34 ASTRONOMICAL and 



9' 22''. 917 = the visible diiFerence of latitude between the 

 Sun and Mercury, at the time of the first external cqntact; 

 from -which substract Mercury's parallax in latitude from the 

 Sun = 2". 322; and the remainder, viz. 9' 20''. 505, will be 

 Mercury's true latitude by observation. The latitude by 

 the tables, being 9' 23". 961, the errour is -^ 3". 366. 



For the difference of longitude of the centres of the Sun 



* * — 



and Mercury at the time of the first external contact, and 

 consequently at the time of the ecliptick conjunction : 



Radius : O ^, 972". 48 : : Sine < O 5 C, 35" 22' 10". 3 



o 



C 0, 792". 995, the visible difference of longitude of the 

 Sun's and Mercury's centre; from which substract 3". 3^3, 

 Mercury's parallax in longitude from the Sun, and the re- 

 mainder, viz. 789". 662, will be the true diflerence of Ion 

 tude by observation. 



Mercury's heliocentrick motion in longitude from S'^ 24' 

 4", A. M. to 10^ 39' 31", A. M. viz. in •2'^ 15' 27", was by the 

 tables, 1689". 9; which will give 788". 29 for his geocen- 

 trick motion on the Sun's disc, reduced to the ecliptick. 

 Therefore, 788". 29 : 2^ 15' 27" = 8127" : : 789". 662 : 

 8141" = 2^ 15' 41" ; which added to the time of the first 

 external contact, viz. 8^ 24' 4", will give 10'^ 39' 45", a. M. 

 for the apparent time of the true ecliptick conjunction by 

 observation ; being 14" later than by the tables. 



w 



At 10^ 39' 45", the Sun's longitude was 7" 13° 40' 57". 1 ; 



consequently, it being the echptick geocentrick conjunction, 

 Mercury's geocentrick longitude was the same, which makes 

 his heliocentrick longitude at that time, to have been V 13° 



40'57". 1 



