Galileo was born in Italy in 1564. Although his father was a musician, he hoped that his son would become a doctor, and sent him to the University of Pisa.
He discovered many things, the first of which was that a pendulum always takes the same length of time to swing, irrespective of how long the arc that it is swinging through, is. Because of this observation, he eventually took the idea on further and developed the first pendulum clock.
At that time, it was generally thought that heavier objects would fall through the air quicker that lighter objects. He spent many days dropping objects off the tower of Pisa and proved that the weight or density of an object did not affect its rate of fall, and that all objects drop at the same rate (when in a vacuum so that air resistance is not a problem).
When solving a problem about where oars should be placed in a galley, he used the same principle to devise a pump that could raise water from a well, using only one horse.
Perhaps his most famous invention was the telescope. At the time, telescopes had been invented that could magnify something 3 times. Galileo's telescope magnified things twenty times. With it he found 4 of Jupiter's moons, and a supernova.
He also verified features on the moon, and on Venus that had been previously suggested.
Copernicus many years previously, had suggested that the sun was stationary, and that the Earth and other planets revolved around it. This was thought to be heresy, and against everything that the bible said, so when Galileo verified Copernicus's theory, he found himself in trouble with the church authorities.
The Catholic Church had set up a committee of men who decided what was for and what was against the teachings in the bible. They had decided that the Copernican theory was against the bible and therefore heresy. It was suggested to Galileo that providing he treated his idea of the sun at the centre of the solar system as a theory, everything would be all right, but Galileo published his discovery in a book, and was immediately arrested.
He was too famous to be thrown in jail, so instead he was given 'house arrest', however, this was to be for the remainder of his life.