International Comet Quarterly

Comet Names and Designations; Comet Naming and Nomenclature; Names of Comets

When a comet is discovered and confirmed, it is announced by the Central Bureau for Astronomical Telegrams (CBAT) -- on behalf of the International Astronomical Union (IAU) -- through its publications (the printed and electronic IAU Circulars, and occasionally also first via its electronic only CBETs). At the time of this announcement, it is usually given a designation relating to the year of discovery, and (upon publication of the first orbit) also usually a name that corresponds to the person, persons, or observing program that is credited with the discovery. (Sometimes naming is delayed for days or many weeks due to debate over a proper name within the 16-member [as of July 2003] IAU Committee on Small Bodies Nomenclature, which advises the CBAT on comet-naming issues.) Note that short-period comets are often recovered via prediction (or re-discovered accidentally) after having not been seen for some years, and if only making a second observed return to perihelion, such comets will also get a provisional year-and-letter/number designation for the current date of recovery (with a permanent-number designation usually to follow soon thereafter).

While comets dating back to Charles Messier of Paris in the late 18th century are now listed in catalogues as having names, the situation regarding recognized naming of comets has been very complicated and non-uniform over the last couple of centuries. The first definite comet to be credited internationally as "belonging" to an astronomer was not until after 1759, when the comet that had been predicted by Edmund Halley in 1705 to return in a few decades (being the same comet that had been seen in 1607 and 1682) was indeed re-discovered in 1758. And, indeed, Halley's comet is the most famous of all comets today (though the name "Halley" is pronounced with a short "a", so that it rhymes with the English word "alley", not like the long "a" in "Haley" of "Bill Haley and the Comets" fame). But in the 19th century, it was typical to follow the example of Halley's comet whereby (in general) comets were given names only after they were observed at their second apparition (or return to perihelion), meaning that most long-period comets were referred to only by a number and year (as the third comet of a given year) or by a year-and-letter or year-and-Roman-numeral designation. Beginning in the middle of the 19th century, astronomical publications sometimes gave names of discoverers in titles referring to observations of a given comet, but those names were usually put in parentheses: comet designations were given primacy, the names being given in parentheses (if at all) after the year designation. This practice of referring only to multiple-apparition comets by the discoverers' names, and to single-apparition comets by the designation, continued until the second world war (1940s).

In the late 19th century and early 20th century, some astronomers began to refer to short-period comets carrying the same discoverer name (e.g., Tempel, Brooks, Schwassmann-Wachmann, etc.) -- such as the first and second Tempel comets, or Tempel I and Tempel II, or Tempel (1) and Tempel (2), and finally Tempel 1 and Tempel 2. For the last 50 years or so, this practice of using suffixed Arabic numerals has been widely employed, but with the adoption in the 1990s of a new designation scheme (with the practical scheme of permanent numberings of multi-apparition short-period comets), such numerals are redundant and unnecessarily complicated. Indeed, beginning in 1995, numerals were no longer added to newly discovered comets, and with the change in name of "Shoemaker 2" to "Shoemaker-LINEAR" in late 2000 upon its accidental rediscovery, there is a gap leaving short-period comets named "Shoemaker 1", "Shoemaker 3", and "Shoemaker 4". (The acronym "LINEAR" stands for the "Lincoln Near-Earth Asteroid Research" project, operated by the Lincoln Laboratory of the Massachusetts Institute of Technology.) Thus, at the end of 2000, the ICQ adopted the practice of eliminating the suffixed numerals to comet names; this does mean, however, that permanent numbers or year/letter/number designations must be used with the name (of course, it is sufficient to use the permanent numbers or designations alone without the name, but not vice versa). The IAU adopted a policy of *optionally* including or excluding suffixed numerals to comet names when it approved the new set of comet-naming guidelines in March 2003. Thus, the Minor Planet Center and the Central Bureau for Astronomical Telegrams also no longer use comet-name suffixed numerals. (While a few people who have dealt a lot with comets over the last several decades will wish to continue using the redundant suffixed numerals for nostalgic reasons, such use will fade into oblivion during the next generation of astronomers -- so one might as well stop using them now and concentrate instead on the more logical prefixed numerals. This will make it easier on astronomers of the future, who will soon be wondering what the suffixed numerals stood for in the "old" literature, as the old suffixed numerals fall rapidly into dis-use.)

While it is assumed by many that all comets are named for their discoverers, this is not strictly true. Some comets discovered long ago (1P/Halley, 2P/Encke, 27P/Crommelin) were named for astronomers who actually worked arduosly on their orbits to show that observations at different "apparitions" were one and the same comet. Others in the past have been erroneously named for non-discoverers due to lack of accurate information. For comets that do not receive the names of the individual people involved, it is perhaps better thought of as "labelling" such comets (rather than "naming"). Some comets, then, have been "labelled" for spacecraft that have detected them (including hundreds of comets labelled "SOHO" for the sun-orbiting coronagraph spacecraft that has imaged many tiny comets very close to the sun that are usually invisible from the ground). Today there are numerous professional CCD survey programs that scan the sky most clear nights of each month looking for near-earth objects (NEOs), which are usually minor planets (or more rarely, comets) that pass -- or can pass -- within about 0.1 AU of the earth. Convention is now that most comets found by such surveys, which each employ numerous people, are labelled/named usually (but not always) for the survey program name; thus, we have some 117 comets labelled "LINEAR" as of Sept. 2003, 35 comets labelled for "NEAT", nine comets labelled for "LONEOS", seven comets labelled "Spacewatch", four comets labelled "Catalina", and three comets labelled "Tsuchinshan", Sometimes two-member teams will get both names on a comet, but two names is a firm limit for such teams; occasionally a single astronomer from a large-survey program will be the observer, discoverer, and communicator of a new comet discovery, and such individual names are in such cases sometimes allowed to go with the comet rather than the team name. The general rule today is that, once a comet has been announced widely via the Internet (WWW or e-mail), it is no longer eligible for additional names to be added due to "independent discoveries". Some comets have never been named, including most comets from the 18th century and earlier, and many other comets discovered years after they were visible (from photographs, for example) and many comets that have no possible orbit determinations (due to few available observations).

In the 1930s and 1940s, it became customary on IAU Circulars to put the discoverer's name after the word "Comet", with the year designation given parenthetically. But in 1995, with the official IAU change to a new comet-designation scheme that more closely resembles the minor-planet designation scheme in place for most of the 20th century (having both "provisional unnumbered" and "permanent numbered" designations), the CBAT and its allied Minor Planet Center reverted to the standard 19th-century format of giving year designations first with names (if at all) parenthetically. This is because scientists dealing with a lot of data in computer form will invariably want to use a designation only for a given comet in a form that will have a uniform number of sortable columns. (It may have made some sense to refer to comets by name when there were not so many comets around, but nowadays with so very many comets discovered every year, it is really easier and more certain and safe to refer to comets by their designation primarily and by their names in a more secondary manner.) Thus, comets are assigned provisionally a year-letter-number designation that specifies the year, half-month, and sequence within that half-month of discovery (the year and half-month/sequence-number being separated by a single space) -- and this is preceded by a "C/" for single-apparition comet or a "P/" for either a multiple-apparition comet or a single-apparition comet with orbital period less than about 30 years. (Sometimes a "D/" is used for short-period comets that cannot be predicted accurately and are essentially considered lost, due to limited observations.) The permanent numbers are given in chronological (historical) order in which the comets are confirmed as returning to perihelion two or more times, and are given as "nnnP" or "nnnP/name", where nnn is the number.

Many people use a popular approach and use only the name (and not the designation) when referring to a comet. It is natural in everyday speech (discussions between two or three people), when talking about a given comet, to refer to it by name alone, as there will usually not be any ambiguity amongst those partaking in the discussion (and one can ask the other if not clear). But in formal talks or lectures and in print, one should always use the comet's designation (with or without the name) for absolute clarity. Even scientists get sloppy in this manner in the literature, and one will find that few editors seem to understand the usefulness of maintaining a unified approach to comet nomenclature in a journal or conference proceedings -- so that one finds the authors (and, by default, the editors) carelessly using their own personal preferences (which may number a dozen different possible ways of referring to a comet!) in their own writings about a comet or comets. We strongly discourage this sloppiness and encourage all writers (especially scientists, but also regular journalists) to follow the official CBAT/MPC/ICQ format for comet nomenclature, as is uniformly visible on all of our web pages and all our publications without exception. Thus, we have unnumbered, single-apparition comets like "C/1995 O1 (Hale-Bopp)" and numbered comets like "1P/Halley", which can also be called comet "1P/1982 U1 (Halley)" for its most recent apparition, or simply "1P (Halley)" or even just "1P". This last term is often useful for use in tables and graphs/charts in particular, especially in the case of a comet with a really long name, where (for example) "57P" alone may be preferable to "57P/du Toit-Neujmin-Delporte" when mentioned many times in a given paper (of course, the full designation and name should be given at the beginning of any paper on a particular comet, for clarity). [Note that such unofficial abbreviations as "d-N-D" or "57P/d-N-D" are to be avoided, though a few scientists have a habit of using such awkward shorthand.]

In the case of discussing pre-1995 comets, it is recommended to use both the new-style and old-style designations when referring to such comets in text, for the convenience of readers who may either be familiar with older designations or who may be consulting older literature containing only the old-style designations. Thus, comet "C/1973 E1 (Kohoutek) = 1973 XII = 1973f", or "C/1973 E1 (Kohoutek; O.S. 1973 XII = 1973f)".

Further reading:

Green, D. W. E. (1997). ICQ Guide to Observing Comets, p. 11.

Marsden, B. G. (1995). ICQ 17, 3.

Index to the CBAT/MPC/ICQ pages.