432 Memoir oil the Diminution of the 



given by the observed lengths of the shadow at the two 

 solstices. This coincidence is a remarkable confirmation 

 of the value of the masses of Venus and Mars, which M. 

 Delambre has determined by the comparison of a very great 

 number of observations of the sun by means of the formu- 

 las, and of the perturbations of the earth's motion I have 

 given in the 3d vol. of Mac. Cel. 



Tcheou-Kong, by his observations, had determined the 

 moment of the winter solstice, but they have not been 

 transmitted to us. We only know that he fixed this solstice 

 at 2° Chinese from V, a constellation which begins at s of 

 Aquarius, (vol. xxvi. of Edifying Letters, p. 124.) We 

 shall also fix the epochas of this determination at 1000 years 

 before our sera. Tcheou-Kong and the Chinese astronomers 

 at that tkftft referred the constellation to the equator; be- 

 sides 2° Chinese = 1° 58' 17": subtracting this from 270°, 

 the difference 268° l' 43" was the right ascension of s of 

 Aquarius at the epocha of 1 100 years before our aera. 



In the beginning of 1750, the longitude of s of Aquarius 

 was 308° 14' 10"; its latitude was north 8° 6' 20'. 



Comparing Bradley and Mayer's catalogues with Piazzy's, 

 this star does not appear to have any sensible motion of 

 its own, and its annual precession is 50", 1. 



I find by the formulas of c. xii. of book vi. of Mec. Cel, 

 for the epocha of 1100 years before our sera, 

 rj/ - J 40" 2' 43" 

 V = 23° 32' 49", 

 jt being the precession of the equinoxes from that epocha 

 till 1750. This precession being referred to the equinox of 

 1 750, is the obliquity of ihe equator on this ecliptic at 

 the same epocha. Thus at this epocha the longitude of £ 

 of Aquarius, computed from the intersection of the equator 

 with the. ecliptic of 1750, was in the year 3 100 before our 

 aera 268° J l' 27"; whence I conclude its right ascension, 

 relatively to the same intersection, to be equal to 268° 9' 2". 



I aiterwards find, by the formulas of the quoted chapter, 

 • o" = 3.5' 44"; 9 = — 1° 33' 25" : 

 <p" being the ecliptic's inclination from that time, above 

 that of the ecliptic of 1750, and 9 being the longitude of 

 its node upon that said ecliptic, computed from the fixed 

 equinox of 1750. Whence I conclude that the right as- 

 cension of the true equinox with the preeeding; that is to 

 sav, the equator's intersection with the fixed ecliptic of 1750, 

 was in the year 1 100 before our sera equal to —-42' 12"; 

 the right ascension of s, relative to the true equinox, was 

 therefore then 26S a 51' 14", greater by 49' 31'' than Tcheou- 



Kong's 



