Halley's Comet or Comet Halley, officially designated 1P/Halley, is a short-period comet visible from Earth every 7576 years. Halley is the only known short-period comet that is regularly visible to the naked eye from Earth, and thus the only naked-eye comet that can appear twice in a human lifetime. Halley last appeared in the inner parts of the Solar System in 1986 and will next appear in mid-2061.Halley's periodic returns to the inner Solar System have been observed and recorded by astronomers around the world since at least 240 BC. But it was not until 1705 that the English astronomer Edmond Halley understood that these appearances were reappearances of the same comet. As a result of this discovery, the comet is named after Halley.During its 1986 visit to the inner Solar System, Halley's Comet became the first comet to be observed in detail by spacecraft, providing the first observational data on the structure of a comet nucleus and the mechanism of coma and tail formation. These observations supported a number of longstanding hypotheses about comet construction, particularly Fred Whipple's "dirty snowball" model, which correctly predicted that Halley would be composed of a mixture of volatile icessuch as water, carbon dioxide, ammonia, and dust. The missions also provided data that substantially reformed and reconfigured these ideas; for instance, it is now understood that the surface of Halley is largely composed of dusty, non-volatile materials, and that only a small portion of it is icy.

An apsis (from Ancient Greek ἁψίς (hapsís) 'arch, vault'; pl. apsides AP-sih-deez) is the farthest or nearest point in the orbit of a planetary body about its primary body. The apsides of Earth's orbit of the Sun are two: the aphelion, where Earth is farthest from the sun, and the perihelion, where it is nearest. "Apsides" can also refer to the distance of the extreme range of an object orbiting a host body.