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1P/Halley - Halley's Comet
Discovered by
Prehistoric observation; Edmond Halley (recognition of periodicity)
Semi-major axis
17.83414429255373 AU
Eccentricity
0.9671429084623044
Orbital period
75.315892782197
Mean anomaly
38.384264476436°
Inclination
162.2626905791606°
Longitude of ascending node
58.42008097656843°
Argument of perihelion
111.3324851045177°
Mass
2.2×1014 kg
Mean density
0.6 (estimates range from 0.2 to 1.5 g/cm3)
Escape velocity
~0.002 km/s
Albedo
0.04
Apparent magnitude
28.2 (in 2003)[
Epoch
2449400.5 (17 February 1994)
Dimensions
15×8 km, 11 km (mean diameter)
Aphelion
35.08231047359055 AU
Perihelion
0.5859781115169086 AU
Last Perihelion(s)
9 February 1986
Next Perihelion
28 July 2061
Computation of Orbit

Halley was the first comet to be recognized as periodic. Until the Renaissance, the philosophical consensus on the nature of comets, promoted by Aristotle, was that they were disturbances in Earth's atmosphere. This idea was disproved in 1577 by Tycho Brahe, who used parallax measurements to show that comets must lie beyond the Moon. Many were still unconvinced that comets orbited the Sun, and assumed instead that they must follow straight paths through the Solar System.

In 1687, Sir Isaac Newton published his Principia, in which he outlined his laws of gravity and motion. His work on comets was decidedly incomplete. Although he had suspected that two comets that had appeared in succession in 1680 and 1681 were the same comet before and after passing behind the Sun (he was later found to be correct; see Newton's Comet), he was unable to completely reconcile comets into his model.

Ultimately, it was Newton's friend, editor and publisher, Edmond Halley, who, in his 1705 Synopsis of the Astronomy of Comets, used Newton's new laws to calculate the gravitational effects of Jupiter and Saturn on cometary orbits. This calculation enabled him, after examining historical records, to determine that the orbital elements of a second comet that had appeared in 1682 were nearly the same as those of two comets that had appeared in 1531 (observed by Petrus Apianus) and 1607 (observed by Johannes Kepler). Halley thus concluded that all three comets were, in fact, the same object returning every 76 years, a period that has since been amended to every 75–76 years. After a rough estimate of the perturbations the comet would sustain from the gravitational attraction of the planets, he predicted its return for 1758. Halley died in 1742 before he could observe this himself.

Halley's prediction of the comet's return proved to be correct, although it was not seen until 25 December 1758, by Johann Georg Palitzsch, a German farmer and amateur astronomer. It did not pass through its perihelion until 13 March 1759, the attraction of Jupiter and Saturn having caused a retardation of 618 days. This effect was computed prior to its return (with a one-month error to 13 April) by a team of three French mathematicians, Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute. The confirmation of the comet's return was the first time anything other than planets had been shown to orbit the Sun. It was also one of the earliest successful tests of Newtonian physics, and a clear demonstration of its explanatory power. The comet was first named in Halley's honour by French astronomer Nicolas Louis de Lacaille in 1759.

Some scholars have proposed that first-century Mesopotamian astronomers already had recognized Halley's Comet as periodic, Jews among them. This theory notes a passage in the Bavli Talmud that refers to "a star which appears once in seventy years that makes the captains of the ships err."

 

Apparitions

Halley's calculations enabled the comet's earlier appearances to be found in the historical record. The following table sets out the astronomical designations for every apparition of Halley's Comet from 240 BC, the earliest documented widespread sighting. For example, "1P/1982 U1, 1986 III, 1982i" indicates that for the perihelion in 1986, Halley was the first period comet known (designated 1P) and this apparition was the first seen in half-month U (the second half of October) in 1982 (giving 1P/1982 U1); it was the third comet past perihelion in 1986 (1986 III); and it was the ninth comet spotted in 1982 (provisional designation 1982i). The perihelion dates of each apparition are shown. The perihelion dates farther from the present are approximate, mainly because of uncertainties in the modelling of non-gravitational effects. Perihelion dates of 1531 and earlier are in the Julian calendar, while perihelion dates 1607 and after are in the Gregorian calendar.

Last modified
08/01/2019 - 11:46