Caption:
Halley's Comet is the best known of the short-period comets and is visible from Earth every 75-76 years. It is the only short-period comet that is clearly visible to the naked eye from Earth. It was observed and recorded by astronomers since at least 240 BC. The comet's periodicity was first determined in 1705 by English astronomer Edmond Halley, after whom it is now named. Lithograph by Jules Joseph Guillaume Bourdet, 1835-37 (cropped and cleaned).