en the Planets Venus and Mars . 2 3 
According to the different dimenfions, given to the orbit of 
the planet in the different tables, the radius vector at a given 
time will alfo be fomewhat different. Thefe differences in the 
tables of Dr. Halley and M. de la Lande are but fmall: 
thus, 17S4, September 20, at o h. 37' if' mean time, the 
obferved and apparent geocentric longitude of Venus 6 s. 9® 
54^, the aberration and nutation + 33", the corre&ed 
and true geocentric longitude 6 s. 9 0 46' 27", the fun’s geo- 
centric longitude 5 s. 28° 5' 5 1" ; hence the elongation T = 
os. ii° 34' 3 ^'- ^ now the logarithm of the radius vector 
SP is taken out of Dr. Halley’s tables =4.858251, then 
f] n§ p — and P=i6° if 52"; but if the logarithm 
SP 
SP is taken out of the tables of M. de la Lande = 4,858 1 68, 
the angle P will be found =i'6° I2 / the difference is 12". 
This uncertainty in the commutation, and confequently in the 
heliocentric longitude, would have been {till greater if the 
calculations had been made only from the tables, or from the 
planet’s geocentric longitude by the tables ; thus this angle P 
is, according to Dr. Halley, = 16 0 io' 52" ; and from the 
tables of M. de la Lande :=:i6° io' 13". 
■t 
Mean 
