Ok, hist of sci people out there, help me out: In Hilchot
Yesodei Hatorah chapter 3, Maimonides lays out the geocentric
model of the cosmos, epicycles and all. At 3:8, he notes that
Earth is about 40 times larger than the moon, the sun is about
170 times larger than Earth, the sun is the largest of the
"stars" (a category that includes planets too), and Mercury is
the smallest. Comparing this with the actual data we know now,
the sun is of course larger than any of the planets, and Mercury
is the second smallest of the heavenly bodies known at that time
(the moon is smaller). The sun's radius is 109 times Earth's
radius, so the Rambam's number is pretty decent, within a factor
of 2. Earth's radius is 3.67 times the radius of the moon, so
that figure is considerably further off (by an order of
magnitude). The ratio of Earth's volume to the moon's volume is
49, much closer to the Rambam's number of 40 (and he doesn't
actually specify which dimension he's talking about), though if
we understand the Rambam's rati