372 Ohsertationi on the value of the cipher, ^c. [Och 



of that body, and that of the mean Sun at the monient^f 

 its tra?isit. — The astronomical tables furnish us with the 

 light ascensions of the true Sun at apparent noon, which 

 with the aid of the equation of time can be reduced to 

 that of the mean Sun at mean Noon. Thediflerence of 

 tjtie right ascensions of the mean Sun and the Moon at 

 any time is the angle at the pole which the two objects 

 subtend — on the succeeding noon by a similar process, 

 we determine another angle, and by applying a propor- 

 tional part of the difference of these angles, we find the 

 angle corresponding to the time of the Moon's culmina- 

 tion whielx is-th« mean time of the transit of the 

 Moon. 



^ -Lgt^^ pTepresejit the A. R. of the true Sun at apparent 

 noon. 



tbe-Equation of time 

 S' the A. R. of the mean Sun at mean ndbn "'^"^j^ 

 When thie' true Sun is on the meridian (e) is the hour 

 angle- of the mean Sun, but the interval of time in which 

 the mean Sun performs one revolution round the earth 

 (for such is the natural appearance of the diurnal phe- 

 nomenon) is 24h. 3m. 56.555 therefore the time of 

 the mean Sun describing e will be. 



or c 4- acceleration due to e = 



being the siderial interval between the transits of the 

 true and mean Sun, the right ascension of the mean 

 ' «lm S' =: S + 



Here the mean Sun is considered to be advanced in 

 right ascension, - which makes e" a positive quantity, but 

 all the Ephemerides have the equatipn of time with, the 

 sign minus when the mean Sun has a greater right as- 

 cension than the true, therefore to compute S' from an 

 almanack,. the formula will be S'^r: S — e' 



Although it is 9, slight deviation from my present sub- 

 ject, yet as it occurs in the course of enquiry, it may no t 



