Galileo’s most important contribution to astronomy was his
a. discovery of evidence that the sun is the center of the solar system.
b. set of calculations of the size of each of the planets.
c. discovery of the planet mars.
d. description of the behavior of moving objects.

When Galileo got ahold of a little telescope ... no more than
a kid's toy today ... he soon turned it towards the night sky. 
The story says that he was the first one to do that.  Apparently
everyone else who had these things used them to look at their
neighbors' windows.  Anyway, When Galileo looked at Jupiter,
he saw four tiny dots.  As he watched them night after night, he
saw that the four dots always stayed with Jupiter, but got nearer
and farther from it.  He eventually realized that they were small
bodies in orbit around Jupiter.  And then ... because he was Galileo ...
he made the big intellectual jump:  He reasoned that if there were
smaller bodies in orbit around Jupiter, then Jupiter and the other
planets might also be in orbit around something, and not necessarily
around the Earth.  He understood this as evidence that the sun is
the center of the solar system.