Comet Halley, officially designated 1P/Halley, more generally known as Halley's Comet after Edmond Halley, is the best-known and the brightest of the "short-period" comets from the Kuiper belt that visit the inner solar system in years or decades-long orbits rather than the millennial periods of comets from the Oort Cloud.
The most standard pronunciation of "Halley" is [hælɪ] (IPA), to rhyme with "valley". The pronunciation [heɪlɪ] (to rhyme with "Bailey") is thought to have originated by association with the rock group Bill Haley & His Comets.
Having perceived that the elements of the comet of 1682 were nearly the same as those of two comets which had appeared in 1531 (observed by Petrus Apianus) and 1607 (observed by Johannes Kepler in Prague), Halley concluded that all three comets were in fact the same object returning every 76 years. After a rough estimate of the perturbations the comet would sustain from the attraction of the planets, he predicted its return for 1757. Halley's prediction of the comet's return proved to be correct, although it was not seen until December 1758, and did not pass through its perihelion until March 1759, the attraction of Jupiter and Saturn having caused, as was computed by Alexis Clairault previously to its return, a retardation of 618 days. Halley did not live to see the comet's return, having died in 1742.
Halley's calculations enabled the comet's earlier appearances to be found in the historical record:
- When the comet was observed in 1456, it passed very near to the Earth; its tail extended over 60° of the heavens, and its tail took the form of a sabre.
- In 1066, the comet was thought to be an omen: later that year Harold II of England died at the Battle of Hastings. It is shown on the Bayeux Tapestry, and the accounts which have been preserved represent it as having then appeared to be four times the size of Venus, and to have shone with a light equal to a fourth of that of the Moon.
- It is calculated that Comet Halley may have passed as close as 0.03 AU from Earth in the year 837.
- Some have suggested that the comet's appearance in 11 BC might explain the Biblical story of the Star of Bethlehem.
The comet returned in 1835 and 1910 and 1986. The 1910 approach of the comet was notable for several reasons: as well as the first orbit for which photographs of the comet exist, it was a relatively close approach to Earth making the comet a spectacular sight and indeed the Earth passed through the tail of the comet.
The 1986 approach was less favorable for Earth observers: the comet did not achieve the spectacular brightness of some previous approaches, and with increased light pollution from urbanization, many people never saw the comet at all. However, the development of space travel allowed scientists the opportunity to study a comet at close quarters, and several probes were launched to do so. Most spectacularly, the Giotto space probe, launched by the European Space Agency, made a close pass of the comet's nucleus. Other probes included the Soviet Union's Vega 1 and Vega 2, and two Japanese probes, Suisei and Sakigake .
Halley is expected to return in 2061.
- Two of the comet's visits - 1835 and 1910 - are the same years as the birth and death of the American novelist Mark Twain.
Halley's Comet in fiction
- Arthur C. Clarke's novel 2061: Odyssey Three includes a detailed description of a manned mission to Halley's Comet.
- In Heart of the Comet , a novel by Gregory Benford and David Brin (1987), a multinational team colonizes Halley's Comet, building a habitat within the ice.