Halley's Comet, officially designated 1P/Halley and also referred to as Comet Halley after Edmond Halley, is a comet that can be seen every 75-76 years. It is the most famous of all periodic comets. Although in every century many long-period comets appear brighter and more spectacular, Halley is the only short-period comet that is visible to the naked eye, and thus, the only naked-eye comet certain to return within a human lifetime.[2] Its many appearances over the centuries have had a notable effect on human history, despite the fact that they were not recognized as the same object until the 17th century. Halley's Comet last appeared in the inner Solar System in 1986, and will next appear in mid 2061.

The most standard pronunciation of "Halley" — and the pronunciation that the astronomer himself probably used — is [hæli] (IPA), to rhyme with "valley". The once-standard alternate pronunciation [heɪli] (to rhyme with "Bailey") led to rock and roll singer Bill Haley naming his band Bill Haley and the Comets.

Edmond Halley's study

Halley's Comet was the first to be recognized as periodic. Having perceived that the observed characteristics 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 (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 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 25 December 1758 by Johann Georg Palitzsch, a German farmer and amateur astronomer, and did not pass through its perihelion until March 1759; the attraction of Jupiter and Saturn having caused a retardation of 618 days, as was computed by a team of three French mathematicians, Alexis Clairault, Joseph Lalande, and Nicole-Reine Lepaute, previous to its return. Halley did not live to see the comet's return, having died in 1742.

Notable appearances

Halley's calculations enabled the comet's earlier appearances to be found in the historical record.

Early appearances

• 240 BCE and earlier: Historical records show that Chinese astronomers observed the comet's appearance in 240 BCE and possibly as early as 2467 BCE. Habitual observations and calculations of appearances after 240 BC are recorded by Chinese, Japanese, Babylonian, Persian, and other mesopotamian astronomers.
• 87 BCE: According to Armenian and Italian researchers, the "Symbol on Tigranes the Great's crown that features a star with a curved tail may represent the passage of Halley's comet in 87 BC. Tigranes could have seen Halley's comet when it passed closest to the Sun on Aug. 6 in 87 BC according to the researchers, who said the comet would have been a 'most recordable event' -- heralding the New Era of the brilliant King of Kings.^[1]
• 12 BCE: Some theologians have suggested that the comet's appearance in 12 BC might explain the Biblical story of the Star of Bethlehem.
• 66 CE: In the Talmud, it is mentioned that "There is a star which appears once in seventy years that makes the captains of the ships err" (Horioth, chap. III). It probably refers to the AD 66 perihelion.
• 837: In this year, it is calculated that Comet Halley may have passed as close as 0.03 AU (3.2 million miles) from Earth, by far its closest approach. Its tail may have stretched 90 degrees across the sky.[3]
• 1066: The comet was seen in England and thought to be a bad 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 quarter of that of the Moon. This appearance of the comet is also noted in the Anglo-Saxon Chronicle. Having first seen it as a young boy in 989, Eilmer of Malmesbury declared prophetically in 1066: "You've come, have you?...You've come, you source of tears to many mothers. It is long since I saw you; but as I see you now you are much more terrible, for I see you brandishing the downfall of my country" (William of Malmesbury, Deeds of the English Kings, Ch. 225, ISBN 0-19-820678-X).
• Chaco Native Americans in New Mexico recorded this 1066 comet in their petroglyphs. [4]
• 1301: The artist Giotto di Bondone could have observed the comet and his depiction of the Star of Bethlehem in the Nativity in the Arena Chapel cycle completed in 1305 is a candidate for an early depiction.
• 1456: The comet passed very close to the Earth; its tail extended over 60° of the heavens and took the form of a sabre. According to one story, first appearing in a posthumous biography in 1475 and later embellished and popularized by Pierre-Simon Laplace, Pope Callixtus III excommunicated the 1456 apparition of the comet, believing it to be an ill omen for the Christian defenders of Belgrade, who were at that time being besieged by the armies of the Ottoman Empire. However, no known primary source supports the authenticity of this account.

Recent history

The most recent appearances have been in 1835, 1910, and 1986. Halley will next return in 2061.

1835

American satirist and writer Mark Twain was born on November 30, 1835; exactly two weeks after the comet's perihelion. In his biography, he said, "I came in with Halley's comet in 1835. It's coming again next year (1910), and I expect to go out with it. The Almighty has said no doubt, 'Now here are these two unaccountable freaks; they came in together, they must go out together.' " Twain died on April 21, 1910, the day following the comet's subsequent perihelion. [5] [6] The 1985 fantasy film The Adventures of Mark Twain is inspired by this.

1910

The April 1910 approach was notable for several reasons: it was the first approach of which photographs exist, and the comet made a relatively close approach, making it a spectacular sight. Indeed, on May 18, the comet transited the Sun's disk, and the Earth actually passed through its tail. At the time, the comet's tail was thought to contain poisonous cyanogen and gas. The popular media picked up this fact and, despite the pleas of astronomers, wove sensational tales of mass cyanide poisoning engulfing the planet. In reality, the gas is so diffuse that the world suffered no ill-effects from the passage through the tail.

Many people who claim to remember seeing the 1910 apparition are in fact remembering a different comet, the Great Daylight Comet of 1910, which surpassed Halley in brilliance and was actually visible in broad daylight for a short time about four months before Halley made its appearance.

1986

The 1986 approach was the least favourable for Earth observers of all recorded passages of the comet throughout history: 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. Further, the comet appeared brightest when it was almost invisible from the northern hemisphere in March and April, prompting many amateur astronomers to travel to the southern hemisphere for a glimpse of the interloper. 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/France joint projects Vega 1 and Vega 2, and two Japanese probes, Suisei and Sakigake. The probes were unofficially known as the Halley Armada.

It was Stephen Edberg (then serving as the Coordinator for Amateur Observations at NASA's Jet Propulsion Laboratory (JPL) and Charles Morris who were the first to observe Comet Halley with the naked eye in its 1986 apparition.

The comet was also observed from space by the International Cometary Explorer, which was in a solar orbit at the time. Originally International Sun-Earth Explorer 3 (ISEE-3), it was renamed and retooled after it was freed from its L1 Lagrangian point location to observe comet 21P/Giacobini-Zinner.

Two Space Shuttle missions--the ill-fated STS-51-L and STS-61-E--were scheduled to observe Comet Halley from low Earth orbit. 61-E, which would have been flown by Challenger in March 1986, would have carried the ASTRO-1 platform to study the comet, among other things. The Challenger disaster thwarted all such plans. ASTRO-1 would not fly until late 1990 on STS-35. [7]

The Soviet Union's Salyut 7 space station was unoccupied during Halley's 1986 visit, and Mir, though launched during the visit, did not have a crew during the time of the comet's stay.

Designation

This table sets out the astronomical designation for various apparitions of Halley's Comet. For example, "(1P/1982 U1, 1986 III, 1982i" indicates that for the perihelion in 1986, Halley's Comet was the first period comet known (designated 1P) and this apparition was the first seen in "half-month" U (the first half of November) 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. [8]. Note, however, that the perihelion dates farther from the present are approximate, mainly because of uncertainties in the modeling of non-gravitational effects.

Note that perihelion dates 1607 and later are in the Gregorian calendar, whilst perihelion dates of 1531 and earlier are in the Julian calendar. [9]

Meteor Showers

Because its orbit comes close to Earth's orbit in two places, Comet Halley is the parent body of two meteor showers: the Eta Aquarids in early May, and the Orionids in late October.[10]. The Eta Aquarids show orbital similarities approaching Earth as they do of Mars and so a meteor shower at Mars is anticipated there as well[11] but this time appearing to come from Lambda Gemini.