THE PLANET VENUS 527 



of the inception of the investigation at Flagstaff and its execution and 

 thought he had detected a short rotation for it. 



Belopolski made his attempt at elongation in spite of knowing what 

 I shall now explain. For elongation, although the time when the planet 

 is easiest seen, is not that in which it is best examined spectrograph- 

 ically for rotation. 



In the case of a body reflecting light, the shift varies from what it 

 is if the body be emitting it, from twice as much in some positions to 

 nothing at all in others. This is because the reflecting surface itself 

 moves to or from the waves. Thus if a planet be on the side of the sun 

 away from the earth, the rim of it which is approaching the earth ad- 

 vances to meet each new wave and so shortens it by just the amount 



The Planet Venus. 

 October 15, 1896. February 12, 189T. Match 26, 1897. 



of its own advance, thus doubling the shortage which would result if it 

 emitted the waves itself. The receding side in the same manner 

 doubles the recession. If the planet be at right angles to the sun the 

 waves are affected as if they were emitted and we have a single shorten- 

 ing or lengthening, as the case may be. If the planet be between us 

 and the sun, the rim is running from the sun at just the speed it is 

 approaching us and the total effect is nil. 



Thus in the case of Venus, the evidence of tilt obtained depends 

 entirely on where you take her. Superior conjunction or when she lies 

 beyond the sun is the best time spectrographically and it was this that 

 Dr. Slipher chose. He caught the planet just as she was coming out 

 from behind the sun as evening star. In this he was abetted by the 

 clear and steady air of Flagstaff, which enabled him to get her while 

 she was still not far from the sun himself. 



