An Etymological Dictionary of Astronomy and Astrophysics
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A small body of → gas and → dust which revolves around the → Sun in a usually very → elliptical or even → parabolic → orbit. It is seen to be composed of a → head, or → coma, and often with a spectacular gaseous → tail extending a great distance from the head. The rocky-icy head is called the → comet nucleus. As the comet nears the Sun, the increased temperature causes the → ice in the nucleus to → sublimate and form a gaseous halo around the nucleus, called the coma. Comets often possess two tails, a → dust tail that lies in the orbit behind the comet generated by surface activity, and a brighter, ionized → gas tail, that points away from the Sun, driven by → solar wind. → Long-period comets are thought to originate in the → Oort cloud, at distances exceeding 50,000 → astronomical units (AU). They are perturbed by the planets (especially → Jupiter) to fall in toward the Sun. Their orbits typically have random inclinations and a very large → eccentricity; some → hyperbolic orbits have been observed. → Short-period comets apparently arise in the → Kuiper belt in the zone from 20 to 50 AU. Their orbits typically have small eccentricities. Both cometary reservoirs are thought to represent primordial solar system material. A comet with a dust coating on its surface that inhibits gas production might be classified as an → asteroid. Because of this ambiguity, objects such as → Chiron, a → Centaur asteroid, have been reclassified as comets. Comets are primarily composed of amorphous → water ice, but also contain → carbon dioxide (CO₂), → carbon monoxide (CO), → formaldehyde (H₂CO), → methanol (CH₃OH), → methane (CH₄) at a few percent level (with respect to water), and many other molecules at a lower level. See also → comet designation.

From O.Fr. comète, from L. cometa, from Gk. (aster) kometes, "long-haired (star)," from kome "hair of the head," so called from resemblance of the comet's tail to streaming hair.

Dombâledâr, from dombâlé "tail," from domb, dom (Mid.Pers. dumb, Av. duma- "tail") + -âlé, -âl resemblance suffix, → -al + dâr "having, possessor," (from dâštan "to have, to possess," O.Pers./Av. root dar- "to hold, keep back, maitain, keep in mind," Skt. dhr-, dharma- "law," Gk. thronos "elevated seat, throne," L. firmus "firm, stable," Lith. daryti "to make," PIE *dher- "to hold, support").

A → comet with an irregular → nucleus of roughly 3 × 5 km across orbiting the Sun between → Jupiter and → Earth with a period of 6.45 years. The comet has been observed from Earth on seven approaches to the Sun: in 1969, 1976, 1982, 1989, 1996, 2002, and 2009. It was also imaged by the → Hubble Space Telescope in 2003, which allowed estimates of its size and shape. It arrived at → perihelion on 13 August 2015. In 2014 the → European Space Agency probe → Rosetta, launched in 2004, was placed on an orbit around 67P/Churyumov-Gerasimenko. Over an entire year, as it approached the Sun, Rosetta mapped the comet's surface and studied changes in its activity.

→ comet; Named after its discoverers, Klim Churyumov and Svetlana Gerasimenko, Ukrainian astronomers, who first noticed the comet in 1969.

A → nomenclature system for naming → comets. In early 1995, a new comet designation system was established by the → International Astronomical Union. The main rules are as follows:
a) If the comet is a newly discovered one, it first gets a provisional name, which closely matches the → asteroid designation system. For example, the first comet discovered in the first half of 1998 January is designated 1998 A1, the second 1998 A2, etc.
b) The name of the person(s) who discovered the comet may be added to this designation (limited, however, to three names). For example, comet → Hale-Bopp (C/1995 O1) has its full name as Hale-Bopp C/1995 O1, whereas its designation is C/1998 O1. If several people are involved with a discovery at an observatory, the comet may be named after the observatory instead of the individuals.
c) → Long-period comets and one-apparition → periodic comets receive only a provisional designation.
d) A → short-period comet would get the P/designation until it is recovered in a second → apparition. At this point, the P/Year designation would be replaced with a number followed immediately by an upper case P, and a slash followed by the name of the discoverer(s). The number here is one more than the number of known periodic comets that have reappeared. For example, the comet Hug-Bell (P/1999 X1) was given the full name 178P/Hug-Bell after it reappeared in 2007. Previously, 177 periodic comets had got assigned numbers.
e) Long-period comets are indicated by the prefix C.
f) If the comet is destroyed, or if it fails to appear after several apparitions, it would be prefixed D/ (→ defunct comet) followed by the year of its discovery. For example, → Comet Shoemaker-Levy 9 has been assigned D/1993 F2 since it was discovered in the second half of March in 1993 and was destroyed when it crashed into Jupiter in 1994.
g) Comets that lack sufficient position measurements for an orbital determination are given the designation of X/ followed by the year of their discovery and the appropriate letter and number code.
h) When a → comet nucleus nucleus splits, each fragment is given the comet designation followed by A, B, C, etc (for fragments).

→ comet; → designation.

A group of comets with similar orbital characteristics. They result from perturbations by planets which change the diverse original orbits of the comets into those they now occupy.

→ comet; → family.

One of the brightest comets seen in the twentieth century, even though it came no closer to Earth than 1.32 AU (on 22 March 1997). It was visible to the naked eye for many months. The → nucleus of Hale-Bopp was estimated to be about 30 to 40 km across. Hale-Bopp has an orbital period of 2,380 years and is predicted to be seen again in AD 4377.

Discovered independently by American amateur astronomers Alan Hale and Thomas Bopp on July 22, 1995; → comet.

A → long-period comet found in January 1996, which became the brightest comet since → Comet West in 1976. It was a bright naked-eye object and remained so in March, April, and May of 1996. At closest approach to Earth on March 25, is was only 0.10 AU away, displaying a long tail of up to 100 degrees. Small fragments were observed to break off the main nucleus. Hyakutake was the first comet from which X-ray emission was detected.

→ comet; Named after the Japanese amateur astronomer Yuuji Hyakutake (1951-2002), who discovered this comet in the morning of January 30, 1996.

The solid, centrally located part of a → comet. The nucleus is a mass of dust and frozen gases. When heated by the → Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the → coma, which is later swept into an elongated tail. Reliable measurements of cometary nuclei indicate sizes from a few km to 10 or 20 km. The nucleus of → Comet Hale-Bopp is one of the largest (perhaps 40 km). The composition of the nucleus is determined by measuring the composition of the coma (except for 67P/Churyumov-Gerasimenko). The dominant → volatile is → water, followed by → carbon dioxide (CO₂), → carbon monoxide (CO), → formaldehyde (H₂CO), → methanol (CH₃OH), → methane (CH₄) at a few percent level (with respect to water) and many other molecules at a lower level.

→ comet; → nucleus.

A comet, formally designated D/1993 F2, whose shattered nucleus crashed into the planet → Jupiter over the period of July 16-22, 1994, several months after its discovery. The collision produced scars that were visible from Earth even in small telescopes. The cataclysmic event was the first collision between two → solar system bodies ever observed. The comet had been discovered on March 24, 1993, from photographs taken using the 0.46 m → Schmidt telescope at → Palomar Observatory. The appearance of the comet was reported as "most unusual": the object appeared as a "dense linear bar'' with a "fainter, wispy tail.'' The comet's brightness was reported as about magnitude 14, more than a thousand times too faint to be seen with the naked eye. Later observations revealed that the "bar'' was made up of as many as 21 pieces "strung out like pearls on a string,'' according to one researcher. Orbit calculations show that on July 7, 1992, the comet had passed only 25,000 km above Jupiter. The differential pull of the planet's enormous → gravitational force on the near and far sides of the comet fragmented it into the 21 or more pieces with sizes estimated at up to 2 km and an enormous amount of smaller debris. The comet had been in a rapidly changing orbit around Jupiter for some time before this, probably for at least several decades.

→ comet; Named after the husband and wife scientific team of American Carolyn S. (1929-) and Eugene M. Shoemaker (1928-1997) and Canadian amateur astronomer David H. Levy (1948-)

A spectacular comet that at its closest approach to Earth reached a brightness of -1 magnitude. It was so bright that could be seen even at sunrise. The comet reached → perihelion on 1976 Feb. 25 at 0.20 A.U. and had a fan-shaped tail of dimensions 25° x 25° x 15° on the sky. A few days after perihelion, the nucleus split in four fragments. The → carbon monoxide (CO) molecule in comets was first detected in West. The comet's orbit has a period of about 500,000 years. Formerly designated 1976 VI.

After the Danish astronomer Richard M. West (1941-), who worked at the → European Southern Observatory (ESO); → comet.

Of or relating to or resembling a → comet.

→ comet; → -ary.

The appearance of → gas and → dust features from the rocky-icy nucleus of a comet when approaching the Sun (→ cometary atmosphere, → cometary tail). The → sublimation of → water can explain cometary activity at distances from the Sun up to about 4 → astronomical units. At larger distances, the average temperature of the → comet nucleus' surface is less than 140 K, too low for efficient sublimation of water → ice. However, there are many examples of cometary activity at larger distances. This can probably be due to the sublimation of more → volatile → chemical species. Indeed, radio spectroscopic observations of comets at large distances have revealed an important → outgassing of → carbon monoxide (CO), which can sublimate at temperatures as low as 25 K.

→ cometary; → activity.

The envelope of → gas and → dust around a → comet nucleus, also known as → coma. As the comet approaches the → Sun, the frozen materials → sublimate and give rise to an expanding atmosphere. The atmosphere is composed of dust, → molecules, → radicals, and molecular → ions released from the inner coma with velocities ~ 0.5 to 1 km s^-1, well above the → escape velocity for the nucleus. The → chemical species observed in cometary spectra can be divided into several categories: (i) atoms and molecules related to → water (H, O, OH, OH⁺, H2O, H2O⁺), (ii) carbon and related molecules (C, C⁺, CO, CO⁺, CO₂⁺, C₂, CH, CH⁺, HCO, H₂CO), (iii) → nitrogen and related molecules (CN, CN⁺, HCN, CH₃CN, NH, NH₂, N₂⁺, NH₃, NH₄), (iv) → sulphur and related molecules (S, CS, S₂, H₂S⁺), (v) → metals (Na, K, Ca, Co, Cr, Cu, V, Fe, Mn, Ni). For a typical average comet the neutral atmosphere is first seen when the heliocentric distance is d ≤ 3 → astronomical units.

→ cometary; → atmosphere.

A relatively small cloud of → dust and → gas in the → interstellar medium shaped like a comet with a bright-rimmed head. Cometary globules are situated near young → massive stars with a strong → stellar wind. The wind ionizes gases on the side facing the → O stars and sweep away the low-density gas toward the tail. Cometary globules are believed to be → molecular cloud condensations, which are so dense that they are not disrupted when an → H II region expands into the molecular cloud surrounding it. The → Rosette nebula is a good example of an H II region which shows an abundance of cometary globules.

→ comet; → globule.

→ comet nucleus.

→ cometary; → nucleus.

The → path followed by a → comet in the → solar system around the → Sun. Most cometary orbits appear to be → elliptical, or in some cases → parabolic. The orbits of → short-period comets are elliptical, carrying them out to a region lying from → Jupiter to beyond the orbit of → Neptune. Those of → long-period comets are very elliptical. The orbits may be strongly influenced if they pass near the Jovian planets, particularly Jupiter itself. The cometary orbits are also influenced to some degree by gases shooting out of comets, so their orbits are primarily but not completely determined by gravity. Newton (1644-1727) was the first to compute a cometary orbit. He found that the comet of 1680 was following a parabolic orbit around the Sun. Edmond Halley (1656-1742), following the methods of Newton, computed the → orbital elements of 24 comets. He realized that the comets of 1531, 1607 and 1682 had very similar elements and postulated that they were in fact the same object, orbiting an elongated ellipse. He predicted the next return to occur in 1758 or early 1759. The return of what is now called Halley's comet was observed after his death, This first observation of a "predicted" comet is manifestly one of the major successes of → celestial mechanics.

→ cometary; → orbit.

A formation of → gas and/or → dust that streams away from the → coma of many comets under the influence of the Sun's → radiation pressure and the → solar wind. See also: → dust tail, → gas tail, → ion tail, → plasma tail, → sodium tail, → Type I tail, → Type II tail, → antitail.

→ cometary; → tail.

1) Eating together at the same table.
2) Ecology: Of a form of → symbiosis in which two different species of plant or animal live in close association without injury to either.

From M.L. commensalis, from → com- "with, together" + mensa (genitive mensalis) "table," → Mensa.

Ham-miz "(eating together) at the same table," from ham-, → com-, + miz "table," → Mensa.

A mode of → survey particularly in → radio astronomy such that two different observing goals are achieved simultaneously. Commensal survey necessitates compromises whenever necessary.

→ commensal; → survey.

(adj.) Of the same size, extent, or duration as another; proportionate.

L.L. commensuratus, from → com- "together, with" + mensuratus, p.p. of mensurare "to measure," from menusra "measure."

Hammasâ, from ham- "together," → com- + masâ "size, greatness," from Mid.Pers. masây, masâk "size," Av. masah- "size, greatness, length," maz-, masan-, mazant- "great, important," mazan- "greatness, majesty," mazišta- "greatest," cf. Skt. mah-, mahant-, Gk. megas, L. magnus; PIE *meg- "great."

Of two bodies orbiting around a common barycenter, when the orbital period of one is an exact fraction, for example one-half or two-thirds, of the other.

→ commensurate; → orbit.