Halley's Comet

Halley's Comet

Halley's Comet on 8 March 1986
Discovery
Discovered by	Prehistoric (observation) Edmond Halley (recognition of periodicity)
Discovery date	1758 (first predicted perihelion)
Orbital characteristics[1]
Epoch 4 August 2061 (2474040.5)
Aphelion	35.14 au[2] (aphelion: 9 December 2023)[2][3]
Perihelion	0.59278 au[4] (last perihelion: 9 February 1986) (next perihelion: 28 July 2061)[4]
Semi-major axis	17.737 au
Eccentricity	0.96658
Orbital period (sidereal)	74.7 yr 75y 5m 19d (perihelion to perihelion)
Mean anomaly	0.07323°
Inclination	161.96°
Longitude of ascending node	59.396°
Time of perihelion	28 July 2061[4][5] ≈27 March 2134[6][5]
Argument of perihelion	112.05°
Earth MOID	0.075 au (11.2 million km) (epoch 1968)[7]
TJupiter	-0.598
Physical characteristics
Dimensions	14.42 km × 7.4 km × 7.4 km (Giotto)[8] 15.3 km × 7.21 km × 7.21 km (Vega)[8]
Mean diameter	11 km[8]
Mass	(2.2±0.9)×1014 kg[9]
Mean density	0.55±0.25 g/cm3[10] 0.2–1.5 g/cm3 (est.)[11]
Escape velocity	~0.002 km/s
Synodic rotation period	2.2 d (52.8 h) (?)[12]
Albedo	0.04[13]
Apparent magnitude	2.1 (in 1986)[14] 28.2 (in 2003)[15]

Halley's Comet is the only known short-period comet that is consistently visible to the naked eye from Earth,^[16] appearing every 72–80 years,^[17] though with the majority of recorded apparations (25 of 30) occurring after 75–77 years. It last appeared in the inner parts of the Solar System in 1986 and will next appear in mid-2061. Officially designated 1P/Halley, it is also commonly called Comet Halley, or sometimes simply Halley.

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 re-appearances 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 a spacecraft, Giotto, 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 ices—such 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.

Comet Halley is usually pronounced /ˈhæli/, rhyming with valley, or sometimes /ˈheɪli/, rhyming with daily.^[18][19] As to the surname Halley, Colin Ronan, one of Edmond Halley's biographers, preferred /ˈhɔːli/, rhyming with crawly.^[20] Spellings of Halley's name during his lifetime included Hailey, Haley, Hayley, Halley, Haly, Hawley, and Hawly, so its contemporary pronunciation is uncertain, but the version rhyming with valley seems to be preferred by current bearers of the surname.^[21]

Halley was the first comet to be recognised 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 disproven 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.^[22] In 1687, Sir Isaac Newton published his Philosophiæ Naturalis Principia Mathematica, 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),^[23] he was initially unable to completely reconcile comets into his model.^[24]

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.^[25] Having compiled a list of 24 comet observations, he calculated 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).^[25] Halley thus concluded that all three comets were, in fact, the same object returning about every 76 years, a period that has since been found to vary between 72 and 80 years.^[17] After a rough estimate of the perturbations the comet would sustain from the gravitational attraction of the planets, he predicted its return for 1758.^[26] While he had personally observed the comet around perihelion in September 1682,^[27] Halley died in 1742 before he could observe its predicted return.^[28]

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. Other observers from throughout Europe and its colonies sent in confirmations to Paris after the comet brightened the following spring. In the Americas, John Winthrop lectured at Harvard University to explain the implications of the comet's reappearance for Newtonian mechanics and natural theology.^[29]

Another independent recognition that the comet had returned was made by the Jamaican astronomer Francis Williams, but his observations did not reach Europe.^[30][31] A unique portrait commissioned by Williams demonstrates the impact of the comet's return on period astronomers. Williams' hand rests on the page of Newton's Principia with procedures to predict comet sightings. The white smudge in the sky is a depiction of Halley's comet relative to the constellations in March 1759. The chord hanging above the book represents the comet's orbit.^[32][31] In 2024, using X-ray imaging, the painting was shown to depict the field of stars in which the comet would have been visible in 1759. Williams likely commissioned the portrait to commemorate his observations.^[31]

The comet did not pass through its perihelion until 13 March 1759, the attraction of Jupiter and Saturn having caused a delay of 618 days.^[33] This effect was computed before its return (with a one-month error to 13 April)^[34] by a team of three French mathematicians, Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute.^[35] The confirmation of the comet's return was the first time anything other than planets had been shown to orbit the Sun.^[36] It was also one of the earliest successful tests of Newtonian physics, and a clear demonstration of its explanatory power.^[37] The comet was first named in Halley's honour by French astronomer Nicolas-Louis de Lacaille in 1759.^[37]

Some scholars have proposed that first-century Mesopotamian astronomers already had recognised Halley's Comet as periodic.^[38] This theory notes a passage in the Babylonian Talmud, tractate Horayot^[39] that refers to "a star which appears once in seventy years that makes the captains of the ships err".^[40] Others doubt this idea based on historical considerations about the exact timing of this alleged observation, and suggest it refers to the variable star Mira.^[41]

Researchers in 1981 attempting to calculate the past orbits of Halley by numerical integration starting from accurate observations in the seventeenth and eighteenth centuries could not produce accurate results further back than 837 owing to a close approach to Earth in that year. It was necessary to use ancient Chinese comet observations to constrain their calculations.^[42]

The orbital path of Halley, against the orbits of the planets (animation)

Orionid meteor originating from Halley's Comet streaking the sky below the Milky Way and to the right of Venus

Halley's orbital period has varied between 74 and 80 years since 240 BC.^[43] Its orbit around the Sun is highly elliptical, with an orbital eccentricity of 0.967 (with 0 being a circle and 1 being a parabolic trajectory). The perihelion, the point in the comet's orbit when it is nearest the Sun, is 0.59 au (88 million km). This is between the orbits of Mercury and Venus. Its aphelion, or farthest distance from the Sun, is 35 au (5.2 billion km), roughly the orbital distance of Pluto. Unlike the overwhelming majority of objects in the Solar System, Halley's orbit is retrograde; it orbits the Sun in the opposite direction to the planets, or, clockwise from above the Sun's north pole.^[44] The orbit is inclined by 18° to the ecliptic, with much of it lying south of the ecliptic.^[45] This is usually represented as 162°, to account for Halley's retrograde orbit.^[46][1] The 1910 passage was at a relative velocity of 70.56 km/s (157,800 mph).^[7] Because its orbit comes close to Earth's in two places, Halley is associated with two meteor showers: the Eta Aquariids in early May, and the Orionids in late October.^[47]

Halley is classified as a periodic or short-period comet: one with an orbit lasting 200 years or less.^[48] This contrasts it with long-period comets, whose orbits last for thousands of years. Periodic comets have an average inclination to the ecliptic of only ten degrees, and an orbital period of just 6.5 years, so Halley's orbit is atypical.^[37] Most short-period comets (those with orbital periods shorter than 20 years and inclinations of 30 degrees or less) are called Jupiter-family comets.^[48] Those resembling Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets.^[48][49] As of 2024^[update], 105 Halley-type comets have been observed, compared with 816 identified Jupiter-family comets.^[50][51]

The orbits of the Halley-type comets suggest that they were originally long-period comets whose orbits were perturbed by the gravity of the giant planets and directed into the inner Solar System.^[48] If Halley was once a long-period comet, it is likely to have originated in the Oort cloud,^[49] a sphere of cometary bodies around 20,000–50,000 au from the Sun. Conversely the Jupiter-family comets are generally believed to originate in the Kuiper belt,^[49] a flat disc of icy debris between 30 au (Neptune's orbit) and 50 au from the Sun (in the scattered disc). Another point of origin for the Halley-type comets was proposed in 2008, when a trans-Neptunian object with a retrograde orbit similar to Halley's was discovered, 2008 KV42, whose orbit takes it from just outside that of Uranus to twice the distance of Pluto. It may be a member of a new population of small Solar System bodies that serves as the source of Halley-type comets.^[52]

Halley has probably been in its current orbit for 16,000–200,000 years, although it is not possible to numerically integrate its orbit for more than a few tens of apparitions, and close approaches before 837 AD can only be verified from recorded observations.^[53] The non-gravitational effects can be crucial;^[53] as Halley approaches the Sun, it expels jets of sublimating gas from its surface, which knock it very slightly off its orbital path. These orbital changes cause delays in its perihelion passage of four days on average.^[54]

In 1989, Boris Chirikov and Vitold Vecheslavov performed an analysis of 46 apparitions of Halley's Comet taken from historical records and computer simulations, which showed that its dynamics were chaotic and unpredictable on long timescales.^[55] Halley's projected dynamical lifetime is estimated to be about 10 million years.^[56] The dynamics of its orbit can be approximately described by a two-dimensional symplectic map, known as the Kepler map, a solution to the restricted three-body problem for highly eccentric orbits.^[56][57] Based on records from the 1910 apparition, David Hughes calculated in 1985 that Halley's nucleus has been reduced in mass by 80 to 90% over the last 2,000 to 3,000 revolutions, and that it will most likely disappear completely after another 2,300 perihelion passages.^[58] More recent work suggests that Halley will evaporate, or split in two, within the next few tens of thousands of years, or will be ejected from the Solar System within a few hundred thousand years.^[59][49]

The Giotto and Vega missions gave planetary scientists their first view of Halley's surface and structure. The nucleus is a conglomerate of ices and dust, often referred to as a "dirty snowball".^[16] Like all comets, as Halley nears the Sun, its volatile compounds (those with low boiling points, such as water, carbon monoxide, carbon dioxide and other ices) begin to sublimate from the surface.^[60] This causes the comet to develop a coma, or atmosphere, at distances up to 230,000 kilometres (140,000 mi) from the nucleus.^[61] Sublimation of this dirty ice releases dust particles, which travel with the gas away from the nucleus. Gas molecules in the coma absorb solar light and then re-radiate it at different wavelengths, a phenomenon known as fluorescence, whereas dust particles scatter the solar light. Both processes are responsible for making the coma visible.^[16] As a fraction of the gas molecules in the coma are ionised by the solar ultraviolet radiation,^[16] pressure from the solar wind, a stream of charged particles emitted by the Sun, pulls the coma's ions out into a long tail, which may extend more than 100 million kilometres into space.^[60][62] Changes in the flow of the solar wind can cause disconnection events, in which the tail completely breaks off from the nucleus.^[63]

Despite the vast size of its coma, Halley's nucleus is relatively small: barely 15 kilometres (9.3 mi) long, 8 kilometres (5.0 mi) wide and perhaps 8 kilometres (5.0 mi) thick.^[64][65] Based on a reanalysis of images taken by the Giotto and Vega spacecraft, Lamy et al. determined an effective diameter of 11 kilometres (6.8 mi).^[8][65] Its shape has been variously compared to that of a peanut, a potato, or an avocado.^[66] Its mass is roughly 2.2 × 10¹⁴ kg,^[9] with an average density of about 0.55 grams per cubic centimetre (0.32 oz/cu in).^[10] The low density indicates that it is made of a large number of small pieces, held together very loosely, forming a structure known as a rubble pile.^[67] Ground-based observations of coma brightness suggested that Halley's rotation period was about 7.4 days. Images taken by the various spacecraft, along with observations of the jets and shell, suggested a period of 52 hours.^[10][68] Given the irregular shape of the nucleus, Halley's rotation is likely to be complex.^[60] The flyby images revealed an extremely varied topography, with hills, mountains, ridges, depressions, and at least one crater.^[10]

Halley's day side (the side facing the Sun) is far more active than the night side.^[10] Spacecraft observations showed that the gases ejected from the nucleus were 80% water vapour, 17% carbon monoxide and 3–4% carbon dioxide,^[69] with traces of hydrocarbons^[70] although more recent sources give a value of 10% for carbon monoxide and also include traces of methane and ammonia.^[71] The dust particles were found to be primarily a mixture of carbon–hydrogen–oxygen–nitrogen (CHON) compounds common in the outer Solar System, and silicates, such as are found in terrestrial rocks.^[60] The dust particles ranged in size down to the limits of detection (≈0.001 μm).^[72] The ratio of deuterium to hydrogen in the water released by Halley was initially thought to be similar to that found in Earth's ocean water, suggesting that Halley-type comets may have delivered water to Earth in the distant past. Subsequent observations showed Halley's deuterium ratio to be far higher than that found in Earth's oceans, making such comets unlikely sources for Earth's water.^[60]

Giotto provided the first evidence in support of Fred Whipple's "dirty snowball" hypothesis for comet construction; Whipple postulated that comets are icy objects warmed by the Sun as they approach the inner Solar System, causing ices on their surfaces to sublime (change directly from a solid to a gas), and jets of volatile material to burst outward, creating the coma. Giotto showed that this model was broadly correct,^[60] though with modifications. Halley's albedo, for instance, is about 4%, meaning that it reflects only 4% of the sunlight hitting it – about what one would expect for coal.^[73] Thus, despite astronomers predicting that Halley would have an albedo of about 0.17 (roughly equivalent to bare soil), Halley's Comet is in fact pitch black.^[74] The "dirty ices" on the surface sublime at temperatures between 170 K (−103 °C) in sections of higher albedo to 220 K (−53 °C) at low albedo; Vega 1 found Halley's surface temperature to be in the range 300–400 K (27–127 °C). This suggested that only 10% of Halley's surface was active, and that large portions of it were coated in a layer of dark dust that retained heat.^[72] Together, these observations suggested that Halley was in fact predominantly composed of non-volatile materials, and thus more closely resembled a "snowy dirtball" than a "dirty snowball".^[10][75]

Early descriptions of Halley's comet

From left to right, top to bottom:
1. Report of Halley's Comet by Chinese astronomers in 240 BC (Shiji);
2. Observation of Halley's Comet, recorded in cuneiform on a clay tablet between 22 and 28 September 164 BC, Babylon, Iraq;
3. Possible record of Halley's Comet's 66 AD appearance in the Talmud (b. Horayot 10a);
4. The Zuqnin Chronicle's mention of Halley's Comet in 760 AD, with an illustration that includes the relative positions of Aries, Mars and Saturn in the sky;
5. The Annals of Ulster's entry for the year 912 AD, ending with Cometis apparuit ("a comet appeared").

The first certain appearance of Halley's Comet in the historical record is a description from 240 BC, in the Chinese chronicle Records of the Grand Historian or Shiji, which describes a comet that appeared in the east and moved north.^[76] The only surviving record of the 164 BC apparition is found on two fragmentary Babylonian tablets, which were rediscovered in August 1984 in the collection of the British Museum.^[77][78]

The apparition of 87 BC was recorded in Babylonian tablets which state that the comet was seen "day beyond day" for a month.^[77] This appearance may be recalled in the representation of Tigranes the Great, an Armenian king who is depicted on coins with a crown that features, according to Vahe Gurzadyan and R. Vardanyan, "a star with a curved tail [that] may represent the passage of Halley's Comet in 87 BC." Gurzadyan and Vardanyan argue that "Tigranes could have seen Halley's Comet when it passed closest to the Sun on August 6 in 87 BC" as the comet would have been a "most recordable event"; for ancient Armenians it could have heralded the New Era of the brilliant King of Kings.^[79]

The apparition of 12 BC was recorded in the Book of Han by Chinese astronomers of the Han dynasty who tracked it from August through October.^[80] It passed within 0.16 au of Earth.^[81] According to the Roman historian Cassius Dio, a comet appeared suspended over Rome for several days portending the death of Marcus Vipsanius Agrippa in that year.^[82] Halley's appearance in 12 BC, only a few years distant from the conventionally assigned date of the birth of Jesus Christ, has led some theologians and astronomers to suggest that it might explain the biblical story of the Star of Bethlehem. There are other explanations for the phenomenon, such as planetary conjunctions, and there are also records of other comets that appeared closer to the date of Jesus's birth.^[83]

If Yehoshua ben Hananiah's reference to "a star which arises once in seventy years and misleads the sailors"^[84] refers to Halley's Comet, he can only have witnessed the 66 AD appearance.^[85] Another possible report comes from Jewish historian Josephus,^[86] who wrote that in 66 AD "The signs ... were so evident, and did so plainly foretell their future desolation ... there was a star resembling a sword, which stood over the city, and a comet, that continued a whole year".^[a][87] This portent was in reference to the city of Jerusalem and the First Jewish–Roman War.^[88]

The 141 AD apparition was recorded in Chinese chronicles, with observations of a bluish white comet on 27 March and 16, 22 and 23 April.^[89]

The 374 AD and 607 approaches each came within 0.09 au of Earth.^[81] The 451 AD apparition was said to herald the defeat of Attila the Hun at the Battle of Chalons.^[90][91]

The 684 AD apparition was reported in Chinese records as the "broom star".^[92]

The 760 AD apparition was recorded in the Zuqnin Chronicle's entry for iyyōr 1071 SE (May 760 AD), calling it a "white sign":^[93]

The year [SE] one thousand seventy one (AD 759/760).

In the month of iyyōr (May) a white sign was seen in the sky, before early twilight, in the north-east [quarter], in the Zodiac [sign] which is called Aries, to the north from these three stars in it, which are very shining. And it resembled in its shape a broom [...]

And the sign itself remained for fifteen nights, until dawn of the feast of Pentecost.

— Zuqnin Chronicle, fol.136v; Neuhäuser et al. (trans.)

In 837 AD, Halley's Comet may have passed as close as 0.03 astronomical units (2.8 million miles; 4.5 million kilometres) from Earth, by far its closest approach.^[94][81] Its tail may have stretched 60 degrees across the sky. It was recorded by astronomers in China, Japan, Germany, the Byzantine Empire, and the Middle East;^[80] Emperor Louis the Pious observed this appearance and devoted himself to prayer and penance, fearing that "by this token a change in the realm and the death of a prince are made known".^[95]

In 912 AD, Halley is recorded in the Annals of Ulster, which states "A dark and rainy year. A comet appeared."^[96]

In 1066, the comet was seen in England and thought to be an omen: later that year Harold II of England died at the Battle of Hastings and William the Conqueror claimed the throne. The comet is represented on the Bayeux Tapestry and described in the tituli as a star. Surviving accounts from the period describe it as appearing to be four times the size of Venus, and shining with a light equal to a quarter of that of the Moon. Halley came within 0.10 au of Earth at that time.^[81]

This appearance of the comet is also noted in the Anglo-Saxon Chronicle. Eilmer of Malmesbury may have seen Halley in 989 and 1066, as recorded by William of Malmesbury:

Not long after, a comet, portending (they say) a change in governments, appeared, trailing its long flaming hair through the empty sky: concerning which there was a fine saying of a monk of our monastery called Æthelmær. Crouching in terror at the sight of the gleaming star, "You've come, have you?", he said. "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."^[97]

The Irish Annals of the Four Masters recorded the comet as "A star [that] appeared on the seventh of the Calends of May, on Tuesday after Little Easter, than whose light the brilliance or light of The Moon was not greater; and it was visible to all in this manner till the end of four nights afterwards."^[98] Chaco Native Americans in New Mexico may have recorded the 1066 apparition in their petroglyphs.^[99]

The Italo-Byzantine chronicle of Lupus the Protospatharios mentions that a "comet-star" appeared in the sky in the year 1067 (the chronicle is erroneous, as the event occurred in 1066, and by Robert he means William).

The Emperor Constantine Ducas died in the month of May, and his son Michael received the Empire. And in this year there appeared a comet star, and the Norman count Robert [sic] fought a battle with Harold, King of the English, and Robert was victorious and became king over the people of the English.^[100]

The 1145 apparition may have been recorded by the monk Eadwine.^[101]

According to legend, Genghis Khan was inspired to turn his conquests toward Europe by the westward-seeming trajectory of the 1222 apparition.^[102][103] In Korea, the comet was reportedly visible during the daylight on 9 September 1222.^[104]

The 1301 apparition was visually spectacular, and may be the first that resulted in convincing portraits of a particular comet. The Florentine chronicler Giovanni Villani wrote that the comet left "great trails of fumes behind", and that it remained visible from September 1301 until January 1302.^[b][92] It was seen by the artist Giotto di Bondone, who represented the Star of Bethlehem as a fire-coloured comet in the Nativity section of his Arena Chapel cycle, completed in 1305. Giotto's depiction includes details of the coma, a sweeping tail, and the central condensation. According to the art historian Roberta Olson, it is much more accurate than other contemporary descriptions, and was not equaled in painting until the 19th century.^[101][92] Olson's identification of Halley's Comet in Giotto's Adoration of the Magi is what inspired the European Space Agency to name their mission to the comet Giotto, after the artist.^[105]

Halley's 1378 appearance is recorded in the Annales Mediolanenses^[106] as well as in East Asian sources.^[107]

In 1456, the year of Halley's next apparition, the Ottoman Empire invaded the Kingdom of Hungary, culminating in the siege of Belgrade in July of that year. In a papal bull, Pope Callixtus III ordered special prayers be said for the city's protection. In 1470, the humanist scholar Bartolomeo Platina wrote in his Lives of the Popes [la] that,^[108]

A hairy and fiery star having then made its appearance for several days, the mathematicians declared that there would follow grievous pestilence, dearth and some great calamity. Calixtus, to avert the wrath of God, ordered supplications that if evils were impending for the human race He would turn all upon the Turks, the enemies of the Christian name. He likewise ordered, to move God by continual entreaty, that notice should be given by the bells to call the faithful at midday to aid by their prayers those engaged in battle with the Turk.

Platina's account is not mentioned in official records. In the 18th century, a Frenchman further embellished the story, in anger at the Church, by claiming that the Pope had "excommunicated" the comet, though this story was most likely his own invention.^[109]

Halley's apparition of 1456 was also witnessed in Kashmir and depicted in great detail by Śrīvara, a Sanskrit poet and biographer to the Sultans of Kashmir. He read the apparition as a cometary portent of doom foreshadowing the imminent fall of Sultan Zayn al-Abidin (AD 1418/1420–1470).^[110]

After witnessing a bright light in the sky which most historians have identified as Halley's Comet, Zara Yaqob, Emperor of Ethiopia from 1434 to 1468, founded the city of Debre Berhan (tr. City of Light) and made it his capital for the remainder of his reign.^[111]

Petrus Apianus and Girolamo Fracastoro described the comet's visit in 1531, with the former even including graphics in his publication. Through his observations, Apianus was able to prove that a comet's tail always points away from the Sun.^[112]

In the Sikh scriptures of the Guru Granth Sahib, the founder of the faith Guru Nanak makes reference to "a long star that has risen" at Ang 1110, and it is believed by some Sikh scholars to be a reference to Halley's appearance in 1531.^[113]

Halley's periodic returns have been subject to scientific investigation since the 16th century. The three apparitions from 1531 to 1682 were noted by Edmond Halley, enabling him to predict it would return.^[114] One key breakthrough occurred when Halley talked with Newton about his ideas of the laws of motion. Newton also helped Halley get John Flamsteed's data on the 1682 apparition.^[115] By studying data on the 1531, 1607, and 1682 comets, he came to the conclusion these were the same comet, and presented his findings in 1696.^[115]

One difficulty was accounting for variations in the comet's orbital period, which was over a year longer between 1531 and 1607 than it was between 1607 and 1682.^[116] Newton had theorised that such delays were caused by the gravity of other comets, but Halley found that Jupiter and Saturn would cause the appropriate delays.^[116] In the decades that followed, more refined mathematics would be worked on, notable by Paris Observatory; the work on Halley also provided a boost to Newton and Kepler's rules for celestial motions.^[115] (See also computation of orbit.)

At Markree Observatory in Ireland, Edward Joshua Cooper used a Cauchoix of Paris lens telescope with an aperture of 340 millimetres (13.3 in) to sketch Halley's comet in 1835.^[117] The same apparition was sketched by German astronomer Friedrich Wilhelm Bessel.^[118] Observations of streams of vapour prompted Bessel to propose that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit.^[119]

Illustrations of prior comet appearances in the January 1910 Popular Science Monthly magazine

1682

1759

1835

An interview in 1910, of someone who was a teenager at the time of the 1835 apparition had this to say:^[120]

When the comet was first seen, it appeared in the western sky, its head toward the north and tail towards the south, about horizontal and considerably above the horizon and quite a distance south of the Sun. It could be plainly seen directly after sunset every day, and was visible for a long time, perhaps a month ...

They go on to describe the comet's tail as being more broad and not as long as the comet of 1843 they had also witnessed.^[120]

Famous astronomers across the world made observations starting August 1835, including Struve at Dorpat observatory, and Sir John Herschel, who made of observations from the Cape of Good Hope.^[121] In the United States telescopic observations were made from Yale College.^[121] The new observations helped confirm early appearances of this comet including its 1456 and 1378 apparitions.^[121]

At Yale College in Connecticut, the comet was first reported on 31 August 1835 by astronomers D. Olmstead and E. Loomis.^[122] In Canada reports were made from Newfoundland and also Quebec.^[122] Reports came in from all over by later 1835, and often reported in newspapers of this time in Canada.^[122]

Several accounts of the 1835 apparition were made by observers who survived until the 1910 return, where increased interest in the comet led to their being interviewed.^[122]

The time to Halley's return in 1910 would be only 74.42 years, one of the shortest known periods of its return, which is calculated to be as long as 79 years owing to the effects of the planets.^[123]

At Paris Observatory Halley's Comet 1835 apparition was observed with a Lerebours telescope of 24.4 cm (9.6 in) aperture by the astronomer François Arago.^[124] Arago recorded polarimetric observations of Halley, and suggested that the tail might be sunlight reflecting off a sparsely distributed material; he had earlier made similar observations of Comet Tralles of 1819.^[125]

Halley's Comet in April 1910, photographed from Arequipa, Peru

A photograph of Halley's Comet taken at Yerkes Observatory on 29 May 1910

The 1910 approach, which came into naked-eye view around 10 April^[81] and came to perihelion on 20 April,^[81] was notable for several reasons: it was the first approach of which photographs exist, and the first for which spectroscopic data were obtained.^[72] Furthermore, the comet made a relatively close approach of 0.15 au,^[81] making it a spectacular sight. Indeed, on 19 May, Earth actually passed through the tail of the comet.^[126][127] One of the substances discovered in the tail by spectroscopic analysis was the toxic gas cyanogen,^[128] which led press to misquote the astronomer Camille Flammarion by stating he claimed that, when Earth passed through the tail, the gas "would impregnate the atmosphere and possibly snuff out all life on the planet".^[129][130] Despite reassurances from scientists that the gas would not inflict harm on Earth,^[130] the damage had already been done with members of the public panic buying gas masks and quack "anti-comet pills".^[131]

The comet added to the unrest in China on the eve of the Xinhai Revolution that would end the last dynasty in 1911.^[132] As James Hutson, a missionary in Sichuan Province at the time, recorded:

"The people believe that it indicates calamity such as war, fire, pestilence, and a change of dynasty. In some places on certain days the doors were unopened for half a day, no water was carried and many did not even drink water as it was rumoured that pestilential vapour was being poured down upon the earth from the comet."^[133]

The 1910 visitation coincided with a visit from Hedley Churchward, the first known English Muslim to make the Haj pilgrimage to Mecca. His explanation of its scientific predictability did not meet with favour in the Holy City.^[134]

The comet was used in an advertising campaign of Le Bon Marché, a well-known department store in Paris.^[135]

The comet was also fertile ground for hoaxes. One that reached major newspapers claimed that the Sacred Followers, a supposed Oklahoma religious group, attempted to sacrifice a virgin to ward off the impending disaster, but were stopped by the police.^[136]

American satirist and writer Mark Twain was born on 30 November 1835, exactly two weeks after the comet's perihelion. In his autobiography, published in 1909, he said,

I came in with Halley's comet in 1835. It is coming again next year, and I expect to go out with it. It will be the greatest disappointment of my life if I don't go out with Halley's comet. The Almighty has said, no doubt: "Now here are these two unaccountable freaks; they came in together, they must go out together."^[137][138]

Twain died on 21 April 1910, the day following the comet's subsequent perihelion.^[139][140] The 1985 fantasy film The Adventures of Mark Twain was inspired by the quotation.^[141]

Halley's 1910 apparition is distinct from the Great Daylight Comet of 1910, which surpassed Halley in brilliance and was visible in broad daylight for a short period, approximately four months before Halley made its appearance.^[142][143]

Halley's Comet as seen on 21 March 1986

Animation of 1P/Halley orbit - 1986 apparition
  1P/Halley ·   Earth ·   Sun

Daily motion across sky during the 1986 passage of Halley's Comet

The 1986 apparition of Halley's Comet was the least favourable on record. In February 1986, the comet and the Earth were on opposite sides of the Sun, creating the worst possible viewing circumstances for Earth observers during the previous 2,000 years.^[144] Halley's closest approach was 0.42 au.^[145] Additionally, increased light pollution from urbanisation caused many people to fail in attempts to see the comet. With the help of binoculars, observation from areas outside cities was more successful.^[146] Further, the comet appeared brightest when it was almost invisible from the northern hemisphere in March and April 1986,^[147] with best opportunities occurring when the comet could be sighted close to the horizon at dawn and dusk, if not obscured by clouds.

The approach of the comet was first detected by astronomers David C. Jewitt and G. Edward Danielson on 16 October 1982 using the 5.1 m Hale Telescope at Mount Palomar and a CCD camera.^[148]

The first visual observation of the comet on its 1986 return was by an amateur astronomer, Stephen James O'Meara, on 24 January 1985. O'Meara used a home-built 610-millimetre (24 in) telescope on top of Mauna Kea to detect the magnitude 19.6 comet.^[149] The first to observe Halley's Comet with the naked eye during its 1986 apparition were Stephen Edberg (then serving as the coordinator for amateur observations at the NASA Jet Propulsion Laboratory) and Charles Morris on 8 November 1985.^[150]

The 1986 apparition gave scientists the opportunity to study the comet closely, and several probes were launched to do so. The Soviet Vega 1 probe began returning images of Halley on 4 March 1986, captured the first-ever image of its nucleus,^[10] and made its flyby on 6 March. It was followed by the Vega 2 probe, making its flyby on 9 March. On 14 March, the Giotto space probe, launched by the European Space Agency, made the closest pass of the comet's nucleus.^[10] There also were two Japanese probes, Suisei and Sakigake. Unofficially, the numerous probes became known as the Halley Armada.^[151]

Based on data retrieved by the largest ultraviolet space telescope of the time, Astron, in December 1985, a group of Soviet scientists developed a model of the comet's coma.^[152] The comet also was observed from space by the International Cometary Explorer (ICE). Originally launched as the International Sun-Earth Explorer 3, the spacecraft was renamed, and departed the Sun-Earth L₁ Lagrangian point in 1982 in order to intercept the comets 21P/Giacobini-Zinner and Halley.^[153] ICE flew through the tail of Halley's Comet, coming within about 40.2 million km (25.0 million mi) of the nucleus on 28 March 1986.^[154][155]

Two U.S. Space Shuttle missions—STS-51-L and STS-61-E—had been scheduled to observe Halley's Comet from low Earth orbit. The STS-51-L mission carried the Shuttle-Pointed Tool for Astronomy (Spartan Halley) satellite, also called the Halley's Comet Experiment Deployable (HCED).^[156] The mission to capture the ultraviolet spectrum of the comet ended in disaster when the Space Shuttle Challenger exploded in flight, killing all seven astronauts onboard.^[157] Scheduled for March 1986, STS-61-E was a Columbia mission carrying the ASTRO-1 platform to study the comet,^[158] but the mission was cancelled following the Challenger disaster and ASTRO-1 would not fly until late 1990 on STS-35.^[159]

In Japan, the comet was observed by Emperor Hirohito, who was 84. He had already seen it in 1910 when he was 8. He became one of the few people in human history to have seen Halley's Comet on two different cycles.^[160]

On 12 February 1991, at a distance of 14.3 au (2.14×10⁹ km) from the Sun, Halley displayed an outburst that lasted for several months.^[161][60] The comet released dust with a total mass of about 10⁸ kg, which spread into an elongated cloud roughly 374,000 km (232,000 mi) by 269,000 km (167,000 mi) in size.^[162] The outburst likely started in December 1990, and then the comet brightened from about magnitude 25 to magnitude 19.^[163][161] Comets rarely show outburst activity at distances beyond 5 au from the Sun.^[161] Different mechanisms have been proposed for the outburst, ranging from interaction with the solar wind to a collision with an undiscovered asteroid.^[164] The most likely explanation is a combination of two effects, the polymerisation of hydrogen cyanide and a phase transition of amorphous water ice, which raised the temperature of the nucleus enough for some of the more volatile compounds on its surface to sublime.^[161]

Halley was most recently observed in 2003 by three of the Very Large Telescopes at Paranal, Chile, when Halley's magnitude was 28.2. The telescopes observed Halley, at the faintest and farthest any comet had ever been imaged, in order to verify a method for finding very faint trans-Neptunian objects.^[15] Astronomers are now able to observe the comet at any point in its orbit.^[15]

On 9 December 2023, Halley's Comet reached the farthest and slowest point in its orbit from the Sun when it was travelling at 0.91 km/s (2,000 mph) with respect to the Sun.^[2][3]

The next perihelion of Halley's Comet is predicted for 28 July 2061,^[4][5] when it will be better positioned for observation than during the 1985–1986 apparition, as it will be on the same side of the Sun as Earth.^[165] The closest approach to Earth will be one day after perihelion.^[7] It is expected to have an apparent magnitude of −0.3, compared with only +2.1 for the 1986 apparition.^[14] On 9 September 2060, Halley will pass within 0.98 au (147,000,000 km) of Jupiter, and then on 20 August 2061 will pass within 0.0543 au (8,120,000 km) of Venus.^[7]

Halley will come to perihelion on 27 March 2134.^[6][5] Then on 7 May 2134, Halley will pass within 0.092 au (13,800,000 km) of Earth.^[7] Its apparent magnitude is expected to be −2.0.^[14]

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 sighting.^[7][166]

In the designations, "1P/" refers to Halley's Comet; the first periodic comet discovered. The number represents the year, with negatives representing BC. The letter-number combination indicates which it was of the comets observed for a given segment of the year, divided into 24 equal parts.^[167] The Roman numeral indicates which comet past perihelion it was for a given year, while the lower-case letter indicates which comet it was for a given year overall.^[168] 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.^[169] The perihelion dates for some of the early apparitions (particularly before 837 AD) are uncertain by a couple of days.^[169] While Halley's Comet usually peaks at around 0th magnitude, there are indications that the comet got considerably brighter than that in the past.^[170]

• Halley's Comet in fiction
• Kepler orbit
• List of Halley-type comets

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• Needham, Joseph (1959). "Comets, meteors, and meteorites". Science and Civilisation in China: Volume 3, Mathematics and the Sciences of the Heavens and the Earth. Cambridge University Press. pp. 430–433. ISBN 978-0-521-05801-8.
• Rayner, John D. (1998). A Jewish Understanding of the World. Berghahn Books. pp. 108–111. ISBN 1-57181-973-8.
• Sagan, Carl; Druyan, Ann (1985). Comet. Random House. ISBN 0-394-54908-2.
• Schmude, Richard M. (2010). Comets and How to Observe Them. Springer. p. 3. ISBN 978-1-4419-5790-0.
• Shayler, David J.; Burgess, Colin (2007). "Ending of Eras". NASA's Scientist-Astronauts. Praxis. ISBN 978-0-387-21897-7. Retrieved 7 January 2009.
• Siddiqi, Asif A. (2018). Beyond Earth: A Chronicle of Deep Space Exploration. United States: NASA History Program Office. pp. 149–150. ISBN 9781626830424.
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Wikimedia Commons has media related to Comet Halley.

• Synopsis of the Astronomy of Comets (1706 reprint of Halley's 1705 paper)
• Image of Halley's Comet by the Giotto spacecraft
• seds.org, links to images and further information about Halley's Comet
• Photographs of 1910 approach from the Lick Observatory Records Digital Archive
• Ephemeris from JPL