https://education.nationalgeographic.org/resource/galileo-discovers-jupiters-moons/
https://hsm.stackexchange.com/questions/7588/what-was-the-aperture-of-the-first-galileos-telescope#:~:text=Galileo%20made%20his%20first%20telescope,finally%20magnify%20objects%20twenty%20times.
With this telescope, he was able to look at the moon, discover the four satellites of Jupiter, observe a supernova, verify the phases of Venus, and discover sunspots. His discoveries proved the Copernican system which states that the earth and other planets revolve around the sun. Prior to the Copernican system, it was held that the universe was geocentric, meaning the sun revolved around the earth.
