Halley's Comet
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Halley's Comet or Comet Halley, officially designated 1P/Halley, is a
short-period comet A comet is an icy, small Solar System body A small Solar System body (SSSB) is an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The term was first IAU definition of planet, defined in 2006 by ...
visible from
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large list of largest lakes and seas in the Solar System, volumes of water can be found throughout the Solar System, only water distributi ...
every 75–79 years. Halley is the only known short-period
comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or Coma (cometary), coma, and sometimes also a Comet ta ...
that is regularly visible to the
naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnification, magnifying, Optical telescope#Light-gathering power, light-collecting optical instrument, such as a telescope or micr ...
from Earth, and thus the only naked-eye comet that can appear twice in a human lifetime. Halley last appeared in the inner parts of the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
in 1986 and will next appear in mid-2061. Halley's periodic returns to the
inner Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
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 Edmond (or Edmund) Halley (; – ) was an English astronomer, mathematician and physicist. He was the second Astronomer Royal in Britain, succeeding John Flamsteed in 1720. From an observatory he constructed on Saint Helena in 1676–77, Hal ...
understood that these appearances were reappearances 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
spacecraft A spacecraft is a vehicle or machine designed to spaceflight, fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth ...
, providing the first observational data on the structure of a
comet nucleus The nucleus is the solid, central part of a comet, once termed a ''dirty snowball'' or an ''icy dirtball''. A cometary nucleus is composed of Rock (geology), rock, dust, and frozen gases. When heated by the Sun, the gases Sublimation (physics), ...
and the mechanism of
coma A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions. Coma patients exhi ...
and
tail The tail is the section at the rear end of certain kinds of animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able t ...
formation. These observations supported a number of longstanding hypotheses about comet construction, particularly Fred Whipple's "dirty snowball" model, which correctly predicted that Halley would be composed of a mixture of volatile ices—such as
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
,
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon Carbon () is a chemical element with the chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetraval ...
,
ammonia Ammonia is an inorganic chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a dis ...
, and
dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in ho ...
. 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.


Pronunciation

Comet Halley is commonly pronounced , rhyming with ''valley'', or , rhyming with ''daily''. Colin Ronan, one of
Edmond Halley Edmond (or Edmund) Halley (; – ) was an English astronomer, mathematician and physicist. He was the second Astronomer Royal in Britain, succeeding John Flamsteed in 1720. From an observatory he constructed on Saint Helena in 1676–77, Hal ...
's biographers, preferred , rhyming with ''crawly''. Spellings of Halley's name during his lifetime included ''Hailey, Haley, Hayley, Halley, Hawley'', and ''Hawly'', so its contemporary pronunciation is uncertain, but current bearers of this surname appear to prefer the version that rhymes with "valley".


Computation of orbit

Halley was the first comet to be recognized as periodic. Until the
Renaissance The Renaissance ( , ) , from , with the same meanings. is a Periodization, period in History of Europe, European history marking the transition from the Middle Ages to modernity and covering the 15th and 16th centuries, characterized by an e ...
, the philosophical consensus on the nature of comets, promoted by
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical Greece, Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatet ...
, was that they were disturbances in Earth's atmosphere. This idea was disproved in 1577 by
Tycho Brahe Tycho Brahe ( ; born Tyge Ottesen Brahe; generally called Tycho (14 December 154624 October 1601) was a Danish astronomer, known for his comprehensive astronomical observations, generally considered to be the most accurate of his time. He was k ...
, who used
parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to perspective (graphica ...
measurements to show that comets must lie beyond the
Moon The Moon is Earth's only natural satellite. It is the List of natural satellites, fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth ( ...
. Many were still unconvinced that comets orbited the Sun, and assumed instead that they must follow straight paths through the Solar System.Lancaster-Brown, Peter; ''Halley & His Comet'', pp. 14, 25 In 1687,
Sir Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, Theology, theologian, and author (described in his time as a "natural philosophy, natural philosopher"), widely ...
published his ''
Philosophiæ Naturalis Principia Mathematica (English language, English: ''Mathematical Principles of Natural Philosophy'') often referred to as simply the (), is a book by Isaac Newton that expounds Newton's laws of motion and his Newton's law of universal gravitation, law of universa ...
'', in which he outlined his laws of
gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the strong ...
and motion. His work on comets was decidedly incomplete. Although he had suspected that two comets that had appeared in succession in 1680 and 1681 were the same comet before and after passing behind the Sun (he was later found to be correct; see Newton's Comet), he was unable to completely reconcile comets into his model. Ultimately, it was Newton's friend, editor and publisher,
Edmond Halley Edmond (or Edmund) Halley (; – ) was an English astronomer, mathematician and physicist. He was the second Astronomer Royal in Britain, succeeding John Flamsteed in 1720. From an observatory he constructed on Saint Helena in 1676–77, Hal ...
, 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.Lancaster-Brown, Peter; ''Halley & His Comet'', p. 76 Having compiled a list of 24 comet observations, he calculated that the
orbital elements Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are considered in two-body systems using a Kepler orbit. There are many different ways to mathematically describe the same o ...
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 Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, German mathematician, mathematician, astrologer, Natural philosophy, natural philosopher and writer on music. He is a key figure in the 17th-century Scienti ...
). Halley thus concluded that all three comets were, in fact, the same object returning about every 76 years, a period that has since been found to vary between 74 and 79 years. After a rough estimate of the perturbations the comet would sustain from the gravitational attraction of the planets, he predicted its return for 1758. While he had personally observed the comet around
perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any e ...
in September 1682, Halley died in 1742 before he could observe its predicted 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. It did not pass through its
perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any e ...
until 13 March 1759, the attraction of
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
and
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
having caused a retardation of 618 days. This effect was computed before its return (with a one-month error to 13 April) by a team of three French mathematicians,
Alexis Clairaut Alexis Claude Clairaut (; 13 May 1713 – 17 May 1765) was a French mathematician, astronomer, and geophysicist. He was a prominent Newtonian whose work helped to establish the validity of the principles and results that Isaac Newton, Sir Isaac ...
, Joseph Lalande, and Nicole-Reine Lepaute. The confirmation of the comet's return was the first time anything other than planets had been shown to orbit the Sun. It was also one of the earliest successful tests of
Newtonian physics Classical mechanics is a Theoretical physics, physical theory describing the motion of macroscopic objects, from projectiles to parts of Machine (mechanical), machinery, and astronomical objects, such as spacecraft, planets, stars, and galax ...
, and a clear demonstration of its explanatory power. The comet was first named in Halley's honour by French astronomer
Nicolas-Louis de Lacaille Abbé Nicolas-Louis de Lacaille (; 15 March 171321 March 1762), formerly sometimes spelled de la Caille, was a kingdom of France, French astronomer and geodesist who named 14 out of the IAU designated constellations, 88 constellations. From 1750 ...
in 1759. Some scholars have proposed that first-century
Mesopotamia Mesopotamia ''Mesopotamíā''; ar, بِلَاد ٱلرَّافِدَيْن or ; syc, ܐܪܡ ܢܗܪ̈ܝܢ, or , ) is a historical region of Western Asia situated within the Tigris–Euphrates river system, in the northern part of the F ...
n astronomers already had recognized Halley's Comet as periodic. This theory notes a passage in the Babylonian
Talmud The Talmud (; he, , Talmūḏ) is the central text of Rabbinic Judaism and the primary source of Jewish religious law (''halakha'') and Jewish theology. Until the advent of modernity, in nearly all Jewish communities, the Talmud was the center ...
, tractate Horayot that refers to "a star which appears once in seventy years that makes the captains of the ships err." Researchers in 1981 attempting to calculate the past orbits of Halley by
numerical integration In analysis, numerical integration comprises a broad family of algorithms for calculating the numerical value of a definite integral, and by extension, the term is also sometimes used to describe the numerical solution of differential equati ...
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.


Orbit and origin

Halley's orbital period has varied between 74 and 79 years since 240 BC. Its orbit around the
Sun The Sun is the star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other st ...
is highly elliptical, with an
orbital eccentricity In astrodynamics, the orbital eccentricity of an astronomical object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical object, physical entity, association, or structure that ex ...
of 0.967 (with 0 being a circle and 1 being a
parabolic trajectory In astrodynamics or celestial mechanics a parabolic trajectory is a Kepler orbit with the Orbital eccentricity, eccentricity equal to 1 and is an unbound orbit that is exactly on the border between elliptical and hyperbolic. When moving away fr ...
). The perihelion, the point in the comet's orbit when it is nearest the Sun, is . This is between the orbits of Mercury and
Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an Inferior and superior planets, interior planet to Earth, Venus (like Mercury (pl ...
. Its
aphelion An apsis (; ) is the farthest or nearest point in the orbit In celestial mechanics, an orbit is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a ...
, or farthest distance from the Sun, is (roughly the distance of
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
). Unusual for an object 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. The orbit is inclined by 18° to the
ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agai ...
, with much of it lying south of the ecliptic. (Because it is retrograde, the true inclination is 162°.) Owing to the retrograde orbit, it has one of the highest velocities relative to the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large list of largest lakes and seas in the Solar System, volumes of water can be found throughout the Solar System, only water distributi ...
of any object in the Solar System. The 1910 passage was at a
relative velocity The relative velocity \vec_ (also \vec_ or \vec_) is the velocity of an object or observer B in the rest frame of another object or observer A. Classical mechanics In one dimension (non-relativistic) We begin with relative motion in the classic ...
of . Because its orbit comes close to Earth's in two places, Halley is associated with two
meteor shower A meteor shower is a celestial event in which a number of meteors are observed to radiate, or originate, from one point in the night sky. These meteors are caused by streams of cosmic debris called meteoroids entering Earth's atmosphere at extre ...
s: the
Eta Aquariids The Eta Aquariids are a meteor shower associated with Halley's Comet. The shower is visible from about April 19 to about May 28 each year with peak activity on or around May 5. Unlike most major annual meteor showers, there is no sharp peak for t ...
in early May, and the
Orionids The Orionids meteor shower, often shortened to the Orionids, is the most Wiktionary:prolific#English, prolific meteor shower associated with Halley's Comet. The Orionids are so-called because the point they appear to come from, called the Radiant (m ...
in late October. Halley is the parent body to the Orionids, while observations conducted around the time of Halley's appearance in 1986 suggested that the comet could additionally perturb the Eta Aquariids, although it might not be the parent of that shower. Halley is classified as a ''periodic'' or ''short-period
comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or Coma (cometary), coma, and sometimes also a Comet ta ...
''; one with an orbit lasting 200 years or less. 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. Most short-period comets (those with orbital periods shorter than 20 years and inclinations of 20–30 degrees or less) are called Jupiter-family comets. 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. , only 75 Halley-type comets have been observed, compared with 511 identified Jupiter-family comets. 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. If Halley was once a long-period comet, it is likely to have originated in the
Oort cloud The Oort cloud (), sometimes called the Öpik–Oort cloud, first described in 1950 by the Dutch astronomer Jan Oort, is a scientific theory, theoretical concept of a cloud of predominantly volatiles, icy planetesimals proposed to surround the S ...
, 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 The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical unit, astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larg ...
, a flat disc of icy debris between 30 au (Neptune's orbit) and 50 au from the Sun (in the
scattered disc The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small solar system bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc objec ...
). Another point of origin for the Halley-type comets was proposed in 2008, when a
trans-Neptunian object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typically, ...
with a retrograde orbit similar to Halley's was discovered, , 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. 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. The non-gravitational effects can be crucial; 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 An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any e ...
of four days on average. In 1989, Boris Chirikov and Vitold Vecheslavov performed an analysis of 46 apparitions of Halley's Comet taken from historical records and computer simulations. These studies showed that its dynamics were chaotic and unpredictable on long timescales. Halley's projected lifetime could be as long as 10 million years. These studies also showed that many physical properties of Halley's Comet dynamics can be approximately described by a simple symplectic map, known as the Kepler map. 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. Observations by D. W. Hughes suggest that Halley's nucleus has been reduced in mass by 80 to 90% over the last 2,000 to 3,000 revolutions.


Structure and composition

The ''
Giotto Giotto di Bondone (; – January 8, 1337), known mononymously as Giotto ( , ) and latinisation of names, Latinised as Giottus, was an List of Italian painters, Italian painter and architect from Florence during the Late Middle Ages. He worked ...
'' and ''
Vega Vega is the brightest star in the Northern celestial hemisphere, northern constellation of Lyra. It has the Bayer designation α Lyrae, which is Latinisation of names, Latinised to Alpha Lyrae and abbreviated Alpha Lyr or α Lyr. This ...
'' missions gave planetary scientists their first view of Halley's surface and structure. Like all comets, as Halley nears the Sun, its volatile compounds (those with low boiling points, such as
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
,
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
,
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon Carbon () is a chemical element with the chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetraval ...
and other ices) begin to sublimate from the surface of its
nucleus Nucleus (plural, : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...
. This causes the comet to develop a
coma A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions. Coma patients exhi ...
, or atmosphere, up to 100,000 km across. Evaporation of this dirty ice releases
dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in ho ...
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 Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy ...
, whereas dust particles
scatter Scatter may refer to: * Scattering, in physics, the study of collisions * Statistical dispersion or scatter * Scatter (modeling), a substance used in the building of dioramas and model railways * Scatter, in computer programming, a parameter in Br ...
the solar light. Both processes are responsible for making the coma visible. As a fraction of the gas molecules in the coma are
ionized Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive Electric charge, charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged a ...
by the solar
ultraviolet radiation Ultraviolet (UV) is a form of electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic field, electromagnetic (EM) field, which propagate through space and carry momentum and electromag ...
, pressure from the
solar wind The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
, a stream of charged particles emitted by the Sun, pulls the coma's ions out into a long
tail The tail is the section at the rear end of certain kinds of animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able t ...
, which may extend more than 100 million kilometres into space. Changes in the flow of the solar wind can cause disconnection events, in which the tail completely breaks off from the nucleus. Despite the vast size of its coma, Halley's nucleus is relatively small: barely 15 kilometres long, 8 kilometres wide and perhaps 8 kilometres thick. Its shape vaguely resembles that of a peanut shell. Its mass is relatively low (roughly 2.2 kg) and its average density is about 0.6 g/cm3, indicating that it is made of a large number of small pieces, held together very loosely, forming a structure known as a
rubble pile In astronomy, a rubble pile is a celestial body that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity. Rubble piles have low density because there are large cavities between the v ...
. Ground-based observations of coma brightness suggested that Halley's
rotation period The rotation period of a celestial object (e.g., star, gas giant, planet, moon, asteroid) may refer to its sidereal rotation period, i.e. the time that the object takes to complete a single revolution around its axis of rotation relative to the b ...
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. Given the irregular shape of the nucleus, Halley's rotation is likely to be complex. Although only 25% of Halley's surface was imaged in detail during the flyby missions, the images revealed an extremely varied topography, with hills, mountains, ridges, depressions, and at least one crater. Halley is the most active of all the periodic comets, with others, such as
Comet Encke Comet Encke , or Encke's Comet (official designation: 2P/Encke), is a periodic comet that completes an orbit of the Sun once every 3.3 years. (This is the shortest period of a reasonably bright comet; the faint main-belt comet 311P/PanSTARRS ...
and Comet Holmes, being one or two
orders of magnitude An order of magnitude is an approximation of the logarithm of a value relative to some contextually understood reference value, usually 10, interpreted as the base of the logarithm and the representative of values of magnitude one. Logarithmic d ...
less active. Its day side (the side facing the Sun) is far more active than the night side. Spacecraft observations showed that the gases ejected from the nucleus were 80% water vapour, 17% carbon monoxide and 3–4% carbon dioxide, with traces of hydrocarbons although more-recent sources give a value of 10% for carbon monoxide and also include traces of
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
and
ammonia Ammonia is an inorganic chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a dis ...
. 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. The dust particles decreased in size down to the limits of detection (≈0.001 µm). The ratio of
deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two Stable isotope ratio, stable isotopes of hydrogen (the other being Hydrogen atom, protium, or hydrogen-1). The atomic nucleus, nucleus of a deuterium ato ...
to
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the chemical ...
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. ''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 sublimate (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, though with modifications. Halley's
albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
, for instance, is about 4%, meaning that it reflects only 4% of the sunlight hitting it; about what one would expect for coal. Thus, despite appearing brilliant white to observers on Earth, Halley's Comet is in fact pitch black. The surface temperature of evaporating "dirty ice" ranges from at higher albedo to at low albedo; Vega 1 found Halley's surface temperature to be in the range . 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. 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".


History


Before 1066

Halley may have been recorded as early as 467 BC, but this is uncertain. A comet was recorded in ancient Greece between 468 and 466 BC; its timing, location, duration, and associated meteor shower all suggest it was Halley. According to
Pliny the Elder Gaius Plinius Secundus (AD 23/2479), called Pliny the Elder (), was a Roman Empire, Roman author, Natural history, naturalist and Natural philosophy, natural philosopher, and naval and army commander of the early Roman Empire, and a friend of t ...
, that same year a meteorite fell in the town of
Aegospotami Aegospotami ( grc, Αἰγὸς Ποταμοί, ''Aigos Potamoi'') or AegospotamosMish, Frederick C., Editor in Chief. “Aegospotami.” ''Webster's Dictionary#Merriam-Webster's Collegiate Dictionary, Webster’s Ninth New Collegiate Dictionary''. ...
, in
Thrace Thrace (; el, Θράκη, Thráki; bg, Тракия, Trakiya; tr, Trakya) or Thrake is a geographical and historical region in Southeast Europe, now split among Bulgaria, Greece, and Turkey, which is bounded by the Balkan Mountains to th ...
. He described it as brown in colour and the size of a wagon load. Chinese chroniclers also mention a comet in that year. 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 ''Records of the Grand Historian'', also known by its Chinese language, Chinese name ''Shiji'', is a monumental Chinese historiography, history of China that is the first of China's Twenty-Four Histories, 24 dynastic histories. The ''Records'' ...
'' or ''Shiji'', which describes a comet that appeared in the east and moved north. The only surviving record of the 164 BC apparition is found on two fragmentary Babylonian tablets, now owned by the
British Museum The British Museum is a public museum dedicated to human history, art and culture located in the Bloomsbury area of London. Its permanent collection of eight million works is among the list of largest art museums, largest and most comprehens ...
. The apparition of 87 BC was recorded in Babylonian tablets which state that the comet was seen "day beyond day" for a month. This appearance may be recalled in the representation of
Tigranes the Great Tigranes II, more commonly known as Tigranes the Great ( hy, Տիգրան Մեծ, ''Tigran Mets''; grc, Τιγράνης ὁ Μέγας ''Tigránes ho Mégas''; la, Tigranes Magnus) (140 – 55 BC) was King of Kingdom of Armenia (ant ...
, an
Armenia Armenia (), , group=pron officially the Republic of Armenia,, is a landlocked country in the Armenian Highlands of Western Asia.The UNbr>classification of world regions places Armenia in Western Asia; the CIA World Factbook , , and '' ...
n king who is depicted on coins with a crown that features, according to
Vahe Gurzadyan Vahagn "Vahe" Gurzadyan ( hy, Վահագն Գուրզադյան; born 21 November 1955) is an Armenian mathematical physicist and a professor and head of Cosmology Center at Yerevan Physics Institute, Yerevan , Armenia, best known for co-writ ...
and R. Vardanyan, "a star with a curved tail
hat A hat is a head covering which is worn for various reasons, including protection against weather conditions, ceremonial reasons such as university graduation, religious reasons, safety, or as a fashion accessory. Hats which incorporate mech ...
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. The apparition of 12 BC was recorded in the ''
Book of Han The ''Book of Han'' or ''History of the Former Han'' (Qián Hàn Shū,《前汉书》) is a history of China finished in 111AD, covering the Western, or Former Han dynasty from the first emperor in 206 BCE to the fall of Wang Mang in 23 CE. ...
'' by Chinese astronomers of the
Han Dynasty The Han dynasty (, ; ) was an imperial dynasty of China (202 BC – 9 AD, 25–220 AD), established by Emperor Gaozu of Han, Liu Bang (Emperor Gao) and ruled by the House of Liu. The dynasty was preceded by the short-lived Qin dynasty (22 ...
who tracked it from August through October. It passed within 0.16 au of Earth. According to the Roman historian
Cassius Dio Lucius Cassius Dio (), also known as Dio Cassius ( ), was a Roman historian and senator of maternal Greek origin. He published 80 volumes of the History of ancient Rome, history on ancient Rome, beginning with the arrival of Aeneas in Italy. The ...
, a comet appeared suspended over Rome for several days portending the death of
Marcus Vipsanius Agrippa Marcus Vipsanius Agrippa (; BC – 12 BC) was a Roman general, statesman, and architect who was a close friend, son-in-law, and lieutenant to the Roman emperor Augustus. He was responsible for the construction of some of the most notable buildi ...
in that year. 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
theologian Theology is the systematic study of the nature of the Divinity, divine and, more broadly, of religious belief. It is taught as an Discipline (academia), academic discipline, typically in universities and seminaries. It occupies itself with the ...
s and astronomers to suggest that it might explain the biblical story of the
Star of Bethlehem The Star of Bethlehem, or Christmas Star, appears in the nativity of Jesus, nativity story of the Gospel of Matthew Matthew 2, chapter 2 where "wise men from the East" (biblical Magi, Magi) are inspired by the star to travel to Jerusalem. There, ...
. There are other explanations for the phenomenon, such as
planetary conjunction In astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objec ...
s, and there are also records of other comets that appeared closer to the date of Jesus' birth. If, as has been suggested, the reference by Yehoshua ben Hananiah in b. Horayot 10a to "a star which arises once in seventy years and misleads the sailors" refers to Halley's Comet, it may be a reference to the 66 AD appearance, because this apparition was the only one to occur during Yehoshua ben Hananiah's lifetime. The 141 AD apparition was recorded in Chinese chronicles. It was also recorded in the Tamil work ''
Purananuru The ''Purananuru'' (, literally "four hundred
oems An original equipment manufacturer (OEM) is generally perceived as a company that produces non-aftermarket parts and equipment that may be marketed by another manufacturer. It is a common industry term recognized and used by many professional or ...
in the genre puram"), sometimes called ''Puram'' or ''Purappattu'', is a classical Tamil literature, Tamil poetic work and traditionally the last of the Eight Anthologies (''Ettuthokai'') i ...
'', in connection with the death of the south Indian Chera king Yanaikatchai Mantaran Cheral Irumporai. The 374 AD and 607 approaches each came within 0.09  au of Earth. The 451 AD apparition was said to herald the defeat of
Attila the Hun Attila (, ; ), frequently called Attila the Hun, was the ruler of the Huns from 434 until his death in March 453. He was also the leader of a tribal empire consisting of Huns, Ostrogoths, Alans, and Bulgars, among others, in Central Europe ...
at the
Battle of Chalons The Battle of the Catalaunian Plains (or Fields), also called the Battle of the Campus Mauriacus, Battle of Châlons, Battle of Troyes or the Battle of Maurica, took place on June 20, 451 AD, between a coalition – led by the Roman Roman ...
. The 684 AD apparition was recorded in Europe in one of the sources used by the compiler of the 1493
Nuremberg Chronicle The ''Nuremberg Chronicle'' is an illustrated encyclopedia consisting of world historical accounts, as well as accounts told through biblical paraphrase. Subjects include human history in relation to the Bible, illustrated mythological creatures, ...
s, which contains an image 8 centuries after the event. Chinese records also report it as the "broom star". In 837, Halley's Comet may have passed as close as 0.03 au (3.2 million miles; 5.1 million kilometres) from
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large list of largest lakes and seas in the Solar System, volumes of water can be found throughout the Solar System, only water distributi ...
, by far its closest approach. 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; Emperor
Louis the Pious Louis the Pious (german: Ludwig der Fromme; french: Louis le Pieux; 16 April 778 – 20 June 840), also called the Fair, and the Debonaire, was King of the Franks and Holy Roman Emperor, co-emperor with his father, Charlemagne, from 813. He was ...
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." In 912, Halley is recorded in the ''
Annals of Ulster The ''Annals of Ulster'' ( ga, Annála Uladh) are annals of History of Ireland, medieval Ireland. The entries span the years from 431 AD to 1540 AD. The entries up to 1489 AD were compiled in the late 15th century by the scribe Ru ...
'', which state "A dark and rainy year. A comet appeared."


1066

In 1066, the comet was seen in England and thought to be an
omen An omen (also called ''portent'') is a phenomenon that is believed to prediction, foretell the future, often signifying the advent of change. It was commonly believed in ancient times, and still believed by some today, that omens bring divine ...
: later that year Harold II of England died at the
Battle of Hastings The Battle of Hastings nrf, Batâle dé Hastings was fought on 14 October 1066 between the Norman-French army of William the Conqueror, William, the Duke of Normandy, and an English army under the Anglo-Saxons, Anglo-Saxon King Harold Godw ...
and
William the Conqueror William I; ang, WillelmI (Bates ''William the Conqueror'' p. 33– 9 September 1087), usually known as William the Conqueror and sometimes William the Bastard, was the first House of Normandy, Norman List of English monarchs#House of Norman ...
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 Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an Inferior and superior planets, interior planet to Earth, Venus (like Mercury (pl ...
, and shining with a light equal to a quarter of that of the
Moon The Moon is Earth's only natural satellite. It is the List of natural satellites, fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth ( ...
. Halley came within 0.10  au of Earth at that time. This appearance of the comet is also noted in the ''
Anglo-Saxon Chronicle The ''Anglo-Saxon Chronicle'' is a collection of annals in Old English Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language English is a West Germanic languages, West Germanic language of the ...
''.
Eilmer of Malmesbury Eilmer of Malmesbury (also known as Oliver due to a scribe's miscopying, or Elmer, or Æthelmær) was an Timeline of aviation - pre-18th century, 11th-century English Order of Saint Benedict, Benedictine monk best known for his early attempt at ...
may have seen Halley in 989 and 1066, as recorded by
William of Malmesbury William of Malmesbury ( la, Willelmus Malmesbiriensis; ) was the foremost English historians in the Middle Ages, English historian of the 12th century. He has been ranked among the most talented English historians since Bede. Modern historian C. ...
:
"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.’"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
The Irish ''
Annals of the Four Masters The ''Annals of the Kingdom of Ireland'' ( ga, Annála Ríoghachta Éireann) or the ''Annals of the Four Masters'' (''Annála na gCeithre Máistrí'') are chronicles of Middle Ages, medieval Irish history. The entries span from the Flood myt ...
'' recorded the comet as "A star
hat A hat is a head covering which is worn for various reasons, including protection against weather conditions, ceremonial reasons such as university graduation, religious reasons, safety, or as a fashion accessory. Hats which incorporate mech ...
appeared on the seventh of the
Calends The calends or kalends ( la, kalendae) is the first day of every month in the Roman calendar The Roman calendar was the calendar used by the Roman Kingdom and Roman Republic. The term often includes the Julian calendar established by the ref ...
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." Chaco Native Americans in
New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe, New Mexico, Santa Fe , LargestCity = Albuquerque, New Mexico, Albuquerque , LargestMetro = Albuquerque metropolitan area, Tiguex , Offi ...
may have recorded the 1066 apparition in their petroglyphs. 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 icfought a battle with Harold, King of the English, and Robert was victorious and became king over the people of the English.


1145–1378

The 1145 apparition was recorded by the monk Eadwine. The 1986 apparition exhibited a fan tail similar to Eadwine's drawing. Some claim that
Genghis Khan ''Chinggis Khaan'' ͡ʃʰiŋɡɪs xaːŋbr /> Mongol script: ''Chinggis Qa(gh)an/ Chinggis Khagan'' , birth_name = Temüjin , successor = Tolui (as regent) Ögedei Khan , spouse = , issue = , house = Borjigi ...
was inspired to turn his conquests toward Europe by the 1222 apparition. The 1301 apparition may have been seen by the artist
Giotto di Bondone Giotto di Bondone (; – January 8, 1337), known mononymously as Giotto ( , ) and latinisation of names, Latinised as Giottus, was an List of Italian painters, Italian painter and architect from Florence during the Late Middle Ages. He worked ...
, who represented the
Star of Bethlehem The Star of Bethlehem, or Christmas Star, appears in the nativity of Jesus, nativity story of the Gospel of Matthew Matthew 2, chapter 2 where "wise men from the East" (biblical Magi, Magi) are inspired by the star to travel to Jerusalem. There, ...
as a fire-colored comet in the Nativity section of his
Arena Chapel The Scrovegni Chapel ( it, Cappella degli Scrovegni ), also known as the Arena Chapel, is a small church, adjacent to the Augustinian monastery, the ''Monastero degli Eremitani'' in Padua Padua ( ; it, Padova ; vec, Pàdova) is a ...
cycle, completed in 1305. Its 1378 appearance is recorded in the ''Annales Mediolanenses'' as well as in East Asian sources.


1456

In 1456, the year of Halley's next apparition, the
Ottoman Empire The Ottoman Empire, * ; is an archaic version. The definite article forms and were synonymous * and el, Оθωμανική Αυτοκρατορία, Othōmanikē Avtokratoria, label=none * info page on book at Martin Luther University) ...
invaded the
Kingdom of Hungary The Kingdom of Hungary was a monarchy in Central Europe that existed for nearly a millennium, from the Middle Ages into the 20th century. The Principality of Hungary emerged as a Christian kingdom upon the Coronation of the Hungarian monarch, c ...
, culminating in the siege of Belgrade in July of that year. In a papal bull,
Pope Callixtus III Pope Callixtus III ( it, Callisto III, va, Calixt III, es, Calixto III; 31 December 1378 – 6 August 1458), born Alfonso de Borgia ( va, Alfons de Borja), was head of the Catholic Church and ruler of the Papal States from 8 April 1455 to his ...
ordered special prayers be said for the city's protection. In 1470, the
humanist Humanism is a philosophical Philosophy (from , ) is the systematized study of general and fundamental questions, such as those about existence, reason, Epistemology, knowledge, Ethics, values, Philosophy of mind, mind, and Philo ...
scholar Bartolomeo Platina wrote in his '' Lives of the Popes'' that,
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. 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). After witnessing a bright light in the sky which most historians have identified as Halley's Comet,
Zara Yaqob Zara Yaqob ( Ge'ez: ዘርዐ ያዕቆብ; 1399 – 26 August 1468) was Emperor of Ethiopia, and a member of the Solomonic dynasty who ruled under the regnal name Kwestantinos I (Ge'ez: ቈስታንቲኖስ, "Constantine"). He is known fo ...
, Emperor of
Ethiopia Ethiopia, , om, Itiyoophiyaa, so, Itoobiya, ti, ኢትዮጵያ, Ítiyop'iya, aa, Itiyoppiya officially the Federal Democratic Republic of Ethiopia, is a landlocked country in the Horn of Africa. It shares borders with Eritrea to the ...
from 1434 to 1468, founded the city of
Debre Berhan Debre Birhan () is a city in central Ethiopia. Located in the Semien Shewa Zone (Amhara), Semien Shewa Zone of Amhara Region, about 120 kilometers north east of Addis Ababa on Ethiopian highway 2, the town has an elevation of 2,840 meters, wh ...
(tr. City of Light) and made it his capital for the remainder of his reign.


1531

In the
Sikh Sikhs ( or ; pa, ਸਿੱਖ, ' ) are people who adhere to Sikhism (Sikhi), a monotheistic religion that originated in the late 15th century in the Punjab region of the Indian subcontinent, based on the revelation of Guru Nanak. The ter ...
scriptures of the
Guru Granth Sahib The Guru Granth Sahib ( pa, ਗੁਰੂ ਗ੍ਰੰਥ ਸਾਹਿਬ, ) is the central holy religious scripture of Sikhism, regarded by Sikhs as the final, sovereign and Guru Maneyo Granth, eternal Guru following the lineage of the Sikh gur ...
, the founder of the faith
Guru Nanak Gurū Nānak (15 April 1469 – 22 September 1539; Gurmukhi: ਗੁਰੂ ਨਾਨਕ; pronunciation: , ), also referred to as ('father Nānak'), was the founder of Sikhism and is the first of the ten Sikh gurus, Sikh Gurus. His birth is ce ...
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.


1531–1759

Halley's periodic returns have been subject to scientific investigation since the 16th century. The three apparitions from 1531 to 1682 were noted by Edmond Halley, enabling him to predict it would return. One key breakthrough occurred when Halley talked with Newton about his ideas of the laws of motion. Newton also helped Halley get Flamsteed's data on the 1682 apparition. 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. 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.Sagan, Carl; Druyan, Ann; ''Comet'', p. 57 Newton had theorized that such delays were caused by the gravity of other comets, but Halley found that Jupiter and Saturn would cause the appropriate delays. 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. (See also #Computation of orbit)


1835

At Markree Observatory in Ireland, a E. J. Cooper used a Cauchoix of Paris lens telescope with an aperture of to sketch Halley's comet in 1835. The comet was also sketched by F.W. Bessel. Streams of vapour observed during the comet's 1835 apparition prompted astronomer Friedrich Wilhelm Bessel to propose that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit. An interview in 1910, of someone who was a teenager at the time of the 1835 apparition had this to say: 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. 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. In the United States telescopic observations were made from
Yale College Yale College is the undergraduate college of Yale University Yale University is a Private university, private research university in New Haven, Connecticut. Established in 1701 as the Collegiate School, it is the List of Colonial Colleg ...
. The new observations helped confirm early appearances of this comet including its 1456 and 1378 apparitions. At Yale College in Connecticut, the comet was first reported on 31 August 1835 by astronomers D. Olmstead and E. Loomis. In Canada reports were made from Newfoundland and also Quebec. Reports came in from all over by later 1835, and often reported in newspapers of this time in Canada. 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.
Astrophotography Astrophotography, also known as astronomical imaging, is the photography or imaging of astronomical objects, celestial events, or areas of the night sky. The first photograph of an astronomical object (the Moon) was taken in 1840, but it was no ...
was not known to have been attempted until 1839, as photography was still being invented in the 1830s, too late to photograph the apparition of 1P/Halley in 1835. 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. At
Paris Observatory The Paris Observatory (french: Observatoire de Paris ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomy, astronomical observatory of France, and one of the largest astronomical centers in the world. ...
Halley's Comet 1835 apparition was observed with a Lerebours telescope of 24.4 cm (9.6 in) aperture by the astronomer François Arago. Arago recorded polimetric 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.


1910

The 1910 approach, which came into naked-eye view around 10 April and came to perihelion on 20 April, was notable for several reasons: it was the first approach of which photographs exist, and the first for which
spectroscopic Spectroscopy is the field of study that measures and interprets the electromagnetic spectrum, electromagnetic spectra that result from the interaction between Electromagnetism, electromagnetic radiation and matter as a function of the wavelengt ...
data were obtained. Furthermore, the comet made a relatively close approach of 0.15 au, making it a spectacular sight. Indeed, on 19 May, Earth actually passed through the tail of the comet. One of the substances discovered in the tail by spectroscopic analysis was the toxic gas
cyanogen Cyanogen is the chemical compound with the chemical formula, formula (carbon, Cnitrogen, N)2. It is a Transparency and translucency, colorless and highly toxic gas with a pungency, pungent odor. The molecule is a pseudohalogen. Cyanogen molecule ...
, which led astronomer
Camille Flammarion Nicolas Camille Flammarion Fellow of the Royal Astronomical Society, FRAS (; 26 February 1842 – 3 June 1925) was a French astronomer and author. He was a prolific author of more than fifty titles, including popular science works about astrono ...
to claim that, when Earth passed through the tail, the gas "would impregnate the atmosphere and possibly snuff out all life on the planet." His pronouncement led to panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public. In reality, as other astronomers were quick to point out, the gas is so diffused that the world suffered no ill effects from the passage through the tail. The comet added to the unrest in China on the eve of the
Xinhai Revolution The 1911 Revolution, also known as the Xinhai Revolution or Hsinhai Revolution, ended China's last Dynasties in Chinese history, imperial dynasty, the Manchu people, Manchu-led Qing dynasty, and led to the establishment of the Republic of Chi ...
that would end the last dynasty in 1911. As James Hutson, a missionary in
Sichuan Sichuan (; zh, c=, labels=no, ; zh, p=Sìchuān; Postal romanization, alternatively romanized as Szechuan or Szechwan; formerly also referred to as "West China" or "Western China" by Protestantism in Sichuan, Protestant missions) is a Prov ...
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."
The 1910 visitation is also recorded as being the travelling companion of Hedley Churchward, the first known English Muslim to make the Haj pilgrimage to
Mecca Mecca (; officially Makkah al-Mukarramah, commonly shortened to Makkah ()) is a city and administrative center of the Mecca Province of Saudi Arabia, and the Holiest sites in Islam, holiest city in Islam. It is inland from Jeddah on the Red ...
. However, his explanation of its scientific predictability did not meet with favour in the Holy City. The comet was used in an advertising campaign of Le Bon Marché, a well-known department store in Paris. 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. American satirist and writer
Mark Twain Samuel Langhorne Clemens (November 30, 1835 – April 21, 1910), known by his pen name Mark Twain, was an American writer, humorist, entrepreneur, publisher, and lecturer. He was praised as the "greatest humorist the United States has pr ...
was born on 30 November 1835, exactly two weeks after the comet's
perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any e ...
. 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.'
Twain died on 21 April 1910, the day following the comet's subsequent perihelion. The 1985 fantasy film '' The Adventures of Mark Twain'' was inspired by the quotation. Halley's 1910 apparition is distinct from the Great Daylight Comet of 1910, which surpassed Halley in brilliance and was actually visible in broad daylight for a short period, approximately four months before Halley made its appearance.


1986

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. Halley's closest approach was 0.42 au. Additionally, increased
light pollution Light pollution is the presence of unwanted, inappropriate, or excessive use of artificial Visible spectrum, lighting. In a descriptive sense, the term ''light pollution'' refers to the effects of any poorly implemented lighting, during the day ...
from urbanization caused many people to fail in attempts to see the comet. With the help of binoculars, observation from areas outside cities was more successful. Further, the comet appeared brightest when it was almost invisible from the northern hemisphere in March and April 1986, 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 The Hale Telescope is a , F-number, 3.3 reflecting telescope at the Palomar Observatory in San Diego County, California, San Diego County, California, US, named after astronomer George Ellery Hale. With funding from the Rockefeller Foundation in ...
at Mount Palomar and a
CCD camera A charge-coupled device (CCD) is an integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semi ...
. The first visual observance 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 telescope on top of
Mauna Kea Mauna Kea ( or ; ; abbreviation for ''Mauna a Wākea''); is a Volcano#Dormant, dormant volcano on the Hawaii (island), island of Hawaii. Its peak is above sea level, making it the List of U.S. states by elevation, highest point in the state of ...
to detect the magnitude 19.6 comet. (Horizons shows the nucleus @ APmag +20.5; the coma up to APmag +14.3) 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 The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a Federally funded research and development centers, federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, La Cañada Flintridge, California ...
) and Charles Morris on 8 November 1985. Although the comet's retrograde orbit and high inclination made it difficult to send a space probe to it, 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 Nucleus (plural, : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...
, 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 , owners = , headquarters = Paris, Île-de-France, France , coordinates = , spaceport = Guiana Space Centre , seal = File:ESA emblem seal.png , seal_size = 130px , image = Views in the Main Control Room (1205 ...
, made the closest pass of the comet's nucleus. There also were two Japanese probes, ''Suisei'' and ''
Sakigake , known before launch as MS-T5, was Japan's first interplanetary spacecraft, and the first deep space probe to be launched by any country other than the USA or the Soviet Union. It aimed to demonstrate the performance of the new launch vehi ...
''. Unofficially, the numerous probes became known as the
Halley Armada The Halley Armada is the name of a series of space probes, five of which were successful, sent to examine Halley's Comet during its 1986 sojourn through the inner Solar System, connected with Halley's Comet#1986, apparition "1P/1982 U1". The armad ...
. Based on data retrieved by the largest
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
space telescope of the time, ''Astron'', during its Halley's Comet observations in December 1985, a group of Soviet scientists developed a model of the comet's
coma A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions. Coma patients exhi ...
. The comet also was observed from space by the
International Cometary Explorer The International Cometary Explorer (ICE) spacecraft (designed and launched as the International Sun-Earth Explorer-3 (ISEE-3) satellite), was launched 12 August 1978, into a heliocentric orbit. It was one of three spacecraft, along with the mo ...
(ICE). Originally named International Sun-Earth Explorer 3, the probe was renamed and freed from its
Lagrangian point In celestial mechanics, the Lagrange points (; also Lagrangian points or libration points) are points of equilibrium for small-mass objects under the influence of two massive orbit, orbiting bodies. Mathematically, this involves the solutio ...
location in Earth's orbit in order to intercept comets 21P/Giacobini-Zinner and Halley. ICE flew about 40.2 million km (25 million mi) from Halley's Comet on 28 March 1986. Two
Space Shuttle The Space Shuttle is a retired, partially reusable launch system, reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. I ...
missions—the
STS-51-L STS-51-L was the 25th mission of the NASA Space Shuttle program and the final flight of Space Shuttle Challenger, Space Shuttle ''Challenger''. Planned as the first Teacher in Space Project flight in addition to observing Halley's Comet for s ...
and the STS-61-E—had been scheduled to observe Halley's Comet from
low Earth orbit A low Earth orbit (LEO) is an geocentric orbit, orbit around Earth with a orbital period, period of 128 minutes or less (making at least 11.25 orbits per day) and an orbital eccentricity, eccentricity less than 0.25. Most of the artificial object ...
. The STS-51-L mission carried the Shuttle-Pointed Tool for Astronomy (SPARTAN-203) satellite, also called the Halley's Comet Experiment Deployable (HCED). The ill-fated mission was ended by the ''Challenger'' disaster. Scheduled for March 1986, STS-61-E was a ''Columbia'' mission carrying the ASTRO-1 platform to study the comet. Owing to the suspension of the American human spaceflight program after the ''Challenger'' explosion, the mission was canceled and ASTRO-1 would not fly until late 1990 on
STS-35 STS-35 was the tenth flight of Space Shuttle Space Shuttle Columbia, ''Columbia'', the 38th shuttle flight, and a mission devoted to Observational astronomy, astronomical observations with Astronomy, ASTRO-1, a Spacelab, Spacelab observatory co ...
.


After 1986

On 12 February 1991, at a distance of from the Sun, Halley displayed an outburst that lasted for several months, releasing a cloud of dust across. The outburst likely started in December 1990, and then the comet brightened from magnitude 24.3 to magnitude 18.9. Halley was most recently observed in 2003 by three of the
Very Large Telescope The Very Large Telescope (VLT) is a telescope facility operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. It consists of four individual telescopes, each with a primary mirror 8.2 m across, ...
s at Paranal, Chile, when Halley's magnitude was 28.2. The telescopes observed Halley, at the faintest and farthest any comet has ever been imaged, in order to verify a method for finding very faint
trans-Neptunian object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typically, ...
s. Astronomers are now able to observe the comet at any point in its orbit. On 9 December 2023, Halley's Comet will reach the farthest and slowest point in its orbit from the Sun when it will be traveling at with respect to the Sun.


2061

The next perihelion of Halley's Comet is 28 July 2061, 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. The closest approach to Earth will be one day after perihelion. It is expected to have an
apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction (astron ...
of −0.3, compared with only +2.1 for the 1986 apparition. On 9 September 2060, Halley will pass within of Jupiter, and then on 20 August 2061 will pass within of Venus.


2134

Halley will come to perihelion on 27 March 2134. Then on 7 May 2134, Halley will pass within of Earth. Its apparent magnitude is expected to be −2.0.


Apparitions

Halley's calculations enabled the comet's earlier appearances to be found in the historical record. The following table sets out the astronomical designations for every apparition of Halley's Comet from 240 BC, the earliest documented widespread sighting. For example, "1P/1982 U1, 1986 III, 1982i" indicates that for the perihelion in 1986, Halley was the first period comet known (designated 1P) and this apparition was the first seen in half-month U (the second half of October) in 1982 (giving 1P/1982 U1); it was the third comet past perihelion in 1986 (1986 III); and it was the ninth comet spotted in 1982 ( provisional designation 1982i). The perihelion dates of each apparition are shown. The perihelion dates farther from the present are approximate, mainly because of uncertainties in the modelling of non-gravitational effects. Perihelion dates of 1531 and earlier are in the
Julian calendar The Julian calendar, proposed by Roman consul Julius Caesar in 46 BC, was a reform of the Roman calendar. It took effect on , by edict. It was designed with the aid of Greek mathematics, Greek mathematicians and Ancient Greek astronomy, as ...
, while perihelion dates 1607 and after are in the
Gregorian calendar The Gregorian calendar is the calendar used in most parts of the world. It was introduced in October 1582 by Pope Gregory XIII as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years diffe ...
. Halley's Comet is visible from Earth every 74–79 years.


See also

* List of Halley-type comets * Halley's Comet in fiction *
Kepler orbit Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, German mathematician, mathematician, astrologer, Natural philosophy, natural philosopher and writer on music. He is a key figure in the 17th-century Scienti ...


References


Bibliography

* * * *


External links


Synopsis of the Astronomy of Comets
(1706 reprint of Halley's 1705 paper)
Halley's nucleus by Giotto spacecraft (ESA link)

Image of Halley in 1986 by Giotto spacecraft (NASA link)



1P/Halley at CometBase database



Orbital simulation
from JPL (Java)
Ephemeris
*
Donald Keith Yeomans, "Great Comets in History"


( Ian Ridpath)
Photographs of 1910 approach taken from the Lick Observatory from the Lick Observatory Records Digital Archive, UC Santa Cruz Library's Digital Collections
{{DEFAULTSORT:Halley, 001P Astronomical objects known since antiquity Comets visited by spacecraft Halley-type comets Meteor shower progenitors 001P 0001 Periodic comets Star of Bethlehem