Astronomical naming conventions

From Academic Kids

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 recognized by astronomers worldwide as the 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: this reflects the leading role of Arab culture in astronomy while Europe was experiencing the Middle Ages. See List of traditional star names for a list of some of these names.

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 to use designations carefully. For example, Rigel Kentaurus contains three stars in a triple star system, with the other two stars being labelled Rigel Kentarus B and C.

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 acronym SDSSp indicates that the designation is from the "Sloan Digital Sky Survey preliminary objects", and the other characters indicate celestial coordinates.

For a more detailed treatment of the designations of stars, see star designation.

Managing the acronyms 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 "acronym", which is kept globally unique by the IAU. Different star catalogues then have different naming conventions for what goes after the acronym, but modern catalogues tend to follow a set of generic rules for the data formats used.

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. Even though the constellations are scientifically meaningless, they still 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 galaxies

Like stars, most galaxies do not have names. There are a few exceptions such as the Andromeda Galaxy, the Whirlpool Galaxy, and others, but most simply have a catalog number.

In the 19th century, the exact nature of galaxies was not yet understood, and the early catalogs such as the Messier catalog simply grouped together open clusters, globular clusters, nebulas, and galaxies. The Andromeda Galaxy is Messier object 31, or M31; the Whirlpool Galaxy is M51. The New General Catalogue (NGC) was a much larger catalog which contained thousands of objects.

In general, most galaxies are just given catalog designations.

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, although 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 Terra.

Three more planets were discovered later:

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 England. French astronomers began calling it Herschel before German Johann Bode proposed the name Uranus, after the Greek god. The name "Uranus" did not come into common usage until around 1850.

Since the discovery of Pluto, a large number of large trans-Neptunian planetoids have been discovered. Under modern criteria, it is dubious whether Pluto would have been called a planet: its mass is now known to be much smaller than what was once thought, and it may simply be the largest member of a large set of Kuiper belt objects.

Natural satellites of planets

The Earth's moon is simply known as the Moon, or sometimes as 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 William Shakespeare or Alexander Pope.

When satellites are first discovered, they are given provisional designations such as "S/2000 J 11" (the 11th new satellite of Jupiter discovered in 2000) 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. These designations are sometimes written like "S/2003 S1", dropping the second space.

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. See planetary nomenclature for more details.

Minor planets

Minor planets are initially assigned provisional designations when observed, with names like "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, and it becomes known as, for instance, (28978) 2001 KX76. 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 sequential number.

After the sequential number is assigned, the discoverer is given an opportunity to propose a name, which replaces the provisional designation. Thus for instance, (28978) 2001 KX76 is now known as 28978 Ixion. However, 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 discovered from now on will never receive a name.

Sometimes unofficial names are proposed before the above rules are satisfied. For instance, in the case of the highly unusual object 90377 Sedna, it only had the provisional designation "2003 VB12" until its orbit was determined well enough for it to be assigned a sequential number (90377). Shortly thereafter, the IAU officially adopted the name "Sedna".

Names are subject to IAU approval. All asteroid names must be less than 16 letters long and preferably one word (like 5535 Annefrank). Military and political leaders must be dead for over 100 years before their names can be used.

Whimsical names can be used for relatively ordinary asteroids (such as 26858 Misterrogers), but exceptional ones are expected to follow more strictly defined naming schemes. IAU rules specify themes for naming planetoids: for example, all planetoids in Pluto-like orbits ("plutinos") are to be named after creation deities (such as 50000 Quaoar).


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. Halley's Comet was named after Edmund Halley, who had calculated its orbit. Similarly, the second known periodic comet, Comet Encke, was named after the astronomer who had calculated its orbit rather than the original discoverer of the comet. Most bright 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 the first three independent discoverers of it. 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 was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock).

Until 1994, the systematic naming of comets 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 (eg Comet Bennett 1969 i was the 9th comet discovery in 1969). Once an orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral, so that Comet Bennett 1969 i became Comet Bennett 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. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery, so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. 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, and D/ indicating a comet which has broken up or been lost. Periodic comets also have a number indicating the order of their discovery. So Halley's Comet, the first comet to be identified as periodic, has the systemic name 1P/1682 Q1. Comet Hale-Bopp's systemic name is C/1995 O1. The famous Comet Shoemaker-Levy 9 was the ninth comet jointly discovered by Carolyn Shoemaker, Eugene Shoemaker, and David Levy, 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.

See also : 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. [1] (

See also


  • The Bright Star Catalogue, 5th Revised Ed. (Preliminary Version) Hoffleit D., Warren Jr W.H. Astronomical Data Center, NSSDC/ADC (1991), available online at [2] (

External links

Some contents adapted from the public domain document at Please update as necessary.


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