Page:A short history of astronomy(1898).djvu/160

114 at its greatest angular distance from the sun (as at 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 sun; this time is called the synodic period. But as during this time the line has changed its direction, Venus is no longer in the same position relatively to the stars, as seen either from the sun or from the earth. If at first Venus and the

earth are at, 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 ; Venus being again at the greatest distance from the sun will therefore be at , but will evidently be seen in quite a different part of the sky, and will not have performed an exact revolution round the sun. It is important to know how long the line takes to return to the same position, i.e. how long Venus takes to return to the same position with respect to the stars,