As you know Galileo was most known for his astronomy discoveries that highly changed the world and the way humans view it. Besides improving on the telescope which incorporated with most of his discoveries. His most known discovery was the four moons of Jupiter; Lo, Europe, Ganymede and Callisto. On the evening of January 7th, 1610, Galileo made a shocking discovery to the world. He pointed his telescope towards Jupiter and saw two tiny “ Stars “ which is how he described them at first, two east of Jupiter and one west. Galileo never gave up with this discovery. Every day he checked upon Jupiter to verify if the “ 3 fixed stars “ still lay East and west of Jupiter, but he realized the stars were still there but now only west of Jupiter. He then realized the stars were not fixed but they moved around with Jupiter, just like our moon moves with earth. He observed the moons as the days went on some days he would only see two but he would mostly see the three. On January 13, 1610 he then identified all the four “ Galilean Moons “ which is now what we call them. Galileo then quickly published his discovery and results from the six months of astronomical observations in one of the most famous scientific publications Sidereus Nucius. To clarify, all moons were visible the first night he looked at Jupiter ( January 7th ) but they were either too close together, spread far apart or it was cloudy which made it difficult for him to see them right away.
Staying in the year of 1610, Galileo incorporated his telescope once again. With the use of his telescope he found out that Venus went through phases just like our moon. His observations of the phases of Venus proved that it orbits around sun, not earth which disproved the Ptolemaic Model; the theory that all planets orbited earth and that it was the center of universe that didn't move at all. This theory was disproved because as he would observe Venus he realized it had a crescent shape and over the months it disappeared and gradually formed a new phase and he had known that in order for the phases to occur Venus had to orbit the sun rather than earth which most people disagreed with him. The phases of Venus that Galileo observed occur by the sun illuminating half of Venus, when earth and Venus orbit the sun different sections of this area can be seen from earth. When Venus gets to a place in space close to between sun and earth we can see it's atmosphere.
One of the last but important astronomy discoveries by Galileo that ties in with the phases of Venus was the Copernican system; a belief that the sun is centered and all other planets move around it which was formulated by Nicolaus Copernicus and published in 1543. This system changed the way we view our place in the universe. This theory was first discovered by some astronomers but Galileo believed strongly to the Copernicus' theory he proved this theory by saying he saw four moons orbiting Jupiter which proves not everything is orbiting earth. Galileo also saw the phases of Venus which showed that the sun was the center or else there would be no phases of Jupiter. Though others did not trust Galileo's telescope and put into their minds he painted planets into his telescope, he was declared heretic by the cardinals in Rome in 1633. This occurred because the Catholic Teachings strongly believed earth was the center of the universe. Galileo was threatened into denying that earth moves and the sun is the center. Now we all can say that if it weren't for Galileo fighting for his belief we could have still believed up to this day that Earth was the center of the universe, which is incorrect.
"Galileo's Discovery of Jupiters Moons." Galileo's Discovery of Jupiter's Moons. N.p., n.d. Web. 18 May 2017.
Sustermans, Justus. "Galileo and the Phases of Venus." Venus. Justus Sistermans, n.d. Web. 18 May 2017.