120 A SHORT HISTORY OF SCIENCE 



as was Tycho Brahe centuries later by the sudden appearance 

 in 134 B.C. in the supposedly changeless starry firmament of a new 

 star of the first magnitude. He accordingly set himself the heavy 

 task of making a new catalogue, which ultimately included more 

 than 1000 stars, for the part of the sky visible to him, and "re- 

 mained, with slight alterations, the standard for nearly sixteen 

 centuries." His list of constellations is the basis of our own. 



PRECESSION OF THE EQUINOXES. While this great piece of 

 routine work was deliberately planned by Hipparchus, not so much 

 as an end in itself as a necessary basis for future investigators, it 

 nevertheless led to his most remarkable discovery, that of the pre- 

 cession of the equinoxes. In comparing, namely, the positions of 

 certain stars with those observed about 150 years earlier, he de- 

 tected a change of distance from the equinoctial point where the 

 celestial equator and the ecliptic meet amounting in one case 

 to about 2. By an inspiration of genius, he interpreted this 

 correctly as due to a slight progressive shifting of the equinoctial 

 points, corresponding to a slow rotation of the earth's axis, by 

 means of which the celestial pole in many thousand years describes 

 a complete circle. His estimate of 36" per year was considerably 

 below the actual value, which is about 50". 



OTHER ASTRONOMICAL DISCOVERIES. PLANETARY THEORY. 

 Striving always for greater accuracy and completeness of data, he 

 determined the length of the year within about six minutes. In 

 attempting to explain the annual motion of the sun, he was aware 

 that the change of direction is not uniform, and its distance from 

 the earth, as shown by its apparent size, not constant. He de- 

 termined the length of spring as 94 days, that of summer as 92^, 

 and by a somewhat complicated calculation arrived at the value -^ 

 as the eccentricity of the earth's position in the sun's orbit. These 

 determinations were naturally very difficult and imperfect on ac- 

 count of the entire lack of accurate time measurement. Following 

 Apollonius, Hipparchus devised a combination of uniform circular 

 motions which should account for the observed facts within the 

 limits of probable error of observation, and in this undertaking he 

 was successful, the degree of accuracy of his theory corresponding 

 to that of which his instruments were capable. 



