In order to classify the chronological appearance of comets, the Astronomische Nachrichten ("Astronomical Reports") introduced in 1870 a system of preliminary and final designations that was used until 1995. The preliminary designation classified comets according to their order of discovery, using the year of discovery followed by a lowercase letter in alphabetical order, as in 1987a, 1987b, 1987c, and so forth. Comets were then reclassified as soon as possible— usually a few years later—according to their chronological order of passage at perihelion (closest distance to the Sun); a Roman numeral was used, as in 1987 I, 1987 II, 1987 III, and so on.

In 1995 the International Astronomical Union (IAU) simplified the designation of comets since the two chronologies of letters and Roman numerals were often the same, and redesignating a comet after its perihelion was confusing. A newly discovered comet is called by the year in which it was discovered, then by a letter corresponding to the half-month of discovery, and finally a number denoting its order in that half-month. For example, Comet Hale-Bopp was 1995 O1. The official designation generally includes the name(s) of its discoverer(s)—with a maximum of three names—preceded by a P/ if the comet is on a periodic orbit of less than 200 years. If a comet has been observed at two peri-helions, it is given a permanent number. For example, Halley's Comet is 1P/Halley since it was the first comet determined to be periodic. Comets with longer periods or that will not return to the inner solar system are preceded by a C/.

The discoverer's rule has not always been strictly applied: comets 1P/Halley, 2P/Encke, and 27P/Crommelin have been named after the astronomers who proved their periodic character. In the past, some comets became bright so fast that they were discovered by a large number of persons at almost the same time. They are given an arbitrary impersonal designation such as the Great September Comet (C/1882 R1), Southern Comet (C/1947 X1), or Eclipse Comet (C/1948 V1). Finally, comets may be discovered by an unusual instrument without direct intervention of a specific observer, as in the case of the Earth-orbiting Infrared Astronomical Satellite (IRAS). Its initials are used as if it were a human observer, as in C/1983 H1 IRAS-Araki-Alcock.

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