxus of Cnidus, abont the year 370 B.C., obtained Euduxus. great reputation as an astronomer. According to Pliny, ho introduced the year of 365^ days into Greece. Archimedes says that he supposed the diameter of the sun to be nine times greater than that of the moon, which shows that he had in some degree overcome the illusions of sense. The titles of three of his works have been preserved, the Period or Circumference of the Earth, the Phenomena, and the Mirror. His observatory was still standing at Cnidus in the time of Strabo. He is noteworthy for his contempt of the Chaldean predictions, and for having contributed to separate true astronomy from the reveries of judicial astrology. Eudoxus seems to have been the first who attempted to give a mehanical explanation of the apparent motions of the planets. He supposed that each planet occu pies a particular part of the heavens, and that the path which it describes is determined by the combined motion of several spheres performed in different directions. The sun and moon had each three spheres : one revolving round an axis which passes through the poles of the earth, and which occasions the diurnal motion; a second revolving round the poles of the ecliptic, in a contrary direction, and causing the annual and monthly revolutions ; the third revolving in a direction perpendicular to the first, and causing the changes of declination. Each of the planets had a fourth sphere to explain the stations and retrogradations. As new inequalities and motions were discovered, new spheres wero added, till the machinery became so complicated as to be altogether unintelligible.
Although Plato can hardly be cited as an astronomer, yet the progress of the science was accelerated by means of the lights struck out by his penetrating genius. He seems to have had just notions of the causes of eclipses; and he imagined that the celestial bodies originally moved in straight lines, but that gravity altered their directions, and compelled them to move in curves. He proposed to astronomers the problem of representing the courses of the stars and planets by circular and regular motions. Geometry was assiduoiisly cultivated in the school of Plato ; and on this account he claims a distinguished place among the promoters of true astronomy.
Astronomy is also under some obligations to Aristotle. In a treatise which he composed on this science, he recorded a number of observations which he had made ; and, among others, mentions an occupation of Mars by the moon, and another of a star in the constellation Gemini by the planet Jupiter. As such phenomena are of rare occurrence, their observation proves that he had paid considerable attention to the planetary motions.
time whose labours and observations prepared the way for the reformation of the science which was shortly after effected by Hipparchus. Helicon of Cyzicus is renowned for the prediction of an eclipse, which took place, as Plutarch affirms, at the time announced. History records the names of only three individuals in ancient Greece who predicted eclipses, Thales, Helicon, and Eudemus. Eudemus composed a history of astronomy, a fragment
of which, consisting of only a few lines, is preserved by- ↑ "Nicetas Syracusius, ut ait Theophrastus, ccelum, solem, lunam, stellas, supera denique omnia, stare censet; neque, prater terrain, rein ullam in mundo moveri ; quse cum circum axem se summa celeritate convertat et torqueat, eadem effici omnia, quasi, stante terra, ccelum moveretur." (Cicero, Acad. Quccst., lib. iv. cap. 39.) Copernicus himself could not have stated the doctrine with greater precision.
