A great comet is a

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, and sometimes also a tail. These phenomena ar ...

that becomes exceptionally bright. There is no official definition; often the term is attached to comets such as

Halley's Comet Halley's Comet or Comet Halley, officially designated 1P/Halley, is a short-period comet visible from Earth every 75–79 years. Halley is the only known short-period comet that is regularly visible to the naked eye from Earth, and thus the o ...

, which during certain appearances are bright enough to be noticed by casual observers who are not looking for them, and become well known outside the astronomical community. Great comets appear at irregular, unpredictable intervals, on average about once per

decade A decade () is a period of ten years. Decades may describe any ten-year period, such as those of a person's life, or refer to specific groupings of calendar years. Usage Any period of ten years is a "decade". For example, the statement that "du ...

. Although comets are officially named after their discoverers, great comets are sometimes also referred to by the year in which they appeared great, using the formulation "The Great Comet of ...", followed by the year.

Causes

Lieve Verschuier - Staartster (komeet) boven Rotterdam - 11028-A-B - Museum Rotterdam

The vast majority of comets are never bright enough to be seen by the naked eye, and generally pass through 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 ...

unseen by anyone except

astronomers An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, natural satellite, moons, comets and galaxy, g ...

. However, occasionally a comet may brighten to naked eye visibility, and even more rarely it may become as bright as or brighter than the brightest stars. The requirements for this to occur are: a large and active

nucleus Nucleus ( : 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 Nucl ...

, a close approach to the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

, and a close approach to the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

. A comet fulfilling all three of these criteria will certainly be very bright. Sometimes, a comet failing on one criterion will still be bright. For example,

Comet Hale–Bopp Comet Hale–Bopp (formally designated C/1995 O1) is a comet that was one of the most widely observed of the 20th century and one of the brightest seen for many decades. Alan Hale and Thomas Bopp discovered Comet Hale–Bopp separately ...

did not approach the Sun very closely, but had an exceptionally large and active nucleus. It was visible to the naked eye for several months and was very widely observed. Similarly,

Comet Hyakutake Comet Hyakutake (, formally designated C/1996 B2) is a comet, discovered on 31 January 1996, that passed very close to Earth in March of that year. It was dubbed the Great Comet of 1996; its passage near the Earth was one of the closest com ...

was a relatively small comet, but appeared bright because it passed very close to the Earth.

Size and activity of the nucleus

Cometary nuclei vary in size from a few hundreds of metres across or less to many kilometres across. When they approach the Sun, large amounts of

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

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 homes ...

are ejected by cometary nuclei, due to solar heating. A crucial factor in how bright a comet becomes is how large and how active its nucleus is. After many returns to the inner Solar System, cometary nuclei become depleted in volatile materials and thus are much less bright than comets which are making their first passage through the Solar System. The sudden brightening of

Comet Holmes Comet Holmes (official designation: 17P/Holmes) is a periodic comet in the Solar System, discovered by the British amateur astronomer Edwin Holmes on November 6, 1892. Although normally a very faint object, Holmes became notable during its Octo ...

in 2007 showed the importance of the activity of the nucleus in the comet's brightness. On October 23–24, 2007, the comet underwent a sudden outburst which caused it to brighten by factor of about half a million. It unexpectedly brightened from an

apparent magnitude Apparent magnitude () is a measure of the 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 of the object's li ...

of about 17 to about 2.8 in a period of only 42 hours, making it visible to the naked eye. All these temporarily made comet 17P the largest (by radius) object in the Solar System although its nucleus is estimated to be only about 3.4 km in diameter.

Close perihelion approach

The brightness of a simple reflective body varies with the

inverse square In science, an inverse-square law is any scientific law stating that a specified physical quantity is inversely proportional to the square of the distance from the source of that physical quantity. The fundamental cause for this can be understo ...

of its distance from the Sun. That is, if an object's distance from the Sun is halved, its brightness is quadrupled. However, comets behave differently, due to their ejection of large amounts of volatile gas which then also reflect sunlight and may also

fluoresce 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, tha ...

. Their brightness varies roughly as the inverse cube of their distance from the Sun, meaning that if a comet's distance from the Sun is halved, it will become eight times as bright. This means that the peak brightness of a comet depends significantly on its distance from the Sun. For most comets, the

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 ellip ...

of their orbit lies outside the Earth's orbit. Any comet approaching the Sun to within or less may have a chance of becoming a great comet.

Close approach to the Earth

For a comet to become very bright, it also needs to pass close to the Earth.

, for example, is usually very bright when it passes through the inner Solar System every seventy-six years, but during its 1986 apparition, its closest approach to Earth was almost the most distant possible. The comet became visible to the naked eye, but was unspectacular. On the other hand, the intrinsically small and faint

(C/1996 B2) appeared very bright and spectacular due to its very close approach to Earth at its nearest during March 1996. Its passage near the Earth was one of the closest cometary approaches on record with a distance of .

List of great comets

Great comets of the past two millennia include the following: * ''Unnamed'' – 373–372 BC *

– 87 BC *

Caesar's Comet Caesar's Comet (also ''Sidus Iulium'' ("Julian Star"); ''Caesaris astrum'' ("Star of Caesar"); Comet Caesar; the Great Comet of 44 BC; numerical designation C/−43 K1) was a seven-day cometary outburst seen in July 44 BC. It was interpreted by ...

– 44 BC *

– 12 BC *

– 1066 * Great Comet of 1106 *

Great Comet of 1264 The Great Comet of 1264 (C/1264 N1) was one of the brightest comets on record. It appeared in July 1264 and remained visible to the end of September. It was first seen during the evenings after sunset, but appeared in its greatest splendor in weeks ...

* Great Comet of 1402 *

Great Comet of 1556 The Great Comet of 1556 (designated C/1556 D1 in modern nomenclature) was a comet that first appeared in February 1556, and which was observed throughout much of Europe. The comet appears to have been seen in some places before the end of February, ...

Great Comet of 1577 The Great Comet of 1577 (official designation: C/1577 V1) is a non-periodic comet that passed close to Earth during the year 1577 AD. Having an official designation beginning with "C" classes it as a non-periodic comet, and so it is not expected t ...

Great Comet of 1618 The year 1618 in science and technology involved some significant events. Astronomy * March 8 – May 15 – Johannes Kepler formulates the third law of planetary motion. * July 21 – Pluto (not known at this time) reaches an aphelion. It nex ...

Great Comet of 1680 C/1680 V1, also called the Great Comet of 1680, Kirch's Comet, and Newton's Comet, was the first comet discovered by telescope. It was discovered by Gottfried Kirch and was one of the brightest comets of the seventeenth century. Overview The c ...

Great Comet of 1744 The Great Comet of 1744, whose official designation is C/1743 X1, and which is also known as Comet de Chéseaux or Comet Klinkenberg-Chéseaux, was a spectacular comet that was observed during 1743 and 1744. It was discovered independently in lat ...

Great Comet of 1811 The Great Comet of 1811, formally designated C/1811 F1, is a comet that was visible to the naked eye for around 260 days, the longest recorded period of visibility until the appearance of Comet Hale–Bopp in 1997. In October 1811, at its bright ...

Great Comet of 1819 The Great Comet of 1819, officially designated as C/1819 N1, also known as Comet Tralles, was an exceptionally bright and easily visible comet, approaching an apparent magnitude of 1–2, discovered July 1, 1819 by the German astronomer Johann G ...

Great Comet of 1843 The Great Comet of 1843, formally designated C/1843 D1 and 1843 I, was a long-period comet which became very bright in March 1843 (it is also known as the Great March Comet). It was discovered on February 5, 1843, and rapidly brightened to beco ...

* Great Comet of 1844 * Donati's Comet – 1858 * Great Comet of 1861 * Coggia's Comet – 1874 *

Great Comet of 1882 The Great Comet of 1882 formally designated C/1882 R1, 1882 II, and 1882b, was a comet which became very bright in September 1882. It was a member of the Kreutz Sungrazers, a family of comets which pass within of the Sun's photosphere at perih ...

* Great Comet of 1901 *

Great January Comet of 1910 The Great January Comet of 1910, formally designated C/1910 A1 and often referred to as the Daylight Comet,. was a comet which appeared in January 1910. It was already visible to the naked eye when it was first noticed, and many people independent ...

– 1910 *

Comet Skjellerup–Maristany Comet Skjellerup–Maristany, formally designated C/1927 X1, 1927 IX, and 1927k, was a long-period comet which became very bright in 1927. This great comet was observable to the naked eye for about 32 days. It was independently discovered by am ...

– 1927 *

Comet Arend–Roland Comet Arend–Roland was discovered on November 8, 1956 by Belgian astronomers Sylvain Arend and Georges Roland on photographic plates. As the eighth comet found in 1956, it was named Arend–Roland 1956h after its discoverers. Because it was the ...

– 1957 * Comet Mrkos – 1957 * Comet Seki-Lines – 1962 *

Comet Ikeya–Seki :''There are two comets named Ikeya–Seki: C/1965 S1 (this one), and C/1967 Y1, a.k.a. 1968 I, 1967n.'' Comet Ikeya–Seki, formally designated C/1965 S1, 1965 VIII, and 1965f, was a long-period comet discovered independently by Kaoru Ikeya ...

– 1965 * Comet Bennett – 1970 *

Comet West Comet West, formally designated C/1975 V1, 1976 VI, and 1975n, was a comet described as one of the brightest objects to pass through the inner Solar System in 1976. It is often described as a "great comet." History It was discovered photograp ...

– 1976 *

– 1996 *

– 1997 * Comet McNaught – 2007

Notes

External links

''The bright-comet chronicles''.
John E. Bortle (W. R. Brooks Observatory)

Gary W. Kronk.
Brightest comets seen since 1935
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