Aristarchus was an ancient Greek astronomer and mathematician, known for proposing a heliocentric model of the solar system, which suggested that the Sun is at the center, and the Earth and other planets revolve around it. This idea was revolutionary for its time, as most people believed in a geocentric model, where the Earth was the center of the universe. He lived around the 3rd century BCE and is also credited with estimating the sizes and distances of the Sun and Moon. Although his ideas were not widely accepted during his lifetime, they laid the groundwork for later astronomers, including Copernicus and Galileo.