In 1609, Galileo Galilei used an early version of the telescope to discover the Moon was covered in pits and mountains, similar to Earth, this was a huge discovery due to scientists believing the Moon was smooth and round.
Galileo then saw the four “stars” that were surrounding Jupiter, which he discovered were actually moons that were in orbit of Jupiter. When Galileo had looked at Venus he observed that the planet orbited the Sun not the Earth as he had originally believed.
Galileo then was curious about the Sun and pointed his telescope at it, unaware that the Sun would damage his eyesight. He discovered that the sun has dark sunspots on its surface.
Galileo proved that the Sun was the centre of the universe, not the Earth, as it had been believed. His discoveries challenged the beliefs of other scientists of that time.