He was a lecturer in mathematics due to his large interest in mathematics.
He heard about an invention called the telescope and improved upon it leading to many discoveries in astronomy which is the study of the planets and stars.
Some of the things he discovered through astronomy were:
Jupiter has 4 moons.
Saturn has rings.
The moon has craters and mountains.
The sun has spots.
Nicolaus Copernicus wrote a book before he died where he stated that the Earth wasn’t the centre of the solar system and that the sun was, Galileo backed up his point in 1642 in The Dialogue Concerning The Two Chief World Systems.
He was convicted of heresy which is knowingly going against the beliefs of the Catholic Church. He was put under house arrest until his death in 1652.
