bT6 PHILOSOPHXCA.L TRANSACTIONS. [anNO J76I. 



17^^3, to give the true longitude and latitude from the sun*s centre. The true 

 ecliptic conjunction therefore was at 17''42'"28* apparent time at Greenwich, 

 when the true latitude was 9' 31" A. 



Mr. B. has omitted the computation of the longitude, latitude, and of the 

 visible and true conjunction from the internal contact at Greenwich, and the 

 difference of declination, as given in his last letter; because there must have been 

 some mistake in reading the numbers of the micrometer, or in setting them or 

 the times down ; for they differ too much from all the above, which correspond 

 so well with each other, though made at different places, and with different in- 

 struments, and give the true latitude, at the ecliptic conjunction, about 8'' less, 

 that we cannot safely depend on them. 



If therefore we suppose the visible ecliptic conjunction to have happened at 

 17^ 45m 3s apparent time at Greenwich, being the mean of the five foregoing 

 deductions, where the greatest difference is no more than 2™ 1 3* of time, or 8'''' 

 of visible longitude, with 9' 56'.3 of visible south latitude, from the sun's centre: 

 where the greatest difference is no more than 2".7 in latitude, we cannot much 

 err from the truth: and also from the mean of the same deductions, the true 

 ecliptic conjunction, as seen from the earth's centre, will beat 17^41'" 17% with 

 9' 33". 1 of south latitude. The middle of the transit was therefore at 17*^ 20™ 5% 

 and the nearest approach of the centres 9' 26'''.8. The latitude then was 9' 10". Q 

 south ; but the longitude of Venus being augmented by the aberration of light 

 3 ''.7, equivalent to 56s of time, by which the true ecliptic conjunction was acce 

 lerated, the true equated conjunction was at 17** 42*" 13*. The error in latitude, 

 caused by the aberration of light, was \"a^ by which it was diminished; the 

 equated latitude therefore was 9' 34".5. 



The equation of time was then 1' 51", to be subtracted from the apparent 

 time, to give the mean; consequently, the true equated ecliptic conjunction, as 

 seen from the earth's centre, was at 17** 40™ 21* mean time at Greenwich. The 

 true place of the sun, corrected by observation, was at that time 2* 15° 36' 12''; 

 and consequently the heliocentric place of Venus was 8' 15° 36' 12' with the 

 geocentric latitude 9' 34".5. Now in this case the geocentric latitude is to the 

 heliocentric latitude, as the distance of Venus from the sun is to the distance of 

 Venus from the earth ; and therefore the planet's latitude, as seen from the sun, 

 was 3' 48".5. If we suppose the inclination of the orbit of Venus to be 3° 23' 

 20", as determined by Dr. Halley and M. Cassini, the distance of Venus from 

 the node will be l'' 4' 20'; consequently its true place 2* 14° 31' 52'' on the day 

 of the transit. The effect of refraction is not taken into these calculations, be- 

 cause, at the first observations, when its effects would have been greatest, it 

 amounted only to a very small part of a second. 



