Galileo Galilei
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 astronomical objects through a telescope. He discovered that the Milky Way is made of many stars. He discovered that the Moon has hills. He discovered the first moons not counting our own moon, these are the four biggest moons of Jupiter: Io, Europa, Ganymade and Callisto. Those moons are now called the Galilean moons. He discovered sunspots, which are dark areas of the Sun. He saw that the planet Venus has light and dark phases just like the Moon. This helped people to know that the Sun is at the center of the Solar System, as Nicolaus Copernicus had said.