"^ * " C-J ^". 



^^VT-aJ ^ ^f\ 



.v.r ya-" 



ASTRONOMICAL PAPERS. 



99 



" Hence comes the method of inveftlgatmg the Sun's 

 parallax, from obfervations o£ Venus on his diik. At the 

 thi^e of the tranfit, the third day of June, Venus was much 

 nearer to the earth than the Sun was, and, of confequence, 

 was much more affe£led by parallax. This efFe£t was pro- 

 duced in a twofold manner, in refped to us in the nor- 

 thern regions of our earth. Firft, Venus was depredcd 

 upon the Sun, by parallax in longitude, bringing her to a 

 Gonjundion with the Sun fooner to our point of view, than 

 to a fpetSator at the center of the earth. In the feconJ 

 place, Ihe was carried nearer to the center of the Sun, by- 

 parallax in- latitude, thereby lengthening the tranfit-line;, 

 both which effects confpired to accelerate the time of firft 

 interior contact.. Now to an obferver in Great-Britain, > 

 parallax had a ftill greater effed, by what is faid before :- 

 That is, fome minutes pafled after the contact was formed;- 

 to the obferver there, before it was feen by us. Now the 

 difference of longitude, between the two places of obfer-- 

 vation,, being accurately known, the effedt of parallax* 

 between the two places, is likewife known; for the dif- 

 ference of longitude, by thefe obfervations, will be con-- 



fiderably lefs than the true difference, - 



"" The method of calculating the Sun's parallax, from^i 

 thefe obfervations, is by trial; the parallax will be fup-- 

 pofed of that quantity, which the obfervations found it in 



I 76 1 ; hence the total effed: of parallax, at each place of. 



obfcryation, mull: be computed; and if it fl^ould be the 

 fame as given by obfervation, it will prove the aflump- 

 tion to be juft; but if, by obfervation, it fhould be greater 

 or lefs than by calculation, the Sun's parallax will turn 

 out to be greater or lefs in the fame. proportion. — When 

 the Sun's parallax is known, the diftance of the earth,, 

 and of all the planets, from the Sun, will be known like^ 

 wife."' 



OBSERVATIONS 



/^' 



