The naming of comets can often seem confusing. Astronomy is an old science and as it progresses it becomes necessary to change the way that objects in the sky are identified. For example, when the first asteroids were discovered they were treated as planets and given names, but as more and more were found and it was realised that they constituted a population different from the major planets, it became useful to have a means of identification that was more easily arranged in catalogues. The same thing has happened with comets.
In the early days, comets were given names that were either those of their discoverers or those who worked on the determination of their orbits. Later they received a designation consisting of the year of discovery followed by a lower-case letter indicating a chronological sequence Thus the first comet discovered in, for example, 1990 would be C 1990a, the second C 1990b, and so on. An alternative designation used for a time was to give the year the comet reached perihelion followed by a sequential Roman numeral. With the advent of automated searches, the number of comets discovered became too large for this scheme.
An additional complexity arises because of the need for a provisional designation on discovery and a permanent one when the true nature of the discovery is known. This is particularly important for periodic comets which need a permanent name irrespective of the date of an observation. In 1995 the International Astronomical Union agreed on a standard which I summarise below.
C for Comet or P for Periodic followed by a / and the year of discovery. This is followed by an upper-case letter to indicate the half-month when it was discovered, and a sequential number. Finally a name (usually that of the discoverer but not always) in brackets. The month letter is A for the period 1 - 15 January, B for 16 - 31 January, and so on through the alphabet except that I is omitted and Z is not needed. The initial letter of the designation will normally be C until such time that the orbital elements are sufficiently well determined that the period is assured to be less than 200 years. In addition it can be D if the comet has vanished, X if the nature of the object is uncertain, A if it is a minor planet, or S if it is a satellite.
Thus the comet I imaged on 10th January 2007 is called C/2006 P1 (McNaught), which interprets as a non-periodic comet (C/), the first discovered in the first half of August (P1) 2006 by McNaught. It must not be confused with C/2007 T1 (McNaught) discovered by the same observer in the beginning of October 2007. (Robert H. McNaught is a prolific discoverer of comets and asteroids. So far (December 2007) he has discovered 23 comets (11 of them periodic) and is co-discoverer of 11 more. He has also discovered 410 asteroids. Born in Scotland he now lives in Australia.)
The name associated with a comet is generally that of its discoverer or, in the case of the older comets, the name of the scientist who calculated its orbit or predicted its return. When there is joint discovery, both (or at most all three) names may be appended. Nowadays many comets are discovered by teams or automated systems engaged in other tasks. Examples are SOHO (SOlar and Heliospheric Observatory), which frequently spots comets passing very close to the Sun, and LINEAR (LIncoln Near Earth Asteroid Research), one of several projects searching for asteroids passing close to the Earth. Comets discovered by these projects generally bear the name of the project.
Once a comet has been seen to return, or it has been followed through aphelion, its designation is preceded by a sequential number maintained by the Minor Planet Center. Following this designation, the name is often abbreviated to nP/name. For example 1P/1682 Q1 (Halley) would be known as 1P/Halley (or in this famous case as "Halley's Comet"). Thus the second comet I imaged in 2007, 17P/Holmes, was the most recent return of the 17th periodic comet to be discovered, originally by Edwin Holmes in 1892.
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