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A great comet is a comet that becomes exceptionally bright. There is no official definition; often the term is attached to comets such as Halley's Comet, 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. Typically, they are as bright or brighter than a second magnitude star and have tails that are 10 degrees or longer under dark skies.^[1] Great comets appear at irregular, unpredictable intervals, on average about once per decade. 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. It can also be used as a generic name when a very bright comet is discovered by many observers simultaneously.^[2]

Causes

The vast majority of comets are never bright enough to be seen by the naked eye, and generally pass through the inner Solar System unseen by anyone except astronomers. 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, a close approach to the Sun, and a close approach to the Earth. 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 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 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 and dust 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 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 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 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. 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 of their orbit lies outside the Earth's orbit. Any comet approaching the Sun to within 0.5 AU (75 million km) 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. Halley's Comet, 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 Comet Hyakutake (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 0.1 AU (15 million km; 39 LD).

List of great comets

Great comets of the past two millennia include the following:

Aristotle's Comet – 373–372 BC^[3]^[4]
Halley's Comet – 87 BC^[4]
Caesar's Comet – 44 BC^[5]
Halley's Comet – 12 BC^[4]
Great Comet of 178 AC^[4]
Great Comet of 191
Great Comet of 240
Great Comet of 390
Great Comet of 400
Great Comet of 442
Great Comet of 565
Great Comet of 568
Great Comet of 770
Halley's Comet – 837
Great Comet of 905
Halley's Comet – 1066
Great Comet of 1106
Great Comet of 1132^[4]
Great Comet of 1240^[4]
Great Comet of 1264^[6]
Great Comet of 1402^[7]
Great Comet of 1468^[4]
Great Comet of 1471^[8]
Great Comet of 1532^[4]
Great Comet of 1533^[4]
Great Comet of 1556^[9]
Great Comet of 1577
Great Comet of 1618
Great Comet of 1664^[10]
Great Comet of 1665^[4]
Great Comet of 1668^[4]
Great Comet of 1680
Great Comet of 1686^[4]
Great Comet of 1744
Great Comet of 1760
Great Comet of 1769^[4]
Great Comet of 1807^[4]
Great Comet of 1811
Great Comet of 1819
Great Comet of 1823
Great Comet of 1831
Great Comet of 1843
Great Comet of 1844^[11]
Great Comet of 1854^[12]
Donati's Comet – 1858
Great Comet of 1861
Great Southern Comet of 1865^[4]
Coggia's Comet – 1874
Great Southern Comet of 1880
Great Comet of 1881
Great Comet of 1882
Great Southern Comet of 1887
Great Comet of 1901
Great January Comet of 1910
Halley's Comet – 1910
Comet Skjellerup–Maristany – 1927
Southern Comet of 1947^[13]
Eclipse Comet of 1948^[13]
Comet Arend–Roland – 1957
Comet Mrkos – 1957
Comet Seki-Lines – 1962^[12]
Comet Ikeya–Seki – 1965
Comet Bennett – 1970^[4]
Comet West – 1976^[4]
Comet Hyakutake – 1996
Comet Hale–Bopp – 1997
Comet McNaught – 2007
Comet Lovejoy – 2011
Comet NEOWISE - 2020^[14]^[15]^[16]^[17]^[18]
Comet Tsuchinshan–ATLAS - 2024^[19]

Notes

^ Seargent, David A. J. (2009). The greatest comets in history: broom stars and celestial scimitars. New York: Springer. p. vii. ISBN 978-0-387-09512-7.
^ "IAU Comet-naming Guidelines". www.cbat.eps.harvard.edu. Retrieved 8 December 2024.
^ A winter comet reported by Ephorus
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o ^p ^q Donald K. Yeomans (April 2007). "Great Comets in History". Jet Propulsion Laboratory/California Institute of Technology (Solar System Dynamics). Retrieved 2011-02-02.
^ Ramsey, John T. & Licht, A. Lewis (1997), The Comet of 44 B.C. and Caesar's Funeral Games, Atlanta, ISBN 0-7885-0273-5{{citation}}: CS1 maint: location missing publisher (link).
^ The Living Age, Volume 58. Lithotyped by Cowles and Company, 17 Washington St., Boston. Press of Geo. C. Rand & Avery. 1858. p. 879.
^ David A. J. Seargent (2009). The Greatest Comets in History: Broom Stars and Celestial Scimitars. Springer Science + Business Media. p. 99. ISBN 978-0-387-09512-7.
^ "Great Comet of 1471". Atlas of Great Comets. Cambridge University Press: 49–50. 2015. doi:10.1017/CBO9781316145166.010. ISBN 9781107093492.
^ Vsekhsvyatsky, S. K. (1958). Physical Characteristics of Comets. Moscow: Fizmatgiz. p. 102.
^ "Great Comet of 1664". Atlas of Great Comets. Cambridge University Press: 72–77. 2015. doi:10.1017/CBO9781316145166.016. ISBN 9781107093492.
^ Bond, G.P. (1850). "On the great comet of 1844–45". The Astronomical Journal. 1: 97. Bibcode:1850AJ......1...97B. doi:10.1086/100067.
^ ^a ^b Bortle, J., "The Bright Comet Chronicles", harvard.edu, retrieved 2008-11-18
^ ^a ^b "Brightest comets seen since 1935". www.icq.eps.harvard.edu. Retrieved 17 January 2024.
^ "Weekly Information about Bright Comets (2020 July 18: North)". Seiichi Yoshida. July 18, 2020. Archived from the original on July 19, 2020. Retrieved July 18, 2020.
^ "Seiichi Yoshida's Diary of Comet Observations (2020)". Seiichi Yoshida. July 19, 2020. Archived from the original on July 20, 2020. Retrieved July 19, 2020.
^ "Comet NEOWISE Updtae: Easy To See In The Evening! When And How To See Comet NEOWISE". Farmer's Almanac. July 18, 2020. Archived from the original on July 19, 2020. Retrieved July 19, 2020.
^ "APOD: 2022 July 26 - Comet NEOWISE Rising over the Adriatic Sea". NASA. July 26, 2022. Retrieved October 17, 2024.
^ "Great Comets: What Are They, And When Will the Next Comet Be Visible?". Star Walk. October 16, 2024. Retrieved October 17, 2024.
^ "APOD: 2024 October 21 – Comet Tsuchinshan ATLAS over California". apod.nasa.gov. Retrieved 2024-10-27.