Galileo Galilei was an Italian astronomer, engineer and physicist and he was born in Pisa on February 15, 1564. Galileo discovered four of Jupiter's moon's (almost 400 years ago!
Galileo, in 1609, heard about a new invention, they spyglass! The spyglass made objects appear closer than they actually were. Galileo used his maths skills and technical skills to further improve the spyglass and later create the telescope which he used to observe Jupiter's moons.
Before he observed Jupiter's moon's he first observed Earth's moon and discovered that it was not smooth as it may appear to the naked eye, but it actually had mountains and pits similar to earth. He also looked at Saturn and Venus using the telescope.
The Heliocentric model
Galileo's groundbreaking observations put out new evidence for the now widely disbelieved Heliocentric model of the solar system (where plants are in orbit around the sun ). Galileo's observations did not prove the Geocentric model ( Earth is at the centre of the solar system ) wrong, however, it did provide strong evidence for the Heliocentric model.
During the time period of Galileo, many people believed in the Geocentric model which was where the Earth was at the solar system. This was believed as back then people didn't have telescopes and this meant they could only see the stars and sun revolve around them. Since this was such a widespread and believed idea which seemed to make sense, it was very hard for Galileo to spread his observations and make people believe him.