Galileo Galilei made several important discoveries about the moon. Through his observations using a telescope, he was able to determine that the moon had a rough and uneven surface, contrary to the previously held belief that it was a perfectly smooth celestial body. Galileo also observed that the moon had mountains, craters, and valleys. This challenged the Aristotelian view that the moon was a perfect, unchanging sphere. Additionally, Galileo discovered that the moon went through phases, just like the Earth's moon. This observation further supported the heliocentric model of the solar system, which proposed that the Earth and other planets revolve around the sun. Galileo's observations of the moon played a significant role in advancing our understanding of celestial bodies and the nature of our solar system.