152 ON THE VISIBILITY OF VENUS — CAMERON. 



the evening about the time that the other conditions are begin- 

 ning or ceasing to be favorable, it is not so much her al)solute 

 ileclination that is important as the ditference between hers and 

 that of the sun. The longer the interval of time between sunset 

 and the setting of Venus, the easier it is to pick her up at these 

 critical seasons ; and the length of this interval depends notonl}' 

 on the elongation, but also on this ditference of declination. 

 When Venus is farther north than the sun the interval is longer 

 than that due to elongation, and when farther south it is shorter. 

 When the elongation is 15° and the declination of both objects is 

 0", the planet will set an hour after sunset ; but if her declina- 

 tion were then 5° north she would remain above the horizon in 

 this latitude a quarter of an hour longer, if 5 south a quarter 

 of an hour shorter. 



All of the above is just as true for the morning star season as 

 for the evening star season if allo"s^■ance be made for the fact that 

 in the former case the season l)egins with inferior conjunction 

 and ends with superior conjunction, instead of vice versa as in the 

 case considered. 



And now to answer the questions which form the subject of 

 my paper, so far as the observations in hand admit of their being- 

 answered. In giving the particulars of elongation, brilliancy, 

 etc., in connection with the observations, the hj'pothesis of mean 

 distances used in the above prefatory matter is no longer retained. 

 The actual distances of hoth Earth and Venus for each date, as 

 p"iven in the Nautical Almanac, are the ones that have been used. 

 In the matter of brillianc}" the same standard is used as above, 

 and each value given is a percentage of the mean greatest 

 brilliancy. All hours mentioned in the paper are standard time 

 of the 60th meridian W. Long. 



I have said that it is always perfectly easy to see Venus with 

 the naked eye when her elongation from the sun is equal to or 

 greater than 20°, and that this happens on the average at an 

 interval of 78 da3^s from superior conjunction and 13 days from 



