How did Galileo and Jupiter upset 2,000 years of astronomy?
When Galileo got his first telescopes, he made some adjustments to them so he could view the more distant planets of Jupiter and Saturn more easily. When he turned his instrument on Jupiter he noticed three dots of light in a straight line very close to the disc of the planet. Assuming that they were background stars, he did not give them much thought. When he observed Jupiter on another night, much to his total astonishment, the 3 dots were still right next to the planet joined by another, making a total of 4 dots. Subsequent observations proved that these 4 dots were actually moons orbiting around Jupiter.
Galileo had a huge problem. Here was incontrovertible proof that the Earth was not the center of the universe. Who would make problems about this discovery for Galileo? The Inquisition.
Why did Copernicus try to give credit for his Sun-centered theory of the universe to an old, dead Greek?
In the model accepted and enforced by the Catholic Church, the Earth was the center of the universe and the papacy in Rome was the center of the Earth. This geocentric theory taken from the ancient Greeks meant that everything in the universe revolved around the Earth with the Pope at its center.
Now here comes Galileo with something he sees in the telescope that obviously is not turning around the Earth. Each night he observed satellites turning around Jupiter. Each night the pattern changed like clockwork. Galileo reasoned that if there are moons revolving around Jupiter, and the Earth is not the center around which everything rotates, then perhaps there are other things that do not rotate around the Earth, like say the moon, sun and planets?
Copernicus had already published this theory at the end of his life, but others like Bruno had been burned at the stake for espousing this and other heretical beliefs. The Pope had no tolerance for anyone who refuted the cardinal fact that the Pope was the center of the physical universe.