Halley's comet

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Halley's comet

or

Comet Halley

(hăl`ē, hā`lē), periodic cometcomet
[Gr.,=longhaired], a small celestial body consisting mostly of dust and gases that moves in an elongated elliptical or nearly parabolic orbit around the sun or another star. Comets visible from the earth can be seen for periods ranging from a few days to several months.
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 named for Edmond Halley, who observed it in 1682 and identified it as the one observed in 1531 and 1607. Halley did not live to see its return in 1758, close to the time he predicted. It reappeared in 1835 when it was carefully recorded by visual observers, and in 1910, when its long tail and outbursts of dust jets were observed photographically. For its most recent return in 1985 and 1986, astronomers observed it from the ground and from space. A massive observing effort (1982–89) including visual observations, photography, and studies of the area around the nucleus, was coordinated by the International Halley Watch. Japan, the European Space Agency, and the USSR sent spacecraft to study the comet; the Vega and Giotto probes revealed a darker-than-expected nucleus 8 km (5 mi) wide and 15 km (9 mi) long, and shaped like a potato.

Bibliography

See NASA Special Publications, Atlas of Comet Halley (1987); M. Grewing, ed., Exploration of Halley's Comet (1988).

Halley's comet

(hal -eez, hay -leez, haw -) A comet that was first positively recorded in 240 bc and has a period of 76 years on average, one return coinciding with the Battle of Hastings (1066) and leading to the comet's representation on the Bayeux Tapestry. The orbit is retrograde, the comet moving in the opposite direction to the planets. It is named after Edmund Halley, who showed that comets move around the Sun in accordance with Newton's theory of gravitation. He noticed that the orbits of the bright comets of 1531, 1607, and 1682 were very similar and concluded that they were actually one and the same comet. He calculated its orbit and predicted that this comet should return in 1758. Unfortunately Halley died 16 years before the comet returned.

Halley's comet was seen with the naked eye in October and November 1985 and in early spring 1986: perihelion was on Feb. 9, 1986 and it passed through its ascending and descending nodes on Nov. 27, 1985 and Apr. 11, 1986. Detailed observations were made using instruments on Earth and in five spacecraft: ESA's Giotto, the Soviet-led missions Vega 1 and Vega 2, and the Japanese craft Sakigake and Suisei. Pictures were obtained by Giotto of the dark potato-shaped nucleus (16 × 8 km) and of structure and activity near the nucleus. The comet's development as it rounded the Sun was recorded and studies were made of cometary composition, of processes producing and occurring in the coma and tails, and the interaction between comet and solar wind. Halley's comet will return in 2061.

Halley’s Comet

 

a bright comet, the first for which an elliptic orbit was calculated, thereby proving the periodicity of its return to the sun. The English astronomer E. Halley, who compiled the first catalog of the elements of the orbits of comets that appeared between 1337 and 1698, noted the coincidence of the paths of the comets of 1531, 1607, and 1682 and proposed that they corresponded to one and the same comet that revolved around the sun with a period of 75-76 years. In 1705, Halley predicted the return of the comet in 1758. In 1758 the French scientist A. Clairaut developed a method of accounting for the perturbations of the comet’s motion caused by the attraction of the planets Jupiter and Saturn and refined the estimate of the date of the comet’s passage through the perihelion. It occurred on Mar. 12, 1759, within the probable period of time indicated by Clairaut. This was a brilliant confirmation of Newton’s mechanics. The perihelion of the orbit of Halley’s comet is 0.587 astronomical unit and the aphelion, more than 35 astronomical units. The next passage of the comet was in 1835. By this time, the perturbations in the comet’s motion by the planet Uranus, which was recently discovered by the English astronomer W. Herschel were also taken into account. The comet passed through the perihelion on November 16, three days later than calculated. The study of Halley’s comet by the German astronomer F. W. Bessel marked the beginning of the mechanical theory of comet forms, later continued by the Russian astronomer F. A. Bredikhin. Investigations of Halley’s comet during its last appearance (perihelion May 19, 1910), based on numerous observations, made it possible to obtain the first information on the physical nature of comets and stimulated P. Cowell to develop a more complete method of calculating perturbations by planets. Together with A. Crommelin, he traced the future motion of Halley’s comet as well as its past motion. It has turned out that up to 1909, Halley’s comet was observed 29 times, for the first time in A.D. 446. The next passage of Halley’s comet through the perihelion will occur in January 1986. This will afford one of the most convenient opportunities in the second half of the 20th century for sending a probe-rocket to a comet with the aim of directly determining the composition of comets and the state of matter within them.

REFERENCE

Orlov, S. V. O prirode komet. Moscow, 1958.

O. V. DOBROVOL’SKII

Halley's Comet

[′hal·ēz ¦käm·ət]
(astronomy)
A member of the solar system, with an orbit and a period of about 76 years; its nucleus is about 9 miles (15 kilometers) in diameter; next due to appear in 2061. Also known as Comet Halley.
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