You all might check
<http://es.rice.edu/ES/humsoc/Galileo/Things/moon.html>
This is the Galileo project at Rice University, run by
Albert Van Helden. He is perhaps the world's foremost
scholar of Galileo. He was one of my professors in
college.
Briefly it suggest several different accounts, mostly
ones already suggested -- namely that the Earth is
corrupting the moon, that it is differently shaded,
and that gasses exist between the earth and the moon.
What isn't written in this account but he presented in
class is that the ancients just didn't care that the
moon didn't look right. Many believed that we couldn't
trust our senses and that we were often deceived by
them. Very few had our conception of science. Arguably
it is during this time period that our ideas of
science were being put together.
Marc "Zeke" Kossover
The Jewish Community High School of the Bay