454 PHILOSOPHICAL TRANSACTIONS. [aNNO IGQI. 



Of the visible Conjunction of Venus with the Sun. — This sight, which is by 

 far the noblest that astronomy affords, like the secular games, is denied to 

 mortals for a whole century, by the strict laws of motion. It will be afterwards 

 shown, that by this observation alone, the distance of the sun, from the earth, 

 might be determined with the greatest certainty, which, on account of the 

 parallax otherwise quite insensible, has not hitherto been precisely defined. As 

 to the periods, they cannot be so accurately determined as those of Mercury, 

 seeing Venus has only once since the creation been observed within thft sun's 

 disk, and that by our countryman Horrox : after having corrected the motions, 

 as much as the ruder observations of the ancients would permit, the sum of the 

 calculation is as follows : The 



Longitude of Venus's ascending node from the first star of Aries 1' 15° \6' O" 

 Therefore the sun is in conjunction with her in the opposite 



point, that is, in these centuries, about the end of November 7 15 l6 O 



Distance of Venus from the sun 7^997 parts. 



Ditto, from the earth 26438. 



Inclination of Venus's orbit to the ecliptic O 3 23 O 



Venus's motion in 8 sidereal years, over 13 revolutions O 1 30 28^ 



Ditto, in 235 sidereal years, over 381 revolutions 1 1 29 17 39 



Ditto, in 243 sidereal years, over 395 revolutions O 48 8 



From these elements, making a calculus according to the method explained 

 in Mercury, the intervals of the times and distances arise as follows : after 8 

 years, Venus revolves to the sun ; viz. after subtracting from the moment of 

 the former transit 2d. lOh. 524-m. and the planet proceeds in a path 24' 41'" 

 more southerly than the preceding: after 235 years, having added 2d. lOh. 9m. 

 Venus may again enter the sun, but 1 1' 33* more northerly ; but if the preced- 

 ing year be bissextile, 3d. lOh. 9m. are to be added: after 243 years, Venus 

 may likewise pass over the sun, subtracting only 43 m. from the time of 

 the former ; but she passes 13' 8" more southerly ; but if the preceding year be 

 bissextile, add 23 h. 17 m. And in all these appulses of Venus to the sun in 

 the month of November, the angle of the visible way of Venus with the 

 ecliptic is 9? 5', and her horary motion in the sun 4' 7" ; and since the semidia- 

 meter of the sun is 16' 21", the greatest duration of the transit of the centre 

 of Venus is 7h. 56m. 



Again, let the sun and Venus be in conjunction at the descending node in 

 the month of May ; then according to the same numbers, the same intervals 

 are comjmted: after 8 years, 2d. 6h. 55m. are to be subtracted, and Venus 

 will pass 19' 58" more northerly : after 235 years, add 2d. 8h. 18m. or, if the 

 former year have been bissextile, 3d. 8h. 18m. and Venus will pass 9' 21" more 



