Galileo Galilei (Gal-lee-lay-oh Gal-eel-ay) (February 15, 1564 – January 8, 1642) was an Italian astronomer (a person who studies outer space), and a famous thinker.
Some people believe that Galileo was the first person to build a telescope. This is not true, but he was the first person to publish his observations of objects in the sky, like the Moon, stars and other planets. He discovered that the Milky Way is made of many tiny stars, that the Moon has hills, and he found four moons around Jupiter, now called the Galilean Moons. (Later scientists found many others.) He also discovered sunspots, which are dark areas of the Sun.