Halley's Comet

Template:Short description Template:Distinguish {{#invoke:other uses|otheruses}} Template:Featured article Template:Use dmy dates Template:Use British English

{{#invoke:infobox|infoboxTemplate | class = vcard | titleclass = fn org | title = Halley's Comet | image = {{#invoke:InfoboxImage|InfoboxImage|image=Lspn comet halley.jpg|upright={{#if:||1.1}}|alt=A colour image of comet Halley, shown flying to the left moon aligned flat against the sky}} | caption = Halley's Comet on 8 March 1986 | headerstyle = {{#if:#FFE0C2|background-color:#FFE0C2|background-color:#E0CCFF}} | labelstyle = max-width:{{#if:||11em}}; | autoheaders = y

| header1 = Discovery

| header10 = {{#if:|Designations|Designations}}

| header20 = Orbital characteristics{{#ifeq:|yes| (barycentric)}}<ref name=MPC/>

| data21 = | data22 = {{#if:4 August 2061 (2474040.5) |Epoch 4 August 2061 (2474040.5)}} | data23 = {{#if: | Uncertainty parameter {{{uncertainty}}}}} | label24 = Observation arc | data24 = | label25 = Earliest precovery date | data25 = | label26 = {{#switch:{{{apsis}}} |apsis|gee|barion|center|centre|(apsis)=Apo{{{apsis}}} |Ap{{#if:|{{{apsis}}}|helion}}}} | data26 = 35.14 au<ref name="Horizons2023"/>
(aphelion: 9 December 2023)<ref name="Horizons2023"/><ref name=seeker2013/> | label27 = Peri{{#if:|{{{apsis}}}|helion}} | data27 = 0.59278 au<ref name="Horizons2061"/>
(last perihelion: 9 February 1986)
(next perihelion: 28 July 2061)<ref name="Horizons2061"/> | label28 = Peri{{#if:|{{{apsis}}}|apsis}} | data28 = | label29 = {{#switch:{{{apsis}}} |helion|astron=Ap{{{apsis}}} |Apo{{#if:|{{{apsis}}}|apsis}}}} | data29 = | label30 = Periastron | data30 = | label31 = Apoastron | data31 = | label32 = Template:Longitem | data32 = 17.737 au | label33 = Template:Longitem | data33 = | label34 = Eccentricity | data34 = 0.96658 | label35 = Template:Longitem | data35 = 74.7 yr
Template:Time interval (perihelion to perihelion) | label36 = Template:Longitem | data36 = | label37 = Template:Longitem | data37 = | label38 = Template:Longitem | data38 = 0.07323° | label39 = Template:Longitem | data39 = | label40 = Inclination | data40 = 161.96° | label41 = Template:Longitem | data41 = | label42 = Template:Longitem | data42 = 59.396° | label43 = Template:Longitem | data43 = | label44 = Template:Longitem | data44 = 28 July 2061<ref name="Horizons2061"/><ref name="Kinoshita"/>
≈27 March 2134<ref name="Horizons2134"/><ref name="Kinoshita"/> | label45 = Template:Longitem | data45 = 112.05° | label46 = Template:Nowrap | data46 = | label47 = Satellite of | data47 = | label48 = Group | data48 = | label49 = {{#switch: |yes|true=Satellites |Known satellites}} | data49 = | label50 = Star | data50 = | label51 = Earth MOID | data51 = Template:Convert
(epoch 1968)<ref name=jpldata/> | label52 = Mercury MOID | data52 = | label53 = Venus MOID | data53 = | label54 = Mars MOID | data54 = | label55 = Jupiter MOID | data55 = | label56 = Saturn MOID | data56 = | label57 = Uranus MOID | data57 = | label58 = Neptune MOID | data58 = | label59 = T_Jupiter | data59 = -0.598

| header60 = Proper orbital elements

| label61 = Template:Longitem | data61 = {{#if: |{{{p_semimajor}}} AU}} | label62 = Template:Longitem | data62 = | label63 = Template:Longitem | data63 = | label64 = Template:Longitem | data64 = {{#if: |{{{p_mean_motion}}} deg Template:\yr}} | label65 = Template:Longitem | data65 = {{#if:|{{#expr:360/1 round 5}} yr
({{#expr:365.25*360/1 round 3}} d) }} | label66 = Template:Longitem | data66 = {{#if:|{{{perihelion_rate}}} arcsec Template:\yr }} | label67 = Template:Longitem | data67 = {{#if:|{{{node_rate}}} arcsec Template:\yr}}

| header70 = Template:Anchor{{#if:| Physical characteristics|Physical characteristics}}

| label71 = Dimensions | data71 = Template:Val (Giotto)<ref name="Lamy2004"/>
Template:Val (Vega)<ref name="Lamy2004"/> | label72 = Template:Longitem | data72 = Template:Val<ref name="Lamy2004"/> | label73 = Template:Longitem | data73 = | label74 = Template:Longitem | data74 = | label75 = Template:Longitem | data75 = | label76 = Flattening | data76 = | label77 = Circumference | data77 = | label78 = Template:Longitem | data78 = | label79 = Volume | data79 = | label80 = Mass | data80 = Template:Val<ref name="mass"/> | label81 = Template:Longitem | data81 = Template:Val<ref name="situ"/>
Template:Val (est.)<ref name="Peale1989November"/> | label82 = Template:Longitem | data82 = | label83 = Template:Longitem | data83 = | label84 = Template:Longitem | data84 = ~Template:V2 km/s | label85 = Template:Longitem | data85 = 2.2 d (52.8 h) (?)<ref name="Peale1989"/> | label86 = Template:Longitem | data86 = | label87 = Template:Longitem | data87 = | label88 = Template:Longitem | data88 = | label89 = Template:Longitem | data89 = | label90 = Template:Longitem | data90 = | label91 = Template:Longitem | data91 = | label92 = Template:Longitem | data92 = | label93 = {{#if: |Template:Longitem |Albedo}} | data93 = 0.04<ref name="dark"/> | label94 = Temperature | data94 =

| data100 = {{#if:|

Surface temp.	min	mean	max
{{{temp_name1}}}
{{{temp_name2}}}
{{{temp_name3}}}
{{{temp_name4}}}

}}

| header110 = Atmosphere

| below = {{#if:||Template:Reflist }}

}}{{#invoke:Check for unknown parameters|check|unknown=Template:Main other|preview=Page using Template:Infobox planet with unknown parameter "_VALUE_"|ignoreblank=y| abs_magnitude | adjective | adjectives | albedo | allsatellites | alt_names | angular_dist | angular_size | aphelion | apoapsis | apsis | apoastron | arg_peri | asc_node | atmosphere | atmosphere_composition | atmosphere_ref | avg_speed | axial_tilt | background | barycentric | bgcolour | caption | circumference | declination | density | dimensions | discovered | discoverer | discovery_method | discovery_ref | discovery_site | earliest_precovery_date | eccentricity | epoch | equatorial_radius | escape_velocity | exosolar planets | extrasolarplanet | family | flattening | group | image | image_alt | image_scale | inclination | jupiter_moid | label_width | long_periastron | magnitude | mars_moid | mass | max_temp_1 | max_temp_2 | max_temp_3 | max_temp_4 | mean_anomaly | mean_diameter | mean_motion | mean_orbit_radius | mean_radius | mean_temp_1 | mean_temp_2 | mean_temp_3 | mean_temp_4 | mercury_moid | min_temp_1 | min_temp_2 | min_temp_3 | min_temp_4 | minorplanet | moid | moment_of_inertia_factor | mp_category | mp_name | mpc_name | name | named_after | neptune_moid | node_rate | note | observation_arc | orbit_diagram | orbit_ref | p_eccentricity | p_inclination | p_mean_motion | p_orbit_ref | p_semimajor | periapsis | periastron | perihelion | perihelion_rate | period | physical_ref | polar_radius | pole_ecliptic_lat | pole_ecliptic_lon | pronounce | pronounced | right_asc_north_pole | rot_velocity | rotation | satellite_of | satellites | saturn_moid | scale_height | semi-amplitude | semimajor | sidereal_day | single_temperature | spectral_type | star | surface_area | surface_grav | surface_pressure | surface_absorbed_dose_rate | surface_equivalent_dose_rate | symbol | synodic_period | temp_name1 | temp_name2 | temp_name3 | temp_name4 | time_periastron | tisserand | uncertainty | uranus_moid | venus_moid | volume }}

Halley's Comet is the only known short-period comet that is consistently visible to the naked eye from Earth,<ref name="Delehanty"/> appearing every 72–80 years,<ref name="Brady1987"/> though with the majority of recorded apparitions (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 several 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.

Template:TOC limit

PronunciationEdit

Comet Halley is usually pronounced Template:IPAc-en, rhyming with valley, or sometimes Template:IPAc-en, rhyming with daily.<ref name="Webster"/><ref name="Ridpath2015"/> As to the surname Halley, Colin Ronan, one of Edmond Halley's biographers, preferred Template:IPAc-en, rhyming with crawly.<ref>That is, with the vowel of hall and in some accents homophonous with holly.</ref> 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.<ref name="NYT_Q&A"/>

Computation of orbitEdit

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.Template:Sfn 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),Template:Sfn he was initially unable to completely reconcile comets into his model.<ref name="Hughes1988"/>

File:Excerpt of Halley's Letter to Newton About Comets' Orbits (MS Add.3982).jpg

"I must entreat you to procure for me of Mr Flamsteed what he has observed of the Comett of 1682 particularly in the month of September, for I am more and more confirmed that we have seen that Comett now three times, since yͤ Yeare 1531, he will not deny it you, though I know he will me." —Excerpt of Halley's letter to Newton about comet's orbits (28 September 1695)

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.Template:Sfn 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).Template:Sfn Halley thus concluded that all three comets were the same object returning about every 76 years, a period that has since been found to vary between 72 and 80 years.<ref name="Brady1987"/> After a rough estimate of the perturbations the comet would sustain from the gravitational attraction of the planets, he predicted its return for 1758.Template:Sfn He had personally observed the comet around perihelion in September 1682,<ref name="yeo_p81"/> but died in 1742 before he could observe its predicted return.Template:Sfn

File:Francis williams.jpg

Jamaican polymath Francis Williams (portrait attributed to William Williams, Template:Circa). The only contemporary illustration of an astronomer detecting the comet's return.

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 confirmations to Paris after the comet brightened early the following year. In the Americas, John Winthrop lectured at Harvard University to explain the implications of the comet's reappearance for Newtonian mechanics and natural theology.<ref name="Greene1954"/>

Another independent recognition that the comet had returned was made by the Jamaican astronomer Francis Williams, but his observations did not reach Europe.<ref name="VAM"/><ref name="Dabhoiwala2024"/> A unique portrait commissioned by Williams demonstrates the impact of the comet's return on period astronomers. Williams' hand rests on page 521 of the third edition of Newton's Principia with procedures to predict comet sightings. The white smudge in the sky is probably a depiction of Halley's comet relative to the constellations in March 1759, and the chord hanging above the book likely represents the comet's orbit.<ref name="Guardian_17Oct2024"/><ref name="Dabhoiwala2024"/> 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.<ref name="Dabhoiwala2024"/>

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.Template:Sfn This effect was computed before its return (with a one-month error to 13 April)Template:Sfn by a team of three French mathematicians, Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute.Template:Sfn The confirmation of the comet's return was the first time anything other than planets had been shown to orbit the Sun.<ref name="hughes1987"/> It was also one of the earliest successful tests of Newtonian physics, and a clear demonstration of its explanatory power.<ref name="hughes1987"/> The comet was first named in Halley's honour by French astronomer Nicolas-Louis de Lacaille in 1759.<ref name="hughes1987"/>

Some scholars have proposed that first-century Mesopotamian astronomers already had recognised Halley's Comet as periodic.<ref name="Brodetsky"/> This theory notes a passage in the Babylonian Talmud, tractate Horayot<ref name="Horioth"/> that refers to "a star which appears once in seventy years that makes the captains of the ships err".Template:Sfn It has also been suggested that the passage may have referred to the variable star Mira, whose brightness oscillates with a period of sixty years.<ref name="Veron1982"/>

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.<ref>Stephenson, F. Richard; Yau, Kevin K. C., "Oriental tales of Halley's Comet", New Scientist, vol. 103, no. 1423, pp. 30–32, 27 September 1984 {{#if:0262-4079|Template:Catalog lookup link{{#if:Template:Trim|{{#ifeq:Template:Yesno-no|yes|Template:Main other|{{#invoke:check isxn|check_issn|Template:Trim|error=Template:Error-small Template:Main other}}}}{{#if:Template:Trim|{{#ifeq:Template:Yesno-no|yes|Template:Main other|{{#invoke:check isxn|check_issn|Template:Trim|error=Template:Error-small Template:Main other}}}}{{#if:Template:Trim|{{#ifeq:Template:Yesno-no|yes|Template:Main other|{{#invoke:check isxn|check_issn|Template:Trim|error=Template:Error-small Template:Main other}}}}{{#if:Template:Trim|{{#ifeq:Template:Yesno-no|yes|Template:Main other|{{#invoke:check isxn|check_issn|Template:Trim|error=Template:Error-small Template:Main other}}}}{{#if:Template:Trim|{{#ifeq:Template:Yesno-no|yes|Template:Main other|{{#invoke:check isxn|check_issn|Template:Trim|error=Template:Error-small Template:Main other}}}}{{#if:Template:Trim|{{#ifeq:Template:Yesno-no|yes|Template:Main other|{{#invoke:check isxn|check_issn|Template:Trim|error=Template:Error-small Template:Main other}}}}{{#if:Template:Trim|{{#ifeq:Template:Yesno-no|yes|Template:Main other|{{#invoke:check isxn|check_issn|Template:Trim|error=Template:Error-small Template:Main other}}}}{{#if:Template:Trim|{{#ifeq:Template:Yesno-no|yes|Template:Main other|{{#invoke:check isxn|check_issn|Template:Trim|error=Template:Error-small Template:Main other}}}}{{#if:Template:Trim|{{#ifeq:Template:Yesno-no|yes|Template:Main other|{{#invoke:check isxn|check_issn|Template:Trim|error=Template:Error-small Template:Main other}}}}}}}}}}}}}}}}}}}}}}|Template:Error-small}}</ref>

Orbit and originEdit

Template:Multiple image Halley's orbital period has varied between 74 and 80 years since 240 BC.<ref name="yeo"/> 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 Template:Convert. This is between the orbits of Mercury and Venus. Its aphelion, or farthest distance from the Sun, is Template:Convert, 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.<ref name="Hromakina2021"/> The orbit is inclined by 18° to the ecliptic, with much of it lying south of the ecliptic.<ref name="Russell1988"/> This is usually represented as 162°, to account for Halley's retrograde orbit.<ref name="NASA_chapter5"/><ref name=MPC/> The 1910 passage was at a relative velocity of Template:Convert.<ref name="jpldata"/> 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.<ref name="Streams"/>

Halley is classified as a periodic or short-period comet: one with an orbit lasting 200 years or less.<ref name="Morbidelli2006"/> 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.<ref name="hughes1987"/> Most short-period comets (those with orbital periods shorter than 20 years and inclinations of 30 degrees or less) are called Jupiter-family comets.<ref name="Morbidelli2006"/> 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.<ref name="Morbidelli2006"/><ref name="jewitt2002"/> Template:As of, 105 Halley-type comets have been observed, compared with 816 identified Jupiter-family comets.<ref name="jpl_JFC-list"/><ref name="yfernandez"/>

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.<ref name="Morbidelli2006"/> If Halley was once a long-period comet, it is likely to have originated in the Oort cloud,<ref name="jewitt2002"/> 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,<ref name="jewitt2002"/> 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, Template:Mpl, 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.<ref name="Gladman2009"/>

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.<ref name="Olsson1987"/> The non-gravitational effects can be crucial;<ref name="Olsson1987"/> 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.Template:Sfn

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.<ref name="Chirikov1989b"/> Halley's projected dynamical lifetime is estimated to be about 10 million years.<ref name="Chirikov1989"/> 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.<ref name="Chirikov1989"/><ref name="Lages2018"/> 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.<ref name="Hughes1985"/> 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.<ref name="Williams2015"/><ref name="jewitt2002"/>

Structure and compositionEdit

File:Comet Halley close up-cropped.jpg

The nucleus of Halley's Comet, imaged by the Giotto probe on 14Template:NbspMarch 1986. The dark colouration of the nucleus can be observed, as well as the jets of dust and gas erupting from its surface.

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".<ref name="Delehanty"/> 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.<ref name="graw"/> This causes the comet to develop a coma, or atmosphere, at distances up to Template:Convert from the nucleus.<ref name="Altwegg1993"/> 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.<ref name="Delehanty"/> As a fraction of the gas molecules in the coma are ionised by the solar ultraviolet radiation,<ref name="Delehanty"/> 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.<ref name="graw"/><ref name="Biermann1958"/> Changes in the flow of the solar wind can cause disconnection events, in which the tail completely breaks off from the nucleus.<ref name="Brandt1987"/>

Despite the vast size of its coma, Halley's nucleus is relatively small: barely Template:Convert long, Template:Convert wide and perhaps Template:Convert thick.<ref name="Keller1987"/><ref name="Reitsema"/> Based on a reanalysis of images taken by the Giotto and Vega spacecraft, Lamy et al. determined an effective diameter of Template:Convert.<ref name="Lamy2004"/><ref name="Reitsema"/> Its shape has been variously compared to that of a peanut, a potato, or an avocado.<ref name="Mendis1986"/> Its mass is roughly 2.2Template:E-sp kg,<ref name="mass"/> with an average density of about Template:Convert.<ref name="situ"/> 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.<ref name="density"/> 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.<ref name="situ"/><ref name="BriceHoover2004"/> Given the irregular shape of the nucleus, Halley's rotation is likely to be complex.<ref name="graw"/> The flyby images revealed an extremely varied topography, with hills, mountains, ridges, depressions, and at least one crater.<ref name="situ"/>

Halley's day side (the side facing the Sun) is far more active than the night side.<ref name="situ"/> Spacecraft observations showed that the gases ejected from the nucleus were 80% water vapour, 17% carbon monoxide and 3–4% carbon dioxide,<ref name="Woods1986"/> with traces of hydrocarbons<ref name="Chyba1987"/> although more recent sources give a value of 10% for carbon monoxide and also include traces of methane and ammonia.<ref name="ESA_2006"/> 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.<ref name="graw"/> The dust particles ranged in size down to the limits of detection (≈0.001 μm).<ref name="post"/> 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.<ref name="graw"/>

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,<ref name="graw"/> 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.<ref name="Weaver1997"/> 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.<ref name="Belton1982"/> The "dirty ices" on the surface sublime at temperatures between Template:Convert in sections of higher albedo to Template:Convert at low albedo; Vega 1 found Halley's surface temperature to be in the range Template:Convert. 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.<ref name="post"/> 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".<ref name="situ"/><ref name="NASA_explore"/>

HistoryEdit

Before 1066Edit

Template:Multiple image

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 BC, in the Chinese chronicle Records of the Grand Historian or Shiji, which describes a comet that appeared in the east and moved north.Template:Sfn 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.<ref name="Stephenson1985"/><ref name="Walker1985"/>

The apparition of 87 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 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.<ref name="Gurzadyan2004"/>

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.<ref name="kronk"/> It passed within 0.16 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 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.<ref name="Humphreys1995"/>

If 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 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".Template:Efn<ref name="Josephus"/> This portent was in reference to the city of Jerusalem and the First Jewish–Roman War.<ref name="Horowitz1996"/>

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.<ref name="Ravené1897"/> The early Tamil bards of southern India (c. 1st - 4th century CE) also describe a certain relatable event.<ref name=":02">Template:Cite book</ref>

The 374 AD and 607 approaches each came within 0.09 au of Earth.<ref name="greatcomets"/> The 451 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 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 SE (May 760 AD), calling it a "white sign":<ref name="Neuhäuser2021"/>

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.{{#if:Zuqnin Chronicle, fol.136v; Neuhäuser et al. (trans.)|{{#if:|}}

— {{#if:|, in }}Template:Comma separated entries
}}

{{#invoke:Check for unknown parameters|check|unknown=Template:Main other|preview=Page using Template:Blockquote with unknown parameter "_VALUE_"|ignoreblank=y| 1 | 2 | 3 | 4 | 5 | author | by | char | character | cite | class | content | multiline | personquoted | publication | quote | quotesource | quotetext | sign | source | style | text | title | ts }}

In 837 AD, Halley's Comet may have passed as close as Template:Convert from Earth, by far its closest approach.<ref name="Horizons837AD"/><ref name="greatcomets"/> 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;<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"/>

1066Edit

File:Comete Tapisserie Bayeux.jpg

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 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.<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:

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>

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 Native Americans in New Mexico may have recorded the 1066 apparition in their petroglyphs.<ref name="Brazil2005"/>

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.<ref name="Lupus"/>

1145–1378Edit

File:Giotto - Scrovegni - -18- - Adoration of the Magi.jpg

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.

The 1145 apparition may have been recorded by the monk 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 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.Template:Efn<ref name="art"/> 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.<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.Template:Sfn

1456Edit

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 Template:Interlanguage link that,<ref name="Emerson"/>

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.<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 Sultan Zayn al-Abidin (AD 1418/1420–1470).<ref>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>

1531-1759Edit

File:Astronomicum Caesareum (1540), p.89v (comet illustration).jpg

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.Template:Sfn 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.Template:Sfn 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.Template:Sfn 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.)

1835Edit

At Markree Observatory in Ireland, Edward Joshua Cooper used a Cauchoix of Paris lens telescope with an aperture of Template:Convert 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 forces of evaporating material could be great enough to significantly alter a comet's orbit.Template:Sfn

Template:Multiple image 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"/>

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 ...{{#if:|{{#if:|}}
— {{#if:|, in }}Template:Comma separated entries
}}

{{#invoke:Check for unknown parameters|check|unknown=Template:Main other|preview=Page using Template:Blockquote with unknown parameter "_VALUE_"|ignoreblank=y| 1 | 2 | 3 | 4 | 5 | author | by | char | character | cite | class | content | multiline | personquoted | publication | quote | quotesource | quotetext | sign | source | style | text | title | ts }}

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 Template:Convert aperture by the astronomer François Arago.Template:Sfn 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"/>

1910Edit

Template:Multiple image 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".Template:Sfnm<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 quack "anti-comet pills".Template:Sfn

The comet added to the unrest in China on the eve of the Xinhai Revolution that would end the last dynasty in 1911.<ref name="TingChen2022"/> 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."<ref name="Hutson1921"/>

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

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 was inspired by the quotation.<ref name="TwainMovie"/>

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.<ref name="Comet1910"/><ref name="Bortle1998"/>

1986Edit

Template:Multiple image 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 m Hale Telescope at 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 Template:Convert telescope on top of Mauna Kea to detect the 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 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 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 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, in December 1985, a group of Soviet scientists developed a model of the comet's 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 Template: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 Template:Convert of the nucleus on 28 March 1986.<ref name="ISEE-3"/>Template:Sfn

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 spectrum of the comet 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,Template:Sfn 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 1986Edit

File:ESO-Comet Halley at 28 AU-phot-27a-03-fullres.jpg

Halley's Comet observed in 2003 at 28 au from the Sun

On 12 February 1991, at a distance of Template:Convert 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 10⁸ kg, which spread into an elongated cloud roughly Template:Cvt by Template:Cvt 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 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.<ref name="Gronkowski2002"/>

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.<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 Template:Convert with respect to the Sun.<ref name="Horizons2023"/><ref name=seeker2013>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

2061Edit

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 Template:Convert of Jupiter, and then on 20 August 2061 will pass within Template:Convert of Venus.<ref name="jpldata"/>

2134Edit

Halley will come to perihelion on 27 March 2134.<ref name="Horizons2134"/><ref name="Kinoshita"/> Then on 7 May 2134, Halley will pass within Template:Convert of Earth.<ref name="jpldata"/> Its apparent magnitude is expected to be −2.0.<ref name="sten"/>

ApparitionsEdit

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.<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.Template:Sfn 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"/>

Return cycle<ref name="Chang1979"/>	Designation	Year BC/AD	Gap (years)	Date of perihelion<ref name=MPC/>	Observation interval<ref name="yeo_p81"/>Template:Efn	Earth approach<ref name="greatcomets"/>	Maximum brightness<ref name="Chang1979"/>	Description<ref name="Ridpath1985"/>
−29	1P/−239 K1	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	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	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 the Hun at the 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.Template:Sfn
−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 di Bondone and included in his painting 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	Template:Hlist	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	Template:Hlist	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	Template:Hlist	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	Template:Hlist	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 on 9 February, closest to Earth (63 million km) on 10 April. Nucleus photographed by the European space probe Giotto and the Soviet probes Vega 1 and 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

NotesEdit

Template:Notelist

ReferencesEdit

Template:Reflist

BibliographyEdit

External linksEdit

Template:Sister project

Template:JPL Small Body
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
HDR Astrophotography: Simulations Atlas of Past Comets (Antiquity to 1699) by Nicolas Lefaudeux
HDR Astrophotography: Simulations Atlas of Past Comets (1900 to 1999) by Nicolas Lefaudeux

Template:PeriodicComets Navigator Template:Comets Template:Portal bar Template:Authority control