VOL. XVII."| 



PHILOSOPHICAL TRANSACTIONS. 



453 



southerly. Lastly, after 243 years, add ih. 23 m. or if the former year have 

 been bissextile, id. 1 h. 23m. and Venus is again found in conjunction with the 

 sun, but in a path lO' 37* more northerly. 



In all the transits at this node, the angle of the visible way of Venus with 

 the ecliptic, is 8° 28'; and her horary motion 4'; and the semidiameter of 

 the sun subtending 15' 51", the greatest duration of the transist of the centre 

 is 7h. 56m. precisely the same as at the other node. As to the epochs ; it is 

 concluded from the ingress of Venus seen in the setting sun, and which was 

 only observed by Horrox, that Venus was in conjunction with the sun at Lon- 

 don 1639, November, 24d. 6h. 37m. but that she passed 8' 30" towards the 

 south. But in the month of May, Venus has not hitherto been seen by any 

 mortal within the sun ; but from my calculations it appears, that Venus for the 

 next time will enter the sun 1761, May, 25d. I7h. 55m. viz. in the middle of 

 the eclipse, and then be distant from his centre towards the south 4' 15". 

 Hence, and from the premised revolutions, all the phaenomena of this kind are 

 easily computed for a thousand years, as appears from the following table : 



As to the duration of these eclipses of Venus, they may, with respect to the 

 centre, be calculated in the same manner as those of Mercury were ; but as the 

 diameter of Venus is pretty large, and the parallaxes may cause a considerable 

 difference as to the time, there must necessarily be a peculiar calculation for 

 each place. The diameter of Venus is so great, that while she adheres to the 

 sun's limb, almost 20 minutes of time are spent, viz. when she directly enters 

 the sun ; but falling obliquely into him, she makes a longer stay on his limb. 

 Her diameter, according to the observation of Horrox, is 1' 1 8", when she is in 

 conjunction with the sun at the ascending node; and l' 12" at the descending 

 node. 



The principal use of these conjunctions is accurately to determine the 

 distance of the sun from the earth, or his parallax, which astronomers have by 



