An Etymological Dictionary of Astronomy and Astrophysics

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.

Etymology (EN): 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.

Etymology (PE): 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”).