Galileo Galilei was a late renaissance mathematician, physicist, scientist, and astronomer. He was born in Pisa, Italy, and spent time there and in Florence as a child. He played a major role in the Scientific Revolution.
He went to the University of Pisa for a medical degree, but he did not complete this, as he was more interested in mathematics. He also studied fine arts, and spent time in Florence as a professor teaching perspective and chiaroscuro at the Academia della Arti del Disegno. Later he was chair of mathematics at the University of Pisa. A few years later, in 1592, he taught at the University of Padua, where he taught geometry, mechanics and astronomy until 1610.
He was very interested in astronomy, and made improvements to the telescope that was invented in the Netherlands in 1608 by Hans Lippershey. His telescopes were what would be called today a spyglass. They were mainly used to see items on earth and at sea. Galileo used them to look at the night sky. In 1610, Galileo discovered with his telescope the four largest moons of Jupiter, which are commonly known as the Galilean satellites.
Galileo was able to see many new items in the night sky with his telescope. He observed the rings on Saturn, although he thought they were moons, and he observed Neptune, although he thought it was a star and not a planet. He observed that the Milky Way, as we know it, was in fact a large, dense group of stars, not nebulous clouds in the night sky as previously thought. He calculated distances of stars from earth, using their relative brightness as a gage of how distant they were. Although these distances are not far enough to be accurate, it did show that stars were much further away than planets.
His observation and discovery of moons orbiting around Jupiter did not agree with the commonly held theory by the Catholic Church. The Church held to the principles of Aristotelian Cosmology that stated that all heavenly bodies circled the Earth.
He published many books on the subject of heliocentrism, or the belief that the Earth and other planets revolve around the Sun. These works were not received well by the Church, and he was forced to recant his findings. He continued to support heliocentrism, and eventually he had to face an inquisition, which found him “vehemently suspect of heresy” in holding that the Sun was the center of the universe and not the Earth. Due to the findings of the inquisition, he was put under house arrest where he remained for the remainder of his life.