SECT, xn.] ANCIENT ASTKONOMY. 99 



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. 148), which was formerly 

 determined 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 years before the Christian era. From these 

 the distances of the sun from the zenith (N. 149) of the city 

 of Layang are known. Half the sum of these zenith dis- 

 tances 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 veri- 

 fied 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 

 observations 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 Christian 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 preces- 

 sion of the equinoxes ; and, as the law of that variation is 

 known, their positions at any time may be computed. Now 



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