THE ORBIT OF URANUS. 



119 



Camhrithje. 



An extended series of planetary observations was commenced here by Professor 

 Airy, in 1827. The series was continued by him and Professor Challis, liis suc- 

 cessor, until 184:2. During the first three or four years the combined right ascen- 

 sions depend on a few special stars, and mainly on a' Capricorni. Taking the 

 mean correction to the adopted right ascensions of the stars actually compared as 

 they are given in the introduction to cacli annual volume, giving to each star a 

 Aveight proportional to the number of comparisons, the following corrections are 

 deduced : 



1828 — OMO 



1829-31 —0.16 



1832-37 —0.19. 



- In the introduction to the volume for 1838 it is stated that the adopted right 

 ascensions are diminished by the average amount of 0\083, wliich would still 

 leave a correction of — ()M07. Actual comparisons in two subsequent years give 



1840, Aa = — 0^087 

 1842, — .069. 



Although the positions deduced from each year's work were adopted for clock 

 correction the year following, wi'.hout any change of equinox, it seems that there 

 was, effectively, a progressive change of about 0\01 annually in the equinox as 

 adopted. 



No declinations were observed until 1830. On comparing the declinations 

 deduced from several years' work with Auwers, it was evident that the correction 

 increased with the polar distance of the star. The law of increase could be well 

 enough represented by supposing the correction proportional to N. P. D. Thus the 

 following corrections were deduced in three different years. 



1".78 X N. P. D. in degrees 



1834, ,5 dec. = — 

 1840, 



100 

 r'.OO X N.P.D. in degrees 



1842, 



100 

 1".03 X N. P. D. in degrees 



100 



From which the correction for other years was deduced by interpolation. But, 

 on applying these corrections, the results were found systematically different from 

 those of other observatories, and on referring to xVuwers' corrections to Airy's 

 Cambridge Catalogue, it appeared that the mural circle required a large correction 

 near the declination of Uranus during this period. The above results were there- 

 fore altered so as to conform as nearly as practicable to Auwers' law. 



EiUxhvvgli. 



In reducing the observations of 1836 Henderson uses the right ascensions of 

 the Tabute Regiomontanae, to which the general correction is at this epoch -{-'.007. 



