150 ON THE VISIBILITY OF VEXUS — CAMERON. 



While her elongation is changing, her brilliancy is changing^ 

 also. At greatest elongation she is three times as bright as at 

 superior conjunction. This does not mean that it is only three 

 times as easy to see her in the former position as in the latter — 

 it is infinitely more easy to do so. No eye can see her in the 

 one case, and no eye can fail to see her in the other. What is 

 called brilliancy is a something quite independent of elongation^ 

 and it is lack of elongation and no lack of brilliancy that makes 

 Venus invisible at superior conjunction. If, wdien at superior 

 conjunction, she had the brilliancy which she has at greatest 

 elongation, she would still be invisible to the naked eye ; and if, 

 when at greatest elongation, she had only the brilliancy of 

 superior conjunction, she would still be the brightest gem in the 

 sky. 



The actual brilliancy at any moment depends on several con- 

 ditions, some physical and others geometrical. Of the physical 

 conditions we know too little to be able to make them the 

 subjects of calculation : but from the geometrical conditions we 

 can calculate the relative theoretical brilliancy for any position 

 in her orbit, and this is found to agree quite well, as a general 

 rule, with the actual observed brilliancy. These geometrical 

 conditions are three in number : the distance of the planet from 

 the sun, the distance of the planet from the earth, and the phase 

 of the planet — that is, the illuminated part of its disc. To get a 

 general idea of the changes in Venus's brilliancy, we may, as 

 l)efore, suppose her to be always at her mean distance from the 

 sun, and then the changes will depend only on her distance from 

 the earth and her phase. It will be convenient also to select some 

 standard in terms of which to express her different brilliancies. 

 For this I shall take her greatest brilliancy as it always would 

 be if both she and the earth were always at their mean 

 distances and if the reflective powers of all parts of the surface 

 of Venus were equal and constant, and I shall use the number 

 100 as the value of this mean greatest brilliancy. 



At superior conjunction she presents the same face to the 

 earth as she does to the sun, so that the value of her phase is 1 

 — she is " full," as we say of the moon. But her distance from 



