A Short History of Astronomy 



[Ca. IV. 



at its greatest angular distance from the sun (as at v in 

 fig. 43) at intervals of about 584 days. This is therefore 

 the time which Venus takes to return to the same position 

 relatively to the sun, as seen from the earth, or relatively 

 to the earth, as seen from the stln ; this time is called 

 the synodic period. But as during this time the line E s 

 has changed its direction, Venus is no longer in the 

 same posiiion relatively to the stars, as seen either from 

 the sun or from the earth. If at first Venus and the 



FIG. 44. The synodic and sidereal periods of Venus. 



earth are at v,, E I respectively, after 584 days (or about 

 a year and seven months) the earth will have performed 

 rather more than a revolution and a half round the 

 sun and will be at E 2 ; Venus being again at the greatest 

 distance from the sun will therefore be at v 2 , but will 

 evidently be seen in quite a different part of the sky, 

 and will not h we performed an exact revolution round the 

 sun. It is important to know how long the line s v l takes 

 to return to the same position, i.e. how long Venus takes 

 to return to the same position with respect to the stars, 



