SECT. XII. ANCIRNT ASTRONOMY. 83 



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. 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 distances de- 

 termines 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 obliquiryis 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 chronol- 

 ogy 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 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 em- 

 braces 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 pre- 

 cession of the equinoxes ; and as the law of that varia- 

 tion is known, their positions at any time may be com- 

 puted. Now Eudoxus, a contemporary of Plato, men- 

 tions a star situate in the pole of the equator, and it ap- 



