Let the circle _B_ be the Sun as seen from any Planet, at a given
distance; to another Planet, at double that distance, the Sun will
appear just of half that breadth, as _A_; which contains only one fourth
part of the area or surface of _B_. For, all circles, as well as square
surfaces, are to one another as the squares of their diameters. Thus,
the square _A_ is just half as broad as the square _B_; and yet it is
plain to sight, that _B_ contains four times as much surface as _A_.
Hence, in round numbers, the Sun appears 7 times larger to Mercury than
to us, 90 times larger to us than to Saturn, and 630 times as large to
Mercury as to Saturn.
[Sidenote: Fig. V.
Proportional bulks and distances of the Planets.