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PTOLEMY (Claudius Ptolemæus) was a native of Egypt. The place of his birth is uncertain. He lived in the neighborhood of Alexandria, and carried on his astronomical work there during the reigns of Hadrian and Antoninus Pius; his house being on the raised terrace of a temple of Serapis at Canobus, where pillars were afterwards erected to commemorate his achievements.
His great work, called Mathematical Syntaxis, best known by the Arabic form of the Greek word for greatest as Al Magest, is a complete treatise on astronomy as known to the ancients. It consists of thirteen books, of which the first two treat of the earth as centre of the universe, of the motion of the stellar spheres by which the day, the year, and the precession of the equinoxes is explained; his table of chords is described and applied; astronomical time is defined, the dependence of climate on latitude is shown. The third book deals with the theory of the sun; the fourth and fifth with that of the moon; the sixth is given to eclipses; the seventh and eighth to the stars; the last five books to the planets.
Ptolemy frequently mentions Hipparchus; but it has been shown in detail by Delambre and others that he has borrowed from him far more than he has acknowledged. But what remains as his own is of much importance. He discovered the second inequality of the moon's motion called by astronomers evection; the first inequality, due to the excentric position of the earth in the lunar orbit, and called, as in the case of the sun, the equation of the centre, had been discovered by Hipparchus. Ptolemy found that the equation of the centre was diminished when the moon was in conjuncture with or opposition to the sun, and was increased in quadrature, i.e. when the angular distance between the moon and sun was 90°. The amount of this inequality depended also on the combination of the places of the lunar apsides with that of the conjunctions. Ptolemy's estimate of this second irregularity was very nearly accurate, and is a discovery of great value.
Ptolemy represented lunar as well as planetary motions by supposing the body considered to move on the circumference of a small circle, the centre of which was carried round the earth; a mode of representing complicated periodic motions first suggested by Apollonius. By varying the size of the subsidiary circle and the direction of motion, all these irregularities became susceptible of geometrical treatment. The hypothesis was strictly legitimate; the more so that none but circular arcs were amenable to calculation. A further discovery of vital import to astronomy is due to Ptolemy--that of the refraction of rays of light when passing from a thinner to a denser medium. It is spoken of not in his astronomical work, but in his Optics, which therefore must have been written later. Arranging at the extremities of one of the diameters of a circle and at the centre three coloured spots, so that when the circle was half immersed in water, they appeared to be in a straight line, he was able to determine with precision the refraction for each angle of the incident ray. Applying this to astronomy he shows that refraction, greatest at the horizon, and diminishing as the star approaches the zenith, disappears when the ray is vertical.
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| This biography is
reprinted from The New Calendar of Great Men. Ed. Frederic
Harrison. London: Macmillan and Co., 1920. |
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