Answer:
Telescope
Step-by-step explanation:
Galileo was Italian astronomer and physicist. He went to University of Pisa to study medicine but due to his interests he left medicine and became a professor of mathematics.
In 609, he built a telescope and became the first person who looked at the moon through a telescope. He also used the newly discovered telescope to discover four moons of Jupiter, study the sunspots on the sun, observe the phase of Venus and study Saturn.
His observation of various celestial bodies confirmed the Copernicus' theory that Earth and other planets revolve around the Sun (Heliocentric model), while most of the people and Church during that time believed in a Earth centred universe.