Claudius Ptolemy was a 2nd century Greek writer known for his work in mathematics, astronomy, geography, astrology, and poetry. He lived in Alexandria, Egypt and wrote in Greek. He is best known for publishing the Almagest, which supported the theory that the Earth is at the center of the universe. Nicolaus Copernicus was a 16th century mathematician and astronomer who formulated a model of the universe that placed the Sun, not the Earth, at the center. Tycho Brahe was a 16th century Danish astronomer known for his accurate astronomical and planetary observations, which were five times more accurate than previous observations. His data helped advance the Copernican model. Galileo Galilei was a 17th