Editing Halley's Comet (section)

==History==
===Before 1066===
{{multiple image|header=Early descriptions of Halley's comet|perrow=2|total_width=350|image_gap=5
|image1=Chinese report of Halley's Comet apparition in 240 BC from the Shiji (史記).jpg
|image2=Babylonian tablet recording Halley's comet.jpg
|image3=Paris1337_Horayot_10a.png
|image4=Chronicle of Zuqnin (Vat.sir.162), fol.136v (comet excerpt).jpg
|image5=Annals of Ulster (Trinity College Dublin's MS 1282), year 912.jpg
|footer=From left to right, top to bottom:<br>'''1.''' Report of Halley's Comet by Chinese astronomers in 240{{nbsp}}BC (''[[Records of the Grand Historian|Shiji]]'');<br>'''2.''' Observation of Halley's Comet, recorded in [[Cuneiform script|cuneiform]] on a clay tablet between 22 and 28 September 164{{nbsp}}BC, [[Babylon]], Iraq;<br>'''3.''' Possible record of Halley's Comet's 66{{nbsp}}AD appearance in the [[Talmud]] (b. [[Horayot#Aggada|Horayot]] 10a);<br>'''4.''' The [[Zuqnin Chronicle]]'s mention of Halley's Comet in 760{{nbsp}}AD, with an illustration that includes the relative positions of [[Aries (constellation)|Aries]], [[Mars]] and [[Saturn]] in the sky;<br>'''5.''' The [[Annals of Ulster]]'s entry for the year 912{{nbsp}}AD, ending with ''Cometis apparuit'' ("a comet appeared").}}

Due to its intrinsic brightness, about one eighth of all comet sightings mentioned in historic records belong to Halley's Comet.<ref name="hughes1987"/> The first certain appearance of Halley's Comet in the historical record is a description from 240&nbsp;BC, in the Chinese chronicle ''[[Records of the Grand Historian]]'' or ''Shiji'', which describes a comet that appeared in the east and moved north.{{sfn|Kronk|1999|p=6}} The only surviving record of the 164&nbsp;BC apparition is found on two fragmentary Babylonian tablets, which were rediscovered in August 1984 in the collection of the [[British Museum]].<ref name="Stephenson1985"/><ref name="Walker1985"/>

The apparition of 87&nbsp;BC was recorded in Babylonian tablets which state that the comet was seen "day beyond day" for a month.<ref name="Stephenson1985"/> This appearance may be recalled in the representation of [[Tigranes the Great]], an [[Armenia]]n 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&nbsp;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&nbsp;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.<ref name="Gurzadyan2004"/>

The apparition of 12&nbsp;BC was recorded in the ''[[Book of Han]]'' by [[Chinese astronomy|Chinese astronomers]] of the [[Han dynasty]] who tracked it from August through October.<ref name="kronk"/> It passed within 0.16&nbsp;au of Earth.<ref name="greatcomets"/> 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.<ref name="Chambers"/> Halley's appearance in 12&nbsp;BC, only a few years distant from the conventionally assigned [[Chronology of Jesus#Year of birth|date of the birth of Jesus Christ]], has led some [[theologian]]s 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 conjunction]]s, and there are also records of other comets that appeared closer to the date of Jesus's birth.<ref name="Humphreys1995"/>

If [[Joshua ben Hananiah|Yehoshua ben Hananiah]]'s reference to "a star which arises once in seventy years and misleads the sailors"<ref name="Horayot10a"/> refers to Halley's Comet, he can only have witnessed the 66&nbsp;AD appearance.<ref name="Ne'eman1983"/> Another possible report comes from Jewish historian [[Josephus]],<ref name="Jaroff1985"/> 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".{{Efn|[[William Whiston]]: "Whether Josephus means that this star was different from that comet which lasted a whole year, I cannot certainly determine. His words most favour their being different one from another."}}<ref name="Josephus"/> This portent was in reference to the city of Jerusalem and the [[First Jewish–Roman War]].<ref name="Horowitz1996"/>

The 141&nbsp;AD apparition was recorded in Chinese chronicles, with observations of a bluish white comet on 27 March and 16, 22 and 23 April.<ref name="Ravené1897"/> The early Tamil bards of southern India (c. 1st - 4th century CE) also describe a certain relatable event.<ref name=":02">{{cite book |last1=Aiyar |first1=K. G. Sesha |author-link1= |url=https://archive.org/details/in.ernet.dli.2015.277483 |title=Chera Kings of the Sangam Period |publisher=Luzac and Co. |year=1937 |isbn= |location=London |pages=63–66 |chapter=Yanaik-kat-Sey Mantaran-Ceral |orig-year=}}</ref>

The 374&nbsp;AD and 607 approaches each came within 0.09&nbsp;[[astronomical unit|au]] of Earth.<ref name="greatcomets"/> The 451&nbsp;AD apparition was said to herald the defeat of [[Attila the Hun]] at the [[Battle of Chalons]].<ref name="Schultheis"/><ref name="Kronk2009"/>

The 684&nbsp;AD apparition was reported in Chinese records as the "broom star".<ref name="art"/>

The 760 AD apparition was recorded in the ''[[Zuqnin Chronicle]]'''s entry for ''iyyōr'' 1071 [[:en:Seleucid era|SE]] (May 760 [[:en:Anno Domini|AD]]), calling it a "white sign":<ref name="Neuhäuser2021"/>

{{blockquote|
text=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]].
|source=''Zuqnin Chronicle'', fol.136v; Neuhäuser et al. (trans.)
}}

In 837 AD, Halley's Comet may have passed as close as {{convert|0.03|AU|e6mi e6km|abbr=off}} from [[Earth]], by far its closest approach.<ref name="Horizons837AD"/><ref name="greatcomets"/> Its tail may have stretched 60&nbsp;[[degree (angle)|degrees]] across the sky. It was recorded by astronomers in China, Japan, Germany, the Byzantine Empire, and the Middle East;<ref name="kronk"/> 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".<ref name="Cabaniss1961"/>

In 912 AD, Halley is recorded in the ''[[Annals of Ulster]]'', which states "A dark and rainy year. A comet appeared."<ref name="annals_of_ulster"/>

===1066===
[[File:Comete Tapisserie Bayeux.jpg|thumb|Halley's Comet in 1066 depicted on the [[Bayeux Tapestry]]]]
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 [[Bayeux Tapestry tituli#32|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&nbsp;[[Astronomical unit|au]] of Earth at that time.<ref name="greatcomets"/>

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]]:

<blockquote>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."<ref name="William">William of Malmesbury; ''Gesta regum Anglorum / The history of the English Kings'', edited and translated by Mynors, R. A. B.; Thomson, R. M.; and Winterbottom, M.; 2 vols., Oxford Medieval Texts (1998–99), p. 121</ref></blockquote>

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."<ref name="annals_of_the_four_masters"/> [[Chaco Culture National Historical Park|Chaco]] [[Native Americans in the United States|Native Americans]] in [[New Mexico]] may have recorded the 1066 apparition in their petroglyphs.<ref name="Brazil2005"/>

The Italo-Byzantine chronicle of [[Lupus Protospatharius|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).

<blockquote>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.<ref name="Lupus"/></blockquote>

===1145–1378===
[[File:Giotto - Scrovegni - -18- - Adoration of the Magi.jpg|thumb|upright=1.0|The ''[[Adoration of the Magi]]'' (circa 1305) by ''Giotto'', who purportedly modelled the star of Bethlehem on Halley, which had been sighted 4 years before that painting.|alt=The wise men and several animals cluster around the baby Jesus, while a comet-like object streaks overhead]]

The 1145 apparition may have been recorded by the monk [[Eadwine Psalter|Eadwine]].<ref name="Olson1979"/>

According to legend, [[Genghis Khan]] was inspired to turn his conquests toward Europe by the westward-seeming trajectory of the 1222 apparition.<ref name="Johnson1997"/><ref name="Cook2008"/> In Korea, the comet was reportedly visible during the daylight on 9 September 1222.<ref name="Choi2017"/>

The 1301 apparition was visually spectacular, and may be the first that resulted in convincing portraits of a particular comet. The [[Florence (Italy)|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.{{efn|There are doubts about the latter date. It is generally accepted that the comet was visible from about mid September until about early November.}}<ref name="art"/> It was seen by the artist [[Giotto|Giotto di Bondone]], who represented the [[Star of Bethlehem]] as a fire-coloured comet in the [[Nativity of Jesus in art|Nativity]] section of his [[Scrovegni Chapel|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.<ref name="Olson1979"/><ref name="art"/> 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.<ref name="asteroid"/>

Halley's 1378 appearance is recorded in the ''Annales Mediolanenses''<ref>''Rerum Italicarum Scriptores'', ed. Ludovico Antonio Muratori (Milan, 1730) v. 16 col. 770.</ref> as well as in East Asian sources.{{sfn|Kronk|1999|pp=253–255}}

===1456===

In 1456, the year of Halley's next apparition, the [[Ottoman Empire]] invaded the [[Kingdom of Hungary]], culminating in the [[Siege of Belgrade (1456)|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 [[Renaissance humanism|humanist]] scholar [[Bartolomeo Platina]] wrote in his ''{{interlanguage link|Lives of the Popes (Platina)|lt=Lives of the Popes|la|Vitae Pontificum (Platina)}}'' that,<ref name="Emerson"/>

<blockquote>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.</blockquote>

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.<ref name="Botley1971"/>

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 [[Zayn al-Abidin (sultan of Kashmir)|Sultan Zayn al-Abidin]] (AD 1418/1420–1470).<ref>[[Walter Slaje|Slaje, Walter]]; inter alia, realia: "An Apparition of Halley's Comet in Kashmir observed by Śrīvara in AD 1456" in Steiner, Roland (ed.); ''Highland Philology: Results of a Text-Related Kashmir Panel at the 31st DOT, Marburg 2010'', Studia Indologica Universitatis Halensis, 4, Halle 2012: 33–48</ref>

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.<ref>The founding of Debre Berhan is described in the ''Ethiopian Royal Chronicles'' (Pankhurst, Richard; Oxford University Press, Addis Ababa, 1967, pp. 36–38).</ref>

===1531-1759===
[[File:Astronomicum Caesareum (1540), p.89v (comet illustration).jpg|thumb|Illustration of the 1531 appearance in Petrus Apianus' [[Astronomicum Caesareum]], noting that a comet's tail always points away from the sun]]

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.<ref name="Kapoor2017"/>

Halley's periodic returns have been subject to scientific investigation since the 16th century. [[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.<ref name="Barker2008"/> The three apparitions from 1531 to 1682 were noted by Edmond Halley, enabling him to predict it would return.{{sfn|Grier|2005|pp=11–25}} 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.<ref name="1985JHA"/> 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.<ref name="1985JHA"/>

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.{{sfn|Sagan|Druyan|1985|p=57}} 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.{{sfn|Sagan|Druyan|1985|p=57}} 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.<ref name="1985JHA"/> (See also [[#Computation of orbit|computation of orbit]].)

===1835===
At [[Markree Observatory]] in Ireland, [[Edward Joshua Cooper]] used a [[Robert-Aglaé Cauchoix|Cauchoix of Paris]] lens telescope with an aperture of {{convert|13.3|inch|mm|order=flip}} to sketch Halley's comet in 1835.<ref name="Abrahams2009"/> The same apparition was sketched by German astronomer [[Friedrich Wilhelm Bessel]].<ref name="Smithsonian1835"/> Observations of streams of vapour prompted Bessel to propose that the [[jet force]]s of evaporating material could be great enough to significantly alter a comet's orbit.{{sfn|Sagan|Druyan|1985|p=117}}

{{multiple image
|align=right
|direction=vertical
|width=200
|header=Illustrations of prior comet appearances in the January 1910 ''[[Popular Science|Popular Science Monthly]]'' magazine
|image1=PSM V76 D017 Halley comet in 1682 (horizontal).png
|caption1=1682
|image2=PSM V76 D017 Halley comet in 1759 (horizontal).png
|caption2=1759
|image3=PSM V76 D018 Halley comet in 1835.png
|caption3=1835
}}
An interview in 1910, of someone who was a teenager at the time of the 1835 apparition had this to say:<ref name="Todd Saw Halley's Comet in 1835"/>

{{blockquote|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.<ref name="Todd Saw Halley's Comet in 1835"/>

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.<ref name=":0"/> In the United States telescopic observations were made from [[Yale College]].<ref name=":0"/> The new observations helped confirm early appearances of this comet including its 1456 and 1378 apparitions.<ref name=":0"/>

At Yale College in Connecticut, the comet was first reported on 31 August 1835 by astronomers D. Olmstead and E. Loomis.<ref name=":2"/> In Canada reports were made from Newfoundland and also Quebec.<ref name=":2"/> Reports came in from all over by later 1835, and often reported in newspapers of this time in Canada.<ref name=":2"/>

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.<ref name=":2"/>

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.<ref name="NASA_1P-explore"/>

At [[Paris Observatory]] Halley's Comet 1835 apparition was observed with a Lerebours telescope of {{convert|24.4|cm|in|abbr=on}} aperture by the astronomer [[François Arago]].{{sfn|Lequeux|2015|p=180}} 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.<ref name="Levasseur-Regourd2019"/>

===1910===
{{multiple image
|align=right
|direction=vertical
|width=200
|image_gap=10
|image1=Halley's comet 1910 (cropped).jpg
|caption1=Halley's Comet in April 1910, photographed from [[Arequipa]], Peru
|image2=Halley's Comet - May 29 1910.jpg
|caption2=A photograph of Halley's Comet taken at [[Yerkes Observatory]] on 29 May 1910
|alt2=Black-and-white picture of the comet, its nucleus brilliant white, and its tail very prominent, moving up and to the right
}}
The 1910 approach, which came into naked-eye view around 10 April<ref name="greatcomets"/> and came to perihelion on 20 April,<ref name="greatcomets"/> was notable for several reasons: it was the first approach of which photographs exist, and the first for which [[spectroscopic]] data were obtained.<ref name="post"/> Furthermore, the comet made a relatively close approach of 0.15 au,<ref name="greatcomets"/> making it a spectacular sight. Indeed, on 19 May, Earth actually passed through the tail of the comet.<ref name="Ridpath"/><ref name="pfizer"/> One of the substances discovered in the tail by spectroscopic analysis was the toxic gas [[cyanogen]],<ref name="NYTimes_19100208"/> 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".{{sfnm|Goodrich|2023|1p=83|Goodrich|2023|2p=238}}<ref name="smith"/> Despite reassurances from scientists that the gas would not inflict harm on Earth,<ref name="smith"/> the damage had already been done with members of the public [[panic buying]] gas masks and [[Quackery|quack]] "anti-comet pills".{{sfn|Goodrich|2023|p=188}}

The comet added to the unrest in China on the eve of the [[Xinhai Revolution]] that would end the [[Qing dynasty|last dynasty]] in 1911.<ref name="TingChen2022"/> As James Hutson, a missionary in [[Sichuan]] Province at the time, recorded:

<blockquote>"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."<ref name="Hutson1921"/></blockquote>

The 1910 visitation coincided with a visit from [[Hedley Churchward]], the first known English Muslim to make the Haj pilgrimage to [[Mecca]].<ref name="Rosenthal1931"/>

The comet was used in an advertising campaign of [[Le Bon Marché]], a well-known department store in Paris.<ref name="Bibliothèque_numérique"/>

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.<ref name="Johnson1997b"/>

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

<blockquote>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."<ref name="Paine1912"/><ref name="Metcalf2009"/></blockquote>

Twain died on 21 April 1910, the day following the comet's subsequent perihelion.<ref name="Cleere1985"/><ref name="Chautauquan1910"/> The 1985 fantasy film ''[[The Adventures of Mark Twain (1985 film)|The Adventures of Mark Twain]]'' was inspired by the quotation.<ref name="TwainMovie"/>

Halley's 1910 apparition is distinct from the [[Great January Comet of 1910|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.<ref name="Comet1910"/><ref name="Bortle1998"/>

===1986===
{{multiple image
|align=right
|direction=vertical
|width=200
|image_gap=10
|image1=Comethalley-21mar1986.jpg
|caption1=Halley's Comet as seen on 21 March 1986
|image2=Animation of 1P／Halley orbit - 1986 apparition.gif
|caption2=Animation of 1P/Halley orbit - 1986 apparition<br>{{legend2|magenta|1P/Halley}}{{·}}{{legend2|Royalblue|Earth}}{{·}}{{legend2|Yellow|Sun}}
|image3=Halley path 1986.png
|caption3=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.<ref name="Broughton1979"/> Halley's closest approach was 0.42 au.<ref name="JPL_summary"/> 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.<ref name="Australian_Astronomy"/> Further, the comet appeared brightest when it was almost invisible from the northern hemisphere in March and April 1986,<ref name="Ocala"/> 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&nbsp;m [[Hale Telescope]] at [[Mount Palomar Observatory|Mount Palomar]] and a [[CCD camera]].<ref name="Recovery_ESA"/>

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 {{convert|24|in|mm|order=flip|adj=on}} telescope on top of [[Mauna Kea]] to detect the [[apparent magnitude|magnitude]] 19.6 comet.<ref name="Jan24"/> 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]] [[JPL|Jet Propulsion Laboratory]]) and Charles Morris on 8 November 1985.<ref name="Naked_eye_NYT"/>

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 [[Comet nucleus|nucleus]],<ref name="situ"/> 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.<ref name="situ"/> There also were two Japanese probes, [[Suisei (spacecraft)|''Suisei'']] and ''[[Sakigake]]''. Unofficially, the numerous probes became known as the [[Halley Armada]].<ref name="Suisei2008"/>

Based on data retrieved by the largest [[ultraviolet]] space telescope of the time, [[Astron (spacecraft)|''Astron'']], in December 1985, a group of Soviet scientists developed a model of the comet's [[Coma (cometary)|coma]].<ref name="Boyarchuk1986"/> 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.<ref name="Murdin2000"/> ICE flew through the tail of Halley's Comet, coming within about {{convert|40.2|e6km|e6mi|abbr=unit}} of the nucleus on 28 March 1986.<ref name="ISEE-3"/>{{sfn|Siddiqi|2018|pp=149–150}}

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).<ref name="Spartan-203"/> The mission to capture the ultraviolet [[Astronomical spectroscopy|spectrum]] of the comet [[Space Shuttle Challenger disaster|ended in disaster]] when the Space Shuttle ''Challenger'' disintegrated in flight, killing all seven astronauts onboard.<ref name="STS-51L"/> Scheduled for March 1986, STS-61-E was a ''Columbia'' mission carrying the ASTRO-1 platform to study the comet,{{sfn|Shayler|Burgess|2007|pp=431–476}} but the mission was cancelled following the ''Challenger'' disaster and ASTRO-1 would not fly until late 1990 on [[STS-35]].<ref name="STS-35"/>

In Japan, the comet was observed by Emperor [[Hirohito]], who was 84.<ref name="Upi1986"/> He had already seen it in 1910 when he was 8.<ref name="Upi1986"/>

===After 1986===
[[File:ESO-Comet Halley at 28 AU-phot-27a-03-fullres.jpg|thumb|right|Halley's Comet observed in 2003 at 28 au from the Sun|alt=Grainy, white-on-black image showing Halley as a barely distinguishable black dot]]

On 12 February 1991, at a distance of {{convert|14.3|au|km|abbr=on}} from the Sun, Halley displayed an outburst that lasted for several months.<ref name="Gronkowski2002"/><ref name="graw"/> The comet released dust with a total mass of about 10<sup>8</sup> kg, which spread into an elongated cloud roughly {{cvt|374,000|km}} by {{cvt|269,000|km}} in size.<ref name="West1991"/> The outburst likely started in December 1990, and then the comet brightened from about magnitude 25 to magnitude 19.<ref name="Prialnik1992"/><ref name="Gronkowski2002"/> Comets rarely show outburst activity at distances beyond 5 au from the Sun.<ref name="Gronkowski2002"/> Different mechanisms have been proposed for the outburst, ranging from interaction with the solar wind to a collision with an undiscovered [[asteroid]].<ref name="Hughes1991"/> The most likely explanation is a combination of two effects, the [[polymerization|polymerisation]] of [[hydrogen cyanide]] and a [[phase transition]] of [[Phases of ice|amorphous water ice]], which raised the temperature of the nucleus enough for some of the more volatile compounds on its surface to sublime.<ref name="Gronkowski2002"/>

Halley was most recently observed in 2003 by three of the [[Very Large Telescope]]s 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 object]]s.<ref name="ESO2003"/> Astronomers are now able to observe the comet at any point in its orbit.<ref name="ESO2003"/>

On 9 December 2023, Halley's Comet reached the farthest and slowest point in its orbit from the Sun when it was travelling at {{convert|0.91|km/s|mph|abbr=on}} with respect to the Sun.<ref name="Horizons2023"/><ref name=seeker2013>{{Cite web|url=https://www.seeker.com/lets-plan-for-a-rendezvous-with-halleys-comet-1767783657.html|title=Let's Plan For a Rendezvous With Halley's Comet|last=DNews|date=3 September 2013|website=Seeker|access-date=29 October 2019|archive-url=https://web.archive.org/web/20221128171635/https://www.seeker.com/lets-plan-for-a-rendezvous-with-halleys-comet-1767783657.html|archive-date=28 November 2022|url-status=dead}}</ref>

=== 2061 ===
The next perihelion of Halley's Comet is predicted for 28 July 2061,<ref name="Horizons2061"/><ref name="Kinoshita"/> 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.<ref name="Abrams-Planetarium"/> The closest approach to Earth will be one day after perihelion.<ref name=jpldata/> It is expected to have an [[apparent magnitude]] of −0.3, compared with only +2.1 for the 1986 apparition.<ref name="sten"/> On 9 September 2060, Halley will pass within {{convert|0.98|au|km|abbr=on}} of Jupiter, and then on 20 August 2061 will pass within {{convert|0.0543|au|km|abbr=on}} of Venus.<ref name="jpldata"/>

===2134===
Halley will come to perihelion on 27 March 2134.<ref name="Horizons2134"/><ref name="Kinoshita"/> Then on 7 May 2134, Halley will pass within {{convert|0.092|au|km|abbr=on}} of Earth.<ref name="jpldata"/> Its apparent magnitude is expected to be −2.0.<ref name="sten"/>

===Apparitions===
Halley's calculations enabled the comet's earlier appearances to be found in the historical record. The following table sets out the [[Comet#Nomenclature|astronomical designations]] for every apparition of Halley's Comet from 240&nbsp;BC, the earliest documented sighting.<ref name="jpldata"/><ref name="icqdes"/>

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.{{sfn|Schmude|2010|p=3}} 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.<ref name="Marsden1996"/> 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]].<ref name="Sitarski1988"/> The perihelion dates for some of the early apparitions (particularly before 837 AD) are uncertain by a couple of days.<ref name="Sitarski1988"/> While Halley's Comet usually peaks at around 0th magnitude, there are indications that the comet got considerably brighter than that in the past.<ref name="Chang1979"/>

{| class="wikitable" style="width:98%;"
|-valign=top
!Return cycle<ref name="Chang1979"/>
!Designation
!Year BC/AD
!Gap (years)
![[Apsis#Time of perihelion|Date of perihelion]]<ref name=MPC/>
!Observation interval<ref name="yeo_p81"/>{{efn|Prior to the [[invention of the telescope]] in the 17th century, this is the time when the comet was visible to the naked eye.}}
!Earth approach<ref name="greatcomets"/>
!Maximum brightness<ref name="Chang1979"/>
!Description<ref name="Ridpath1985"/>
|-
|−29
|1P/−239 K1
|style="white-space:nowrap;"|[[240 BC]]
| –
|30 March
|May – June
|
|
|First confirmed sighting
|-
|−28
|1P/−163 U1
|[[164 BC]]
|76
|17 November?
|October – November
|
|
|Seen by Babylonians
|-
|−27
|1P/−86 Q1
|[[87 BC]]
|77
|2 August
|9 July – 24 August
|
|
|Seen by the Babylonians and Chinese
|-
|−26
|1P/−11 Q1
|[[12 BC]]
|75
|5 October
|26 August – 20 October
|0.16 au
| −5 mag
|Watched by Chinese for two months
|-
|−25
|1P/66 B1
|[[AD 66|66]]
|77
|26 January
|31 January – 10 April
|
| −7 mag
|May be the comet described in [[Josephus]]'s ''[[The Jewish War]]'' as "A comet of the kind called Xiphias, because their tails appear to represent the blade of a sword" that supposedly heralded the destruction of the [[Second Temple]] in 70 AD.<ref name="Chambers" />
|-
|−24
|1P/141 F1
|[[AD 141|141]]
|75
|22 March
|27 March – late April
|
| −4 mag
|Described by the Chinese as bluish-white in colour
|-
|−23
|1P/218 H1
|[[218]]
|77
|17 May
|early May – mid June
|
| −4 mag
|Described by the Roman historian [[Dion Cassius]] as "a very fearful star"
|-
|−22
|1P/295 J1
|[[295]]
|77
|20 April
|1–30 May
|
| −3 mag
|Seen in China, but not spectacular
|-
|−21
|1P/374 E1
|[[374]]
|79
|17 February
|4 March – 2 April
|0.09 au
| −3 mag
|Comet passed 13.5 million kilometres from Earth.
|-
|−20
|1P/451 L1
|[[451]]
|77
|24 June
|10 June – 15 August
|
| −3 mag
|Comet appeared before the defeat of [[Attila|Attila the Hun]] at the [[Battle of Chalons (451)|Battle of Chalons]].
|-
|−19
|1P/530 Q1
|[[530]]
|79
|26 September
|29 August – 23 September
|
| −3 mag
|Noted in China and Europe, but not spectacular
|-
|−18
|1P/607 H1
|[[607]]
|77
|13 March
|March – April
|0.09 au
| −4 mag
|Comet passed 13.5 million kilometres from Earth.
|-
|−17
|1P/684 R1
|[[684]]
|77
|28 October
|September – October
|
| −2 mag
|First known Japanese records of the comet. Attempts have been made to connect an ancient Maya depiction of [[God L]] to the event.{{sfn|Milbrath|1999|p=684}}
|-
|−16
|1P/760 K1
|[[760]]
|76
|22 May
|17 May – mid June
|
| −2 mag
|Seen in China, at the same time as another comet
|-
|−15
|1P/837 F1
|[[837]]
|77
|28 February
|22 March – 28 April
|0.033 au<ref name="Horizons837AD"/>
| −3 mag
|Closest-ever approach to the Earth (5 million km). Tail stretched halfway across the sky. Appeared as bright as Venus.
|-
|−14
|1P/912 J1
|[[912]]
|75
|9 July
|July
|
| −2 mag
|Seen briefly in China and Japan
|-
|−13
|1P/989 N1
|[[989]]
|77
|9 September
|August – September
|
| −1 mag
|Seen in China, Japan, and (possibly) Korea
|-
|−12
|1P/1066 G1
|[[1066]]
|77
|23 March
|3 April – 7 June
|0.10 au
| −4 mag
|Seen for over two months in China. Recorded in England and depicted on the later [[Bayeux tapestry]] which portrayed the events of that year.
|-
|−11
|1P/1145 G1
|[[1145]]
|79
|21 April
|15 April – 6 July
|
| −2 mag
|Depicted on the [[Eadwine Psalter]], with the remark that such "hairy stars" appeared rarely, "and then as a portent"
|-
|−10
|1P/1222 R1
|[[1222]]
|77
|30 September
|3 September – 8 October
|
| −1 mag
|Described by Japanese astronomers as being "as large as the half Moon... Its colour was white but its rays were red"
|-
|−9
|1P/1301 R1
|[[1301]]
|79
|24 October
|1 September – 31 October
|
| −1 mag
|Seen by [[Giotto|Giotto di Bondone]] and included in his painting ''[[Adoration of the Magi|The Adoration of the Magi]]''. Chinese astronomers compared its brilliance to that of the first-magnitude star [[Procyon]].
|-
|−8
|1P/1378 S1
|[[1378]]
|77
|9 November
|26 September – 11 October
|
| −1 mag
|Passed within 10 degrees of the north celestial pole, more northerly than at any time during the past 2000 years. This is the last appearance of the comet for which eastern records are better than Western ones.
|-
|−7
|1P/1456 K1
|[[1456]]
|78
|9 June
|27 May – 8 July
|
| 0 mag
|Observed in Italy by [[Paolo Toscanelli]], who said its head was "as large as the eye of an ox", with a tail "fan-shaped like that of a peacock". Arabs said the tail resembled a Turkish scimitar. Turkish forces attacked Belgrade.
|-
|−6
|1P/1531 P1
|[[1531]]
|75
|25 August
|1 August – 8 September
|
| −1 mag
|Seen by [[Peter Apian]], who noted that its tail always pointed away from the Sun. This sighting was included in Halley's table.
|-
|−5
|1P/1607 S1
|[[1607]]
|76
|27 October
|21 September – 26 October
|
| 0 mag
|Seen by [[Johannes Kepler]]. This sighting was included in Halley's table.
|-
|−4
|1P/1682 Q1
|[[1682]]
|75
|15 September
|15 August – 21 September
|
| 0 mag
|Seen by [[Edmond Halley]] at Islington
|-
|−3
|{{hlist|1P/1758 Y1|1759 I}}
|[[1758]]
|76
|13 March
|25 December 1758 – 22 June 1759
|
| −1 mag
|Return predicted by Halley. First seen by [[Johann Palitzsch]] on 25 December 1758.
|-
|−2
|{{hlist|1P/1835 P1|1835 III}}
|[[1835]]
|77
|16 November
|5 August 1835 – 19 May 1836
|
| 0 mag
|First seen at the Observatory of the [[Roman College]] in August.<ref name="AsNa1836" /> Studied by [[John Herschel]] at the Cape of Good Hope.
|-
|−1
|{{hlist|1P/1909 R1|1910 II|1909c}}
|[[1910]]
|75
|20 April
|25 August 1909 – 16 June 1911
|0.151 au<ref name=jpldata/>
| 0 mag
|Photographed for the first time. Earth passed through the comet's tail on 20 May.
|-
|0
|{{hlist|1P/1982 U1|1986 III|1982i}}
|[[1986]]
|76
|9 February
|Astronomers are now able to observe the comet at every point in its orbit.<ref name="ESO2003" />
|0.417 au
| +2 mag
|Reached [[Perihelion and aphelion|perihelion]] on 9 February, closest to Earth (63 million km) on 10 April. Nucleus photographed by the European space probe [[Giotto (spacecraft)|''Giotto'']] and the Soviet probes ''[[Vega 1]]'' and ''[[Vega 2|2]]''.
|-
|1
|
|[[2061]]
|75
|28 July<ref name="Horizons2061"/><ref name="Kinoshita"/>
|
|0.477 au
|
|Next return with perihelion on 28 July 2061<ref name="Horizons2061"/><ref name="Kinoshita"/> and Earth approach one day later on 29 July 2061<ref name=jpldata/>
|-
|2
|
|2134
|73
|27 March<ref name="Horizons2134"/><ref name="Kinoshita"/>
|
|0.092 au<ref name=jpldata/>
|
|Subsequent return with perihelion on 27 March 2134 and Earth approach on 7 May 2134
|-
|3
|
|2209
|75
|3 February<ref name="Horizons2209"/>
|
|0.515 au<ref name="Horizons2209"/>
|
|Best-fit for February 2209 perihelion passage and April Earth approach

|}
{{Clear}}