English astronomer and mathematician best known for his study of comets. Using Newton's laws of motion, he determined mathematically that comets move in elliptical orbits around the Sun. Based on his findings, he accurately predicted that a comet observed in 1583 would return in 1758, 1835, and 1910. This comet is now named after him. In 1679 Halley became the first person to catalog the stars in the Southern Hemisphere.