88 ANCIENT ASTRONOMY. SECT. XII. 



tion good, it will verify the accuracy of modern tables, and will 

 show to how many centuries they may be extended without the 

 fear of error. A few examples will show the importance of the 

 subject. 



At the solstices the sun is at his greatest distance from the 

 equator ; consequently his declination at these times is equal to 

 the obliquity of the ecliptic (N. 152), which was formerly deter- 

 mined from the meridian length of the shadow of the stile of a 

 dial on the day of a solstice. The lengths of the meridian shadow 

 at the summer and winter solstices are recorded to have been 

 observed at the city of Layang, in China, 1100 } 7 ears before the 

 Christian era. From these the distances of the sun from the 

 zenith (N. 153) of the city of Layang are known. Half the sum 

 of these zenith distances determines the latitude, and half their 

 difference gives the obliquity of the ecliptic at the period of the 

 observation ; and, as the law of the variation of the obliquity is 

 known, both the time and place of the observations have been 

 verified by computations from modern tables. Thus the Chinese 

 had made some advances in the science of astronomy at that 

 early period. Their whole chronology is founded on the observa- 

 tions of eclipses, which prove the existence of that empire for 

 more than 4700 years. The epoch of the lunar tables of the 

 Indians, supposed by Bailly to be 3000 years before the Chris- 

 tian era, was proved by La Place, from the acceleration of the 

 moon, not to be more ancient than the time of Ptolemy, who 

 lived in the second century after it. The great inequality of 

 Jupiter and Saturn, whose cycle embraces 918 years, is peculiarly 

 fitted for marking the civilization of a people. The Indians had 

 determined the mean motions of these two planets in that part of 

 their periods when the apparent mean motion of Saturn was at 

 the slowest, and that of Jupiter the most rapid. The periods in 

 which that happened were 3102 years before the Christian era, 

 and the year 1491 after it. The returns of comets to their 

 perihelia may possibly mark the present state of astronomy to 

 future ages. 



The places of the fixed stars are affected by the precession of 

 the equinoxes ; and, as the law of that variation is known, their 

 positions at any time may be computed. Now Eudoxus, a con- 

 temporary of Plato, mentions a star situate in the pole of the 

 equator, and it appears from computation that * Draconis was 



