What did Galileo do as a astronomer?

Answer 1

Galileo Galilei discovered the information needed to found the belief that the sun was the center of the universe, and not the Earth, which was the belief at that time in the Ptolemaic/Geocentric Theory.

Galileo discovered that the Moon was covered in blemishes, craters, valleys, and mountains with his telescope. This contradicted the belief of that time that everything outside of the Earth's atmosphere was "heavenly" and therefore, "perfect". After he made this discovery, it prompted his curiousity to look farther into the galaxy.

Galileo then made the discovery that Jupiter had four moons, which orbited around Jupiter, instead of the Earth. The theory of that time stated that all major bodies outside o the Earth revelved around the Earth. By Galileo's observations that they revelved around Jupiter, it further disproved the geocentric theory.

Another major discovery that Galileo found, was the phases of Venus. In the Ptolemaic Theory, Venus would never be fully seen because of it's rotation around the Earth. Galileo saw through his telescope that Venus went through phases and that they corresponded with the site of the sun.