Astronomical naming conventions
In ancient times, only the Sun and Moon, a few hundred stars and the most easily visible planets had names. Over the last few hundred years, the number of identified astronomical objects has risen from hundreds to over a billion, and more are discovered every year. Astronomers need to be able to assign systematic designations to unambiguously identify all of these objects, and at the same time give names to the most interesting objects and, where relevant, features of those objects.
The International Astronomical Union (IAU) is the body officially recognized by astronomers and other scientists worldwide as the de facto naming authority for astronomical bodies. In response to the need for unambiguous names for astronomical objects, it has created a number of systematic naming systems for bodies of various sorts.
Names of stars
According to the IAU, apart from a limited number of bright stars with historic names, stars do not have proper names. Where historic names exist, these names are, with a few exceptions, taken from the Arabic language due to their lead in the beginning of modern astronomy. See List of traditional star names for examples.
There are no more than a few thousand stars that appear sufficiently bright in the Earth's sky to be visible to the naked eye, so this represents the limit of the possible number of stars available to be named by ancient cultures. This limit is approximate, as it varies by the acuity of any given observer's eyes, but ten thousand stars (the naked-eye stars to visual magnitude six) seems to be an upper bound to what is physiologically possible.
Estimates of the number of stars with recognised proper names range from 300 to 350 different stars. These tend to be the brightest stars, or stars that form part of constellation patterns with the brightest stars. The number of proper names for stars is greater than the number of stars with proper names, as many different cultures named stars independently. For example, the star known as Polaris has also at various times and places been known by the names Alruccabah, Angel Stern, Cynosura, the Lodestar, Mismar, Navigatoria, Phoenice, the Pole Star, the Star of Arcady, Tramontana and Yilduz.
With the advent of the increased light-gathering abilities of the telescope, many more stars became visible, far too many to all be given names. Instead, they have designations assigned to them by a variety of different star catalogues. Older catalogues either assigned an arbitrary number to each object, or used a simple systematic naming scheme such as combining constellation names with Greek letters. Multiple sky catalogues meant that some stars had more than one designation. For example, the star with the Arabic name of Rigil Kentaurus also has the Bayer designation of Alpha Centauri.
As the resolving power of telescopes increased, numerous objects that were thought to be a single object were found to be multiple star systems that were too closely spaced in the sky to be discriminated by the human eye. These and other confusions make it essential that great care is taken in using designations. For example, Rigil Kentaurus contains three stars in a triple star system, labelled Rigil Kentaurus A, B and C respectively.
Most modern catalogues are generated by computers, using high-resolution, high-sensitivity telescopes, and as a result describe very large numbers of objects. For example, the Guide Star Catalog II has entries on over 998 million distinct astronomical objects. Objects in these catalogs are typically located with very high resolution, and assign designations to these objects based on their position in the sky. An example of such a designation is SDSSp J153259.96-003944.1, where the initialism SDSSp indicates that the designation is from the "Sloan Digital Sky Survey preliminary objects", and the other characters indicate celestial coordinates.
The star nearest to Earth, our Sun, is typically referred to simply as "the Sun" or its equivalent in the language being used (for instance, if two astronomers were speaking French, they would call it le Soleil). However, it is sometimes called by its Latin name, Sol.
Finally, there are a few stars named after people.
Managing the initialisms of star catalogues
The IAU is the ultimate maintainer of the namespace of astronomical designations in catalogues of astronomical objects. The purpose of this is to ensure that names assigned by these catalogues are unambiguous. There have been many historical star catalogues, and new star catalogues are set up on a regular basis as new sky surveys are performed. All designations of objects in recent star catalogues start with an "initialism", which is kept globally unique by the IAU. Different star catalogues then have different naming conventions for what goes after the initialism, but modern catalogues tend to follow a set of generic rules for the data formats used.
Several for-profit star-naming companies sell the right to list stars in their private registries under whatever name the buyer so chooses. These companies typically disclaim the fact that they are not associated with the scientific community and that such gifts are a novelty and only symbolic in nature. Strong consumer demand for novelty star naming keeps these companies in business even when the customer is well informed. However, the IAU (and, therefore, most astronomers) do not recognize those names as "official". Most astronomical organizations (and most international scientific organizations) say that the IAU is the only body allowed to officially name heavenly objects. (It is important to realize that the companies, in fact, often 'doublebook' prominent stars with each other; and sometimes the same company will assign different names to the same object. The customer, of course, is not informed of any prior claim).
Names and boundaries of constellations
The sky was arbitrarily divided into constellations by historic astronomers, according to perceived patterns in the sky. At first, only the shapes of the patterns were defined, and the names and numbers of constellations varied from one star map to another. Despite being scientifically meaningless, they do provide useful reference points in the sky for human beings, including astronomers. In 1930, the boundaries of these constellations were fixed by Eugène Joseph Delporte and adopted by the IAU, so that now every point on the celestial sphere belongs to a particular constellation.
Names of supernovae
Supernova discoveries are reported to the International Astronomical Union's Central Bureau for Astronomical Telegrams which sends out a circular with the name it assigns to it. The name is formed by the year of discovery, immediately followed by a one- or two-letter designation. The first 26 supernovae of the year get an upper case letter from A to Z. Afterward, pairs of lower-case letters are used, starting with aa, ab, and so on. Four historical supernovae are known simply by the year they occurred (SN 1006, 1054, 1572 (Tycho's Nova), and 1604 (Kepler's Star)); starting with 1885, the letters are used, even if there was only one supernova detected that year (e.g. SN 1885A, 1907A, etc.) —this last happened with SN 1947A. The standard abbreviation "SN" is an optional prefix. As instruments improve and the number of astronomers (both professional and amateur) searching increases, more and more supernovae are observed each year - currently at least 500 a year. For example, the last supernova of 2007 was SN 2007va, indicating that it was the 572nd supernova found in 2007 (a record year, in fact).
Names of galaxies
In the 19th century, the exact nature of galaxies was not yet understood, and the early catalogs simply grouped together open clusters, globular clusters, nebulas, and galaxies: the Messier catalog has 110 in total. The Andromeda Galaxy is Messier object 31, or M31; the Whirlpool Galaxy is M51. The New General Catalogue (NGC, J. L. E. Dreyer 1888) was much larger and contained nearly 8,000 objects, still mixing galaxies with nebulas and star clusters.
Names of planets
The brightest planets in the sky have been named from ancient times. The scientific names are taken from the names given by the Romans: Mercury, Venus, Mars, Jupiter, and Saturn. Our own planet is usually named the Earth, or the equivalent in the language being spoken (for instance, two astronomers speaking French would call it la Terre). However, it is only recently in human history that it has been thought of as a planet. The Earth, when viewed as a planet, is sometimes also called by its Latin name Terra (some older science fiction uses the alternate Tellus).
At least two more bodies were discovered later, and called planets:
- Uranus, discovered by William Herschel in 1781
- Neptune, discovered by Johann Gottfried Galle in 1846 (based on predictions by Urbain Le Verrier and John Couch Adams)
All of these planets were given names from Greek or Roman myth, to match the ancient planet names. However, this was only after some controversy. For example, Sir William Herschel discovered Uranus in 1781, and originally called it Georgium Sidus (George's Star) in honour of King George III of the United Kingdom. French astronomers began calling it Herschel before German Johann Bode proposed the name Uranus, after the Greek and Roman god. The name "Uranus" did not come into common usage until around 1850.
Starting in 1801, asteroids were discovered between Mars and Jupiter. The first few (Ceres, Pallas, Juno, Vesta) were initially considered minor planets and joined the ranks of the planets. As more and more were discovered, they were soon stripped of their planetary status. On the other hand, Pluto was considered to be a planet at the time of its discovery in 1930, as it was found far beyond any then-known asteroid's greatest distance from the Sun.
Following this pattern, several hypothetical bodies were given names:
- Vulcan, for a planet within the orbit of Mercury;
- Phaeton, for a planet between Mars and Jupiter which was the precursor of the asteroids;
- Themis, for a moon of Saturn;
- Persephone (and several other names), for a trans-Plutonian planet.
These names, derived from Classical mythology, are only considered standard in Western discussion of the planets. Astronomers in societies which have alternate traditional names for the planets may use those names in scientific discourse. The IAU does not disapprove astronomers discussing Jupiter in Arabic using the term المشتري Al-Mushtarīy or astronomers speaking in Mandarin Chinese discussing Neptune referring to the planet as 海王星 Hǎiwángxīng.
Some sixty years after the discovery of Pluto, a large number of large trans-Neptunian objects began to be discovered. Under the criteria of classifying these Kuiper belt objects (KBOs), it became dubious whether Pluto would have been called a planet had it been discovered in the 1990s. Its mass is now known to be much smaller than once thought and, with the discovery of Eris, it is simply one of the two largest known trans-Neptunian object. In 2006, Pluto was therefore reclassified to a different class of astronomical bodies known as dwarf planets, along with Eris and others.
Natural satellites of planets
The Earth's moon is simply known as the Moon, or the equivalent in the language being spoken (for instance, two astronomers speaking French would call it la Lune). It is sometimes called Luna (which is simply Latin for "moon"). Natural satellites of other planets are generally named after mythological figures. Satellites of Uranus are named after characters from works by William Shakespeare or Alexander Pope.
When satellites are first discovered, they are given provisional designations such as "S/2010 J 2" (the 2nd new satellite of Jupiter discovered in 2010) or "S/2003 S 1" (the 1st new satellite of Saturn discovered in 2003). The initial "S/" stands for "satellite", and distinguishes from such prefixes as "D/", "C/", and "P/", used for comets. The designation "R/" is used for planetary rings. These designations are sometimes written like "S/2003 S1", dropping the second space. The letter following the category and year identifies the planet (Jupiter, Saturn, Uranus, Neptune; although no occurrence of the other planets is expected, Mars and Mercury are disambiguated through the use of Hermes for the latter). Pluto was designated by P prior to its recategorization as a dwarf planet. When the object is found around a minor planet, the identifier used is the latter's number in parentheses. Thus, Dactyl, the moon of 243 Ida, was at first designated "S/1993 (243) 1". Once confirmed and named, it became (243) Ida I Dactyl. Similarly, the fourth satellite of Pluto, discovered after Pluto was categorized as a dwarf planet and assigned a minor planet number, is designated S/2011 (134340) 1 rather than S/2011 P 1, though the New Horizon's team, who disagree with the dwarf planet classification, use the latter.
Note: The assignation of "H" for Mercury is specified by the USGS Gazetteer of Planetary Nomenclature; since they usually follow IAU guidelines closely, this is very likely the IAU convention, but confirmation is needed.
After a few months or years, when a newly discovered satellite's existence has been confirmed and its orbit computed, a permanent name is chosen, which replaces the "S/" provisional designation. However, in the past, some satellites remained unnamed for surprisingly long periods after their discovery. See Naming of moons for a history of how some of the major satellites got their current names.
The Roman numbering system arose with the very first discovery of natural satellites other than Earth's Moon: Galileo referred to the Galilean moons as I through IV (counting from Jupiter outward), in part to spite his rival Simon Marius, who had proposed the names now adopted, after his own proposal to name the bodies after members of the Medici family failed to win currency. Similar numbering schemes naturally arose with the discovery of moons around Saturn and Mars. Although the numbers initially designated the moons in orbital sequence, new discoveries soon failed to conform with this scheme (e.g. "Jupiter V" is Amalthea, which orbits closer to Jupiter than does Io). The unstated convention then became, at the close of the 19th century, that the numbers more or less reflected the order of discovery, except for prior historical exceptions (see the Timeline of discovery of Solar System planets and their moons).
Geological and geographical features on planets and satellites
In addition to naming planets and satellites themselves, the individual geological and geographical features (craters, mountains, volcanos and so forth) on those planets and satellites also need to be named.
In the early days, only a very limited number of features could be seen on other solar system bodies other than the Moon. Craters on the Moon could be observed with even some of the earliest telescopes, and 19th century telescopes could make out some features on Mars. Jupiter had its famous Great Red Spot, also visible though early telescopes.
In 1919 the IAU was formed, and it appointed a committee to regularize the chaotic lunar and Martian nomenclatures then current. Much of the work was done by Mary Adela Blagg, and the report Named Lunar Formations by Blagg and Muller (1935), was the first systematic listing of lunar nomenclature. Later, "The System of Lunar Craters, quadrants I, II, III, IV" was published, under the direction of Gerard P. Kuiper. These works were adopted by the IAU and became the recognized sources for lunar nomenclature.
The Martian nomenclature was clarified in 1958, when a committee of the IAU recommended for adoption the names of 128 albedo features (bright, dark, or colored) observed through ground-based telescopes (IAU, 1960). These names were based on a system of nomenclature developed in the late 19th century by the Italian astronomer Giovanni V. Schiaparelli (1879) and expanded in the early 20th century by Eugene M. Antoniadi (1929), a Greek-born astronomer working at Meudon, France.
However, the age of space probes brought high-resolution images of various solar system bodies, and it became necessary to propose naming standards for the features seen on them.
Initially, the names given to minor planets followed the same pattern as the other planets: names from Greek or Roman myths, with a preference for female names. With the discovery in 1898 of the first body found to cross the orbit of Mars, a different choice was deemed appropriate, and 433 Eros was chosen. This started a pattern of female names for main-belt bodies and male names for those with unusual orbits.
Over the years as more and more discoveries have been made, this system was eventually recognized as being inadequate and a new one was devised. Currently, the main responsibility for designating and naming minor planets lies on the Committee for Small Body Nomenclature (CSBN). Minor planets are initially assigned provisional designations when observed, of the form "2001 KX76" (the first part is a year; the second part defines a sequential order of discovery within that year, see provisional designation for details). If enough sightings are obtained of the same minor planet to calculate an orbit, the object is assigned a sequential number - its 'designation' - and it can then be cited as, for instance, (28978) 2001 KX76.
After the designation is assigned, the discoverer is given an opportunity to propose a name, which, if it is accepted by the IAU, replaces the provisional designation. Thus for instance, (28978) 2001 KX76 was given the name Ixion and is now known as (28978) Ixion, which is often abridged to 28978 Ixion. The name becomes official after its publication in the Minor Planet Circular with a brief citation explaining its significance. This may be a few years after the initial sighting, or in the case of "lost" asteroids, it may take several decades before they are spotted again and finally assigned a designation. If a minor planet remains unnamed ten years after it has been given a designation, then the right to name it is given also to identifiers of the various apparitions of the object, to discoverers at apparitions other than the official one, to those whose observations contributed extensively to the orbit determination, or to representatives of the observatory at which the official discovery was made. The CSBN has the right to act on its own in naming a minor planet, which often happens when the number assigned to the body is an integral number of thousands.
In recent years automated search efforts such as LINEAR or LONEOS have discovered so many thousands of new asteroids that the Center for Small Body Nomenclature has officially limited naming to a maximum of two names per discoverer every two months. Thus, the overwhelming majority of asteroids currently discovered are not assigned formal names.
Under IAU rules, names must be pronounceable, preferably one word (such as Annefrank (5535 Annefrank)), although exceptions are possible (such as James Bond (9007 James Bond)), and since 1982 limited to a maximum length of sixteen characters, including spaces and hyphens. Letters with diacritics are accepted, although the diacritical marks are usually omitted in everyday usage. 4090 Říšehvězd is an asteroid with the most diacritics (four). Military and political leaders are unsuitable until they are dead for 100 years. Nowadays, names of pet animals are discouraged, but there are some from the past. Names after people, companies or products known only for success in business are not accepted, as well as citations that resemble advertising.
Whimsical names can be used for relatively ordinary asteroids (such as 26858 Misterrogers), but those belonging to certain dynamical groups are expected to follow more strictly defined naming schemes.
- Trojan asteroids (those that librate in 1:1 resonance with Jupiter) are named for heroes of the Trojan War. Asteroids at Lagrangian point L4 are named after Greek warriors (such as 588 Achilles) and asteroids at L5 after Trojans (such as 884 Priamus).
- Trans-Jovian minor planets crossing or approaching the orbit of a giant planet but not in a stabilizing resonance are named for centaurs (such as 2060 Chiron).
- Objects in Neptune's 3:2 mean-motion-reference resonance are given mythological names associated with the underworld (such as 90482 Orcus).
- Classical Kuiper belt objects are given mythological names associated with creation (such as 50000 Quaoar).
- Objects that approach or cross Earth's orbit are still given mythological names (such as 1862 Apollo), preferably male.
The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. The first one to be named was "Halley's Comet" (now officially known as Comet Halley), named after Edmond Halley, who had calculated its orbit. Similarly, the second known periodic comet, Comet Encke (formally designated 2P/Encke), was named after the astronomer, Johann Franz Encke, who had calculated its orbit rather than the original discoverer of the comet, Pierre Méchain. Other comets that bore the possessive include "Biela's Comet" (3D/Biela) and "Miss Herschel's Comet" (35P/Herschel-Rigollet, or Comet Herschel-Rigollet). Most bright (non-periodic) comets were referred to as 'The Great Comet Of...' the year in which they appeared.
In the early 20th century, the convention of naming comets after their discoverers became common, and this remains today. A comet is named after up to its first three independent discoverers. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument (for example, Comet IRAS-Araki-Alcock (C/1983 H1) was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock). Comet 105P/Singer Brewster, discovered by Stephen Singer-Brewster, should by rights have been named "105P/Singer-Brewster", but this would have led most readers to believe it had been a joint discovery by two astronomers named Singer and Brewster, respectively, so the hyphen was replaced by a space. Other comets with spaces in their names, however, reflect their discoverers' name spellings (e.g. 32P/Comas Solá).
Until 1994, the systematic naming of comets (the "Old Style") involved first giving them a provisional designation of the year of their discovery followed by a lower case letter indicating its order of discovery in that year (e.g. the first Comet Bennett is 1969i, the 9th comet discovered in 1969). In 1987, more than 26 comets were discovered, so the alphabet was used again with a "1" subscript, very much like what is still done with asteroids (an example is Comet Skorichenko-George, 1989e1). The record year was 1989, which went as high as 1989h1. Once an orbit had been established, the comet was given a permanent designation in order of time of perihelion passage, consisting of the year followed by a Roman numeral. For example, Comet Bennett (1969i) became 1970 II.
Increasing numbers of comet discoveries made this procedure difficult to operate, and in 1994 the International Astronomical Union approved a new naming system (the "New Style"). Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery (A denotes the first half of January, B denotes the second Half of January, C denotes the first half of February, D denotes the second half of February, etcetera) and a number indicating the order of discovery. To exemplify, the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. "I" and "Z" are not used when describing the half of a particular month the comet was discovered. Prefixes are also added to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/ indicating a comet for which no reliable orbit could be calculated (typically comets described in historical chronicles), D/ indicating a comet which has broken up or been lost, and A/ indicating an object at first thought to be a comet but later reclassified as an asteroid. Periodic comets also have a number indicating the order of their discovery. Thus Bennett's comet has the systematic designation C/1969 Y1. Halley's Comet, the first comet to be identified as periodic, has the systematic name 1P/1682 Q1. Comet Hale-Bopp's systematic name is C/1995 O1. The famous Comet Shoemaker-Levy 9 was the ninth periodic comet jointly discovered by Carolyn Shoemaker, Eugene Shoemaker, and David Levy (the Shoemaker-Levy team has also discovered four non-periodic comets interspersed with the periodic ones), but its systematic name is D/1993 F2 (it was discovered in 1993 and the prefix "D/" is applied, since it was observed to crash into Jupiter).
Some comets were first spotted as minor planets, and received a temporary designation accordingly before cometary activity was later discovered. This is the reason for such comets as Astronomical objects named after people
Designations for extra-solar planets
At the moment, according to the IAU, there is no agreed system for designating planets orbiting around other stars, nor is there any plan to create a naming system for extra-solar planets . A trend that is gaining prominence uses a lower-case letter (starting with "b") to extend the star's designation. For example, 16 Cygni Bb is the first extrasolar planet found around the star 16 Cygni B, itself a member of the triple star system 16 Cygni. Occasionally planet names appear by appending the star name with roman numerals  (e.g. Sol I, Sol II, Sol III), but this is not used much outside of science fiction or roleplay .
- Planetary nomenclature
- List of astronomical objects named after people
- List of basic astronomy topics
- List of brightest stars
- List of minor planets (asteroids)
- Naming of moons
- Provisional designation in astronomy
- Dorrit Hoffleit, and Wayne H. Warren Jr., The Bright Star Catalogue, 5th Revised Ed. (Preliminary Version), Astronomical Data Center, NSSDC/ADC (1991), available online at 
- IAU on Naming Astronomical Objects
- IAU specifications for nomenclature
- Dictionary of nomenclature of astronomical objects
- New- And Old-Style Minor Planet Designations from the Minor Planet Center
- Committee on Small Body Nomenclature
- Cometary designation system from Minor Planet Circulars 23803-4
- Who named the planets and who decides what to name them?
- How do planets and their moons get their names?
- James Kaler on star names
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