12._] [Illustration: _Fig.

13._] [Illustration: _Fig.

14._] [Illustration: _Fig.

15._] It must be noticed that the sun's apparent diameter is not always the same.

He is nearer to us in winter than in summer, and, of course, his apparent diameter is greater at the former than at the latter season.
The variation of the apparent diameter corresponds (inversely) to the variation of distance.