(1656-1742) an English astronomer (= scientist who studies the stars, planets, etc.) and mathematician who was a close friend of Isaac Newton. He is best remembered for Halley's comet, which was named after him. A comet is a bright object that moves through space round the sun with a tail of burning gas and dust. Halley correctly predicted that this one would return regularly to be seen in the night sky approximately every 76 years. It was last visible in 1986.