450 THE POPULAR SCIENCE MONTHLY 



nology after it became out of date we can but surmise. For that the 

 stars and the seasons wliich they ruled did not continue to agree must 

 have been early evident to the astronomer-priests who made a study of 

 the two the basis of their calendar and of all the functions, aratral and 

 religious, appertaining to it. So that the stellar springtime of one year 

 was not the springtime of the next. That the zodiacal constellations 

 were continually moving forward to meet the sun in his yearly round 

 of the sky could hardly, one would have supposed, have escaped the 

 observation of antiquity. Yet we find no mention of the fact; not so 

 much as an ascription of the incongruity to the errors of predecessors. 



To Hipparclras is due the honor of its discovery ; a detection brought 

 about in this wise. Besides watching the heliacal risings of the stars 

 the ancients had another way of determining the date of the vernal 

 equinox : this was by noting by the gnomon of a sun-dial the times w^hen 

 the shadows cast by the sun at noon were longest or shortest. This gave 

 them the dates of the solstices. Hipparchus by comparing his own 

 observations with those that had preceded him — on Spica, chiefly — 

 found that the two methods did not agree but that the equinox as set by 

 the sun stepped forward to meet the stars by about twenty minutes 

 each year. As he perceived that while the longitudes of all the stars 

 were thus changing, their latitudes remained the same, he concluded 

 with the astuteness of genius that it was the equator that was moving, 

 not the ecliptic ; that is, the earth's tilt was shifting not the sun's. 



The merit of Hipparchus in the matter was two-fold : the ability to 

 discover the thing and then the courage to proclaim it. For in the good 

 old times men were no quicker to recognize advance than they are to-day 

 and were just as possessed to denounce it. In consequence Hipparchus's 

 discovery suffered the usual fate of new truths. Some astronomers dis- 

 puted his facts, others explained them away as an oscillation merely, 

 while yet others simply ignored them. In spite of which mundane 

 anathema the slow movement of the equinoxes went obstinately on. 



This mighty revolution of the equinoxial points by which spring 

 opens twenty minutes earlier each year Hipparchus was not able to ex- 

 plain. He noted the fact, which was a feat remarkable enough, con- 

 sidering his means. Indeed, he probably never tried to penetrate further 

 into the mystery. The Greeks were better geometers and more discern- 

 ing reasoners than we were brought up at school to believe, but in 

 astronomical matters a great gulf lies between them and modern 

 thought. They never conceived the principle of universal gravitation. 

 Failing this, it is no wonder they never imagined to what precession 

 could be due. For the realization of this result of gravity is a much 

 more advanced step in celestial mechanics than the accounting for the 

 circuit by the planets of the sun. 



Not wholly easy at first of comprehension, appreciation of the prin- 



