The telescope was invented in the Netherlands. The news of the invention spread rapidly, and by 1609 telescopes were for sale in spectacle-makers’ shops throughout Europe. There were even those who used telescopes to observe the moon. But it was Galileo who best refined the telescope for astronomical viewing, subsequently studying not only our own moon, but also discovering the four satellites of Jupiter.
At the end of 1609, Jupiter was the brightest object in the evening sky besides the moon. After Galileo had tinkered with his telescope and finished his lunar observations, he turned his attention to Jupiter. On January 7, 1610, he observed the planet and saw what he thought were three fixed stars near it, strung out in a line. The next night, he saw all three stars to the west of Jupiter. Over the next week he returned to the formation every night. He discovered that not only did the little stars never leave the planet, but they seemed to be carried along with it, and moreover, kept changing their position with respect to each other and to Jupiter. Also, a fourth companion entered the grouping that apparently had been around the other side of the planet during his initial observations.
By January 15th Galileo figured out that the moving bodies were not stars but four moons that were revolving around Jupiter. This proved that not everything in space circled the Earth. Therefore, to Galileo, our planet might not the absolute center of the universe, as the Catholic Church maintained (based on its understanding of the Bible).
In March of 1610 he published a small book, Sidereus Nuncius (The Starry Messenger), revealing some discoveries that had not been dreamed of in the philosophy of the time: mountains on the Moon, lesser moons in orbit around Jupiter, and the resolution of what had been thought cloudy masses in the sky (nebulae) into collections of stars too faint to see individually. Other observations followed, including the phases of Venus and the existence of sunspots.
These revelations, particularly about the moons of Jupiter, had a major impact on cosmology. Traditional belief held that there was only one center of motion, which was the earth. All heavenly bodies were believed to rotate around it. Copernicus had postulated that the earth went around the sun while the moon went around the earth, but his theories were considered absurd. (From about 1510 to 1514, Copernicus developed the first general outline of his new heliocentric system and presented it in a short manuscript, but it was not for publication; rather, he circulated it among his friends. He began to write the first book of De Revolutionibus Orbium Coelestium (On the Revolutions of the Heavenly Spheres) in about 1515 and again tried to keep it a secret, since he was concerned about the implications of his discovery. But again, word of his work spread among the circles of mathematicians and astronomers.)
The discovery that Jupiter also had moons, and that they rotated around that planet, not only gave credence to the Copernican theory but shook up the foundations of science, and religion as well.
Galileo continued to disturb the universe, and was ordered to stand trial on suspicion of heresy in 1633. He was sentenced to house arrest for the remainder of his life.