ON THE VISIBILITY OF VENUS — CAMERON. 149 



'Oil the very same work for several years and had made and re- 

 corded a large number of observations. These he was good 

 enough to send me, and in this paper I have made use of tliem 

 as well as of my own and of any others that I have been able to 

 lay hands on. 



As seen from the earth, Venus completes a revolution around 

 the sun in 584 days. During one half of this time she is even- 

 ing star, and during the other half morning star. B}^ way 

 of a few preliminary and explanatory remarks, let us consider 

 her motion and the various changes she undergoes during the 

 292 days of her season as evening star, and for the sake of sim- 

 plicity let us suppose that both she and the earth are at their 

 mean distances from the sun. Both orbits differ but veiy little 

 from circles, and the results got from considering onl}^ the mean 

 distance will lie quite correct enough for the present purpose. 



At the l)eoinnino' of an evenine" star season Venus is in 

 superior conjunction on the further side of the sun from us, and 

 is in the same part of the sky as the sun is. 81ie cannot then 

 be seen by day because she is hidden in the sun's rays, and she 

 cannot be seen in the evening because she sets at sunset. After 

 superior conjunction she moves off to the east of the sun. In 

 39 days she is 10° away, in 78 days 20°, in 120 days 30°, in 

 166 days 40°, and 220 days after superior conjunction she 

 reaches her greatest elongation of 46° 20.' Only 72 daj^s are 

 left for her to get back, less than a third of the time she 

 takes to swing out. Half of the 72 days are used up in work- 

 ing back to 40% 14 days more to 30°, 9 days more to 20°, and 

 in another 13 days she is again in line with the sun, this time 

 on the hither side of him and in inferior conjunction. In so far 

 then as her visibility depends on her elongation, it is apparent 

 that she can be seen at a shorter interval of time from inferior 

 than from superior conjunction. It is alwaj^s perfectly easy to 

 see her when 20° out, and if this were the limit of her visi- 

 bility we would have to wait 78 days after superior conjunction 

 before getting a glimpse of her, but we could see her eveiy 

 vevenino- after tliat until 13 daj's before inferior conjunction. 



