ON THE VISIBILITY OF VENUS — CAMERON. 151 



the earth is tlien so iimcli greater than when she is brightest that 

 her brilliancy is only 24. As she moves out from superior con- 

 junction her distance decreases, and so does her phase ; but the in- 

 crease of brilliancy due to the decrease of distance is greater than 

 the decrease of brilliancy due to the lessening phase, and so she 

 grows gradually brighter. When she reaches greatest elongation, 

 her distance is only /^ of what it was at superior conjunction ; 

 and as brilliancy varies inversely as the square of the distance, 

 it would now be six times what it was at superior conjunction 

 if the phase remained full. But at greatest elongation the phase 

 is only i — Venus looks now like a half moon in the telescope — - 

 and so the brilliancy is only three times as great as at superior 

 conjunction ; more precisely, the value in terms of our standard 

 is now 73. 



Not 100 yet, for Venus is not brightest when she is farthest 

 from the sun in the sky. For live weeks after she begins her 

 inward swinp- her brisfhtness continues to increase and reaches 

 its maximum value of 100 when she gets back to elongation 40.° 

 This happens 25G da3^s after superior conjunction and only 36 

 days before inferior conjunction, and when the phase is just about 

 |. The decrease of brilliancy due to the lessening phase is 

 henceforth greater than the increase due to the shortening dis- 

 tance, and the brilliancy goes down, and at a much swifter 

 rate than it went up. In l(j days it goes down to where it was 

 at greatest elongation; in 12 days more it is down to where it 

 was at superior conjunction. Thus in the 27 days after greatest 

 brilliancy Venus loses all the increase she gained in the 256 days 

 before. Nine days later she is at inferior conjunction, and phase 

 and brilliancy are each 0. This last statement is strictly true 

 only when she makes a transit across the sun's face ; at all other 

 inferior conjunctions she appears in the telescope as a very thin 

 crescent, — a mere thread of liaht — a little north or south of the 

 sun. 



Besides elongation and brilliancy, there is one other condition 

 that affects the visibility of Venus, viz., her declination. In 

 northern latitudes the farther north she is, the higher she rises, 

 and the easier it is to see her in daylight. For observation in 



