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look at Fig. 14. You can see that for a telescope at A on one side of the Earth, Venus would cross the Sun's disc along the path CD; while a telescope B on the other side would see Venus travel in a different path EF; and you can see also that if the Sun were brought nearer, the two paths would not be so different: if

Voyage in Space page059.jpg

Fig. 15.—An Ideal Transit of Venus.

the Sun were further away the paths would be more different. In other words, the amount of difference tells us just how far off the Sun is, if we can measure the difference accurately.

Unfortunately astronomers found, when the great events had taken place, that they could not measure the difference as accurately as they had hoped. They hoped to determine the exact second when the transit began or ended. It would have been best for them if there could have been a moment such as that labelled II in Fig. 15, when the black disc of Venus just touched the bright edge of the Sun. Before that moment, as in I, it would have been only partly on the Sun, and after it, as in III, the black disc is completely within the Sun. It would have been nice for the astronomers if between I and III