Galilei Galileo (1564-1642) an Italian physicist and astronomer, was greatly remembered for some very important contributions to astronomy and physics. He was also known for his battle against the authorities for freedom of inquiry. Early in his life, Galileo was taught by monks at Vallombrosa, and then entered the university of Pisa in 1581 to study medicine. He soon turned to philosophy and mathematics and left the University without a degree in 1585. In 1589 he became professor of mathematics at Pisa. He supposedly taught theories that contradicted Aristotle's theories, and in 1592 his contract was not renewed. The same year he was appointed the chair of mathematics at the University of Padua, which he remained at until 1610. While at Padua, he invented a 'calculating compass' for solving mathematics problems. In 1609 he heard that in Holland a spy glass had been invented, and he was inspired to create the first telescope, which was as powerful as a modern day field glass. By December of the same year, he had built another telescope twenty times stronger than the first, which he was able to discovery craters on the moon with, stars in the milky way, and the four largest satellites of Jupiter. He had also observed the phases of Venus by this time. After his great discoveries, he mainly stuck to writing books. In 1613 he published a book about sunspots, 1624 a book called Dialogue on the Tides, which he discusses Ptolemaic and Copernican theories, in which he got himself reprimanded by the Inquisition of Rome.
Nicholas Copernicus (1473 -1543) was a Polish astronomer, well known for his Copernican theory. His theory stated that sun rest near the center of the universe, and the earth, which spun daily on its axis, revolved annually around the sun. Now, this process is known as helocentric, or suncentered, system. Copernicus first began going to the University of Krakow in 1491. He studied liberal arts for four years without receiving a degree, and then went to Italy to study law and medicine. During the January of 1497 he bean to study canon law at University of Bologna while living with a mathematician, Domenico Maria de Novara. Novara sparked Copernicus' interest in geography and astronomy, and the final climax of it came when the two watched the occultation (eclipse by the moon) of the star Aldebaran on March 9, 1497. After this, Copernicus lectured about astronomy in Rome. After pursuing a career in medicine, he started to write books about astronomy. His famous one which he wrote between 1507-1515 was called De Hypthesibus Motuum Coelestium a se Censtitutis Commentariolus (commonly known as Commentariolus).
Edmund Halley (1656-1742) was a British Astronomer, who was the first to calculate a comet's orbit. He went to the University of Oxford, where he studied the theories of Sir Isaac Newton. Because he was so intrigued with these theories, it inspired him to write the Principle which he published with his own money in 1687. In 1721 he was made Astronomer Royal and began an 18 year study of the moon's complete revolution through its ascending and descending nodes. During his life he also wrote another important treatise called Astronomiae Cometicae Synopsis (Synopsis on Cometary Astronomy). It was started in 1682 and published in 1705. In this he mathematically demonstrated that comets move in a elliptic orbits around the sun and how over time they would pass the same point. He had such an accurate prediction that when the comet (now Halley's Comet) returned in 1758, it validated his theory.