Even before telescopes existed, people looked out into space and tried to understand the universe. The Egyptians, Babylonians, Greeks, and Maya were among the first peoples to map the movements of the stars. In Europe, a 16th-century Polish mathematician named Nicolaus Copernicus put forth a revolutionary idea. Copernicus watched the stars and took measurements by eye. He concluded that the view of the universe accepted at the time that the Sun traveled around Earth was wrong, and that it was in fact Earth that traveled around the Sun. Despite widespread objections to his ideas, he published his theory in a book. Later astronomers built on Copernicus' work. Technological improvements led to the development of the telescope in the early 17th century. Using a telescope, the Italian scientist Galileo Galilei discovered the four largest moons of Jupiter and observed craters on Earth's Moon. He also confirmed that Copernicus' theory was correct. Over the centuries, scientific discoveries and information from telescopes advanced our knowledge of how the universe works.