VOL. LV.] PHILOSOPHICAL TKANSACTIONS. 265 



XXX I r. On the Transit of Fenus in 1769. By Tho. Hornsby, Professor of 

 Astronomy at Oxford, and F.R.S. p. 326. 



The observations of the late transit of Venus, though made with all possible 

 care and accuracy, have not enabled us to determine with certainty the real quan- 

 tity of the sun's parallax ; since, by a comparison of the observations made in 

 several parts of the globe, the sun's parallax is not less than 84-", nor does it seem 

 to exceed 10". From the labours of those gentlemen who have attempted to 

 deduce this quantity from the theory of gravity, it should seem that the earth 

 perfomis its annual revolution round the sun at a greater distance than is gene- 

 rally imagined; since Mr. Professor Stewart has determined the sun's parallax to 

 be only 6*. 9, and Mr. Mayer, the late celebrated professor at Gottingen, who 

 has brought the lunar tables to a degree of perfection almost unexpected, is of 

 opinion that it cannot exceed 8".* In this uncertainty, the astronomers of the 

 present age are peculiarly fortunate in being able so soon to have recourse to 

 another transit of Venus in 1769, when, on account of that planet's north lati- 

 tude, a difference in the total duration may conveniently be observed, greater 

 than could possibly be obtained, or was even expected by Dr. Halley, from the 

 last transit. 



The experience which we gained in the year 1761, the knowledge of the errors, 

 from whatever cause they may arise, which must unavoidably be committed in 

 observations of this kind, will enable us to put in practice every method of solv- 

 ing this problem, and to determine with what degree of accuracy, and within 

 what limits, the true quantity of the sun's parallax may be obtained, and conse- 

 quently the dimensions of the whole solar system. 



But, before giving a computation of the effect of parallax at the several places 

 where this transit ought to be observed, it will be necessary to premise the prin- 

 ciples on which the general calculus was formed. Having found, by computing 

 the observations made on the transit in 1761, that when the Abbe de la Caille's 

 solar tables are used, the epoch of the mean motion of Venus for 1 761, as given 

 in Dr. Halley's tables, requires a correction of 4' 52-i-"; and that the place of the 

 ascending node was, at the beginning of the same year, in 2' 14° 31' 10': hav- 

 ing collected also, by computing the observations of Mr. Horrox in 1639, with 

 the assistance of Dr. Halley's tables of Venus, and the solar tables above-men- 

 tioned, that the motion of the planet's mean longitude is 6' 19" 12' 22", and of 

 the node 52' 18', in 100 Julian years. Mr. H. has supposed the mean longitude 

 of Venus, in the beginning of the year 1769, to be 5° 23' 48", and the place of 

 the node to be in 2' 14" 35' 21" —; and has assumed the rest of the planet's ele- 



* Mr. Machin, profesDor of astronomy at Gresham college, deduced the same quantity many 

 years ago. See a tract entitled, The Laws of the Moon's Motion according to Gravity, p. 24. 

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