ASTRONOMY IN SHAKESPEARE'S TIME— ABBOT HQ 



Hipparchiis it was who realized the great value to posterity of a 

 star catalog, and he made one containing 1,080 stars, which, with 

 Ptolemy's additions, remained standard for 1,600 years. 



It was the making of this catalog of stars which led Hipparchus 

 to note that certain stars observed by Timocharis and Aristyllus a 

 century and a half earlier were in his time 2° farther east, measured 

 from the equinox, than they had been 150 years previously. Thus 

 he discovered the precession of the equinoxes, and set for it a value 

 of not less than 36 seconds per annum. Modern observations raise this 

 to about 50 seconds. The causes were found after Sir Isaac Newton 

 laid down the law of gravitation. The earth is a great spinning top. 

 As the attraction of the sun and moon on the earth's equatorial 

 bulge tends to bring the equator into the plane of the ecliptic, they 

 merely set the pole to revolving, just as you do when you try to tip 

 over a spinning top. The attractions of the planets slightly increase 

 the effect. 



Hipparchus had worked out a fairly satisfactory theory of the 

 sun's apparent motion, assuming tlie earth stationary. It involved 

 circular orbits only. He was not so successful with the moon's 

 motion, and finding the motions of the planets too obscure, he set 

 himself deliberately to making regular observations of their positions 

 which could be used by posterity, if not by himself, to ascertain their 

 true behavior. This altruistic attitude well deserves high praise 

 for Hipparchus. 



Ptolemy, living over three centuries later, expressed the highest 

 admiration for Hipparchus and employed that master's observations 

 and discoveries with those of other philosophers to compose the great 

 system known as the Ptolemaic, which is described by him in the 

 Almagest. Ptolemy's greatest original contribution is in his system 

 for the moon's and the planetary motions. Eetaining the hypothesis 

 of circular orbits and assuming a stationary central earth, he required 

 a very great complexity of cycles, epicycles, eccentrics, deferents, 

 and equants. He was content with a mathematical system or fiction 

 by means of which the positions of the planets among the stars 

 could be predicted. It is not to be supposed that he regarded this 

 complexity as a real mechanism. In fact, until the rise of Kepler 

 and Newton, no understanding of the real operation of the solar 

 system was possible. At the cost of immense labor, and great shrewd- 

 ness of mathematical analysis, Ptolemy obtained a very fair repre- 

 sentation of the lunar and planetary motions, as accurate perhaps 

 as the observations available to him. 



Throughout the Middle Ages, Ptolemy was regarded as final au- 

 thority in astronomical matters, and little was added to the astro- 

 nomical edifice built up by the Greek philosophers as finished by 



