The designation of comets

The designation of comets is a complex topic in which many changes have occurred over time. Until the mid-18th century, comets were simply designated by the year of their occurrence. The first to receive a different name was Halley’s comet, so named in 1759 by La Caille (1713-1762). Then, comets often took the name of their discoverer, or of the person who established their period, if periodic; this was the case of comets Encke and Lexell.

In the 20th century, a provisional designation indicated the year of their discovery followed by a lowercase letter giving the order of this discovery in that year. Then a final designation was given after about two years, including the year of perihelion passage followed by a serial number in Roman numerals. Thus, comet West, discovered in 1975 and which passed at perihelion on February 25, 1976, was first called 1975n and later 1976 VI. This system had drawbacks. The number of discovered comets increasing rapidly, it depleted the letters of the alphabet and one had to start the alphabet again and add a number: the 27th comet of year xxxx was affected by the provisional designation xxxxa1. Sometimes comets are discovered after perihelion passage, which leads to inconsistencies. Finally, some short-period comets are observable throughout their orbit, making unfounded object their provisional designation.


Cartes et calques de cartes d'un jeu de comète royale (1748-1774)

Source : / Bibliothèque Nationale de France

Since January  1, 1995, the International Astronomical Union refers to comets as follows :

  • A prefix, C/ for new comets or comets with periods greater than 200 years, P/ for shorter-period comets, and more rarely X/ for those whose orbit could not be determined and D/ for disappeared comets;
  • The year of discovery, followed by a capital letter identifying the half-month of the discovery and a number giving the order of discovery in that half month;
  • Optionally, the name of one or (at most) two discoverers can be added to respect the tradition, possibly with a serial number if the person or persons had previously discovered other comets. As many comets are now discovered by observation with a robotic telescope, an artificial satellite or a space probe, the name of the instrument is added in this case (e.g. LINEAR or SOHO).

Thus the second comet discovered in the second half of January 1996, a long-period comet, is designated C/1996 B2 Hyakutake.


Vue d'un phénomène céleste ressemblant à une comète qui a apparu à Salon en Provence -  Gedruckt zu Nurmberg bey M. Joachim Heller, 16e siècle.

Source : / Bibliothèque Nationale de France

And that is not all! For periodic comets that have been observed at several returns, a sequential number is added to the P/ prefix, which indicates the order in which they were discovered or identified, forgetting the rest of the schedule. The name of the discoverer is generally added in such cases. There were in 2015 325 comets in this class: 1P/Halley, 2P/Encke, 3D/Biela (an extinct comet), 4P/Faye … 325P/Yang Gao.

For example, here is how the comet that crashed into Jupiter in July 1994 was named: 1993e and 1994 X, then D/1993 F2 Shoemaker-Levy 9.



Principal comets, Plate 1, tiré de Astronomy / Jean Rambosson - 1875.

Source : Internet Archive Book Images