Astronomy Answers
AstronomyAnswerBook: Names

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1. Astronomical Names ... 2. Names of Celestial Objects in the Solar System ... 3. The Name of the Sun ... 4. The Names of Planets ... 5. Names of Moons ... 6. The Moon ... 7. Rhea ... 8. The Seven Sisters ... 9. The Names of Galaxies ... 10. Do You Want to Buy a Planet or a Star or a Piece of the Moon? ... 11. Can I Have a Star Named After Someone?

This page answers questions about astronomical names. The questions are:


1. Astronomical Names

The names of astronomical things come from many different sources. Things that are easy to see, such as bright stars and planets and the Milky Way, often have different names in different societies. The names of a particular astronomical thing in different societies may be related, but often are not.


2. Names of Celestial Objects in the Solar System

Many thousands of celestial objects such as planets, moons, asteroids, and comets in our Solar System have been discovered and officially named (by the IAU), and each month a couple of new ones are added. It is not feasible to provide a full list here.


3. The Name of the Sun

The origin of the name "Sun" is probably at least three thousand years old, because it can be traced back all the way to the Indo-European language, which is the ancestor language to many languages of today, including English, French, German, Spanish, Greek, and Persian. There is no hidden or other meaning to the name "Sun": It is just the name of that big bright light that is in the sky during the day.

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4. The Names of Planets

The names of the planets of our Solar System are, counting from the Sun: Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto. All of those names except for Earth and Uranus are of gods and goddesses from Roman mythology, and Uranus is a god from Greek mythology.

The word "Earth" comes from an ancient word that means earth or ground. When that word was invented, people did not know that the Earth was a planet.

The first four planets resemble the Earth: they are rather small (for planets) and are made of metals and rocks. The next four planets are a lot larger than the Earth, look like giant balls of gas, and each have a large number of moons. The last planet, Pluto, is smallest of all and is made up of dirty ice, just like a comet.


Pluto was discovered in 1930. The name "Pluto" is the name of the Roman god of the underworld, who was equated with the Greek god "Hades". It is said that the name Pluto was chosen for the planet because it begins with the first letter of the given and family names of the discoverer of the planet, Percival Lowell.

In the last couple of years a number of other things have been discovered near the distance of Pluto that resemble Pluto. Because of this, and because Pluto is so small and made of different things than the other planets, some people think that Pluto isn't a real planet.

In the last few years, over 100 planets have been discovered beyond our Solar System, orbiting around other stars. There are no pictures of those planets. They were detected through the small wobble they cause in the motion of the stars that they orbit. Only planets similar to Jupiter can be found in that way. Planets like the Earth cannot (yet) be disscovered like that.

The planets that have been found in this way do not have names of their own (yet), because we know so little about them (and for some we're not even quite sure that they really exist), and because there are so many of them.

In a list of those planeets, at, they are named after the star that they orbit around (for example, HD 38529) followed by a lower-case letter. The star itself is called component "a" (for example, HD 38529 a). The planet that is on average closest to the star gets the letter "b" (HD 38529 b). The next further planet gets "c", and so on. The problem with this way of assigning names is that tomorrow someone may discover a planet that is even closer to the star than the planet that is now called "b", and then that new planet should be called "b", so the names of all further planets shift by one. We have to invent a better way for this.

The Milky Way is a translation of the name that the Greek astronomers of over 2000 years ago gave to the Milky Way. They thought that the Milky Way looked like a river of milk.

5. Names of Moons

The IAU assigns a name to a newly discovered moon when the orbit of that moon is known well enough that the future positions of that moon can be predicted well enough that the moon can be found again.

The names of the moons of Jupiter come from Greek and Roman mythology and are taken from partners of the Roman god Jupiter / Greek god Zeus, or their descendants. It became necessary to allow names of descendants because more moons were discovered around Jupiter than there are names of known partners of Jupiter/Zeus.

The names of the moons of Saturn are the names of giants and their descendants from the mythology of the Greeks, Romans, Gauls, Inuits, and Norse. It became necessary to allow names of descendants and from other mythologies than the Greek and Roman ones because more moons were discovered around Saturn than there are names of known Greek and Roman giants. Gallic, Inuit, and Norse names are assigned to three different groups of moons that each have similar orbits.

The names of the moons of Uranus are those of characters from the works of William Shakespeare and Alexander Pope.

The names of the moons of Neptune are those of (lesser) gods of the sea. Neptune himself is the chief god of the sea in Roman mythology.


6. The Moon

It seems unfair that all moons of other planets have their own name (such as Io or Titan or Phobos) but our Moon does not, but if you know the history of astronomy then you can understand it.

A very long time ago, people did not know what the planets and the Moon and the Sun were, except that they were lights in the sky that moved between the stars, and that's what "planet" meant originally: a light in the sky that moves between the stars. The Moon moved between the stars just like the planets did, so the Moon was regarded as a planet. Nobody had discovered any other moons yet, so there was just one thing called "Moon", and that was our Moon.

In the 16th century, astronomers discovered that the planets and the Sun do not orbit around the Earth as everybody had thought, but that actually the Earth and the planets orbit around the Sun and only the Moon orbits around the Earth. It turned out that the Moon was a special case: the only celestial object (as far as people knew then) that orbits around a planet (namely the Earth) and not around the Sun. There was then still only one object that was called "Moon".

In the year 1610, Galileo Galilei discovered four small points of light that orbited around the planet Jupiter. These newly discovered objects orbited around a planet and not around the Sun, exactly like the Moon. To quickly explain this to someone, you could then say that it was "a Moon of Jupiter" (i.e., just like the Moon, but around Jupiter), just like you can call a very good soccer player from Leeuwarden the "Cruijff of Leeuwarden" (after the famous soccer player Johan Cruijff) or a luxury bicycle the "Ferrari of bicycles" (after the famous sports cars of the Ferrari brand). After some time, "moon" did not mean only "the celestial object that orbits around the Earth", but more generally "a celestial object that orbits around a planet".

In a similar manner, some brand names can be so successful that people use them not just for the products of that particular brand, but also for all similar products made by different brands. The brand name has then turned into a type name.

You can usually still determine which celestial object is meant if someone writes about a moon: If the word "Moon" is written with a capital M, then it means our Moon which orbits around the Earth. If the word "moon" is written with a small m, then it does not mean our Moon but some other moon or moons in general. You can write: The Moon is a moon, and although there are many moons, there is only one Moon.


7. Rhea

Rhea is a moon of the planet Saturn. According to mythology, Rhea was the sister and wife of Saturn, and the mother of Jupiter, Neptune, and Pluto (among others). For more information about the moon Rhea, visit


8. The Seven Sisters

The Seven Sisters is another name for the Pleiades, an open cluster (a loose group of stars in the same region of space) in the constellation of the Bull, the 45th object in the famous list by Messier (so it is also called M 45).

Names with a number in them are not very convenient, because more groups of that many stars can be found, so such names can be applied on other groups of stars as well. That's why "the Pleiades" is better, because there is only one group of stars with that name.

In Greek mythology, the Pleiades were the seven daughters of Atlas, who were eventually placed in the sky as stars. The story goes that even people with keen eyesight can see only six stars, because one of the sisters disappeared. Pictures of the Pleiades that are taken through powerful telescopes show many more than six or seven stars.


9. The Names of Galaxies

Galaxies are usually listed in a catalog of observed things. The name of the catalog (or an abbreviation of it) and the identification of the galaxy in the catalog together form a name for the galaxy. For example, the famous Andromeda Nebula is a galaxy. It is the 31st object in the catalog made by Mr. Messier, so it is known as Messier 31, usually abbreviated to M 31. If you type in M 31 as the identifier at and press "submit", then you get a list with more than 20 different names that that galaxy has. For example, M 31 is also NGC 224 (i.e., the 224th object in the New General Catalog), and IRAS 00400+4059 (i.e., it is called "00400+4059" in the catalog of objects observed by the IRAS satellite), and so on. And that galaxy may appear in many less well-known catalogs as well. Whoever makes the catalog gets to decide what the identification of each object in the catalog should be. The Messier catalog just counts the objects, but the IRAS catalog uses the coordinates of the object in the sky (right ascension 00 hours 40.0 minutes, declination +40 degrees, 59 minutes) to identify the object.

Some galaxies are so special (so bright, or with such a strange shape) that they have one or more proper names of their own, like "the Andromeda Nebula" or "the Whirlpool Galaxy". However, there is no boss who decides what that name should be, so anybody can invent any name for any galaxy, but you cannot force anyone to use the name that you invented.


Newly discovered galaxies hardly ever get pretty names anymore. I can think of three reasons for this:

  1. Because there are so very many galaxies. More than 500,000 galaxies have been discovered so far, and it is just not possible to invent pretty names for all of them.

  2. Because obvious and easy-to-remember names have already been used for earlier discovered galaxies. There are already a "Whirlpool Galaxy" and an "Andromeda Nebula", so those names are no longer available to be used for other galaxies that resemble a whirlpool or that can be found in the constellation Andromeda, because otherwise people get confused about which one you mean.

  3. Because very few of those galaxies are seen by more than a few people. The fewer people see a particular galaxy, the smaller is the chance that someone will invent a pretty name for it, and the smaller is the chance that that name will become so well-known that everybody will know which galaxy you are talking about.

The pretty names of galaxies are not officially designated. This means that everyone can invent other names for any galaxy, or to invent a pretty name for a galaxy that does not yet have one. However, you cannot force others to use "your" name for that galaxy.


10. Do You Want to Buy a Planet or a Star or a Piece of the Moon?

Nobody on Earth owns natural celestial bodies such as moons, stars, or planets, because such bodies are beyond the jurisdiction of earth-bound courts of law, so papers of ownership of such bodies have no legal value. I think that I've once heard that many countries signed a treaty that states that nobody may claim ownership of natural celestial bodies or parts of those. If someone still seriously offers ownership of such bodies for sale, then they could be committing fraud: they're selling on their own accord something that they don't own. You may still encounter such offers now and again.


11. Can I Have a Star Named After Someone?

Anyone is free to invent a name for a celestial body (or for a thing on Earth, for that matter), but you cannot force other people to use your name for that thing. The same planet or star may have many different names, for example in different languages (for planets or bright stars), or in different catalogs (for all stars). The International Astronomical Union (IAU) is the only organization that gives names outside of the Earth that are used a lot. The IAU assigns names to celestial bodies and to geographical things outside of the Earth but inside of our Solar System, such as to craters or mountains on moons and planets, or to asteroids and comets. The names that the IAU gives are used by scientists and are also used in most atlases of such celestial bodies.

If you want to see your friend's name officially attached to something in the sky, then there are a few ways:

  1. Your friend must discover a new comet. Comets are named by the IAU for the person or persons who discover it the first.
  2. Your friend must discover a new asteroid, or become good friends with someone who has discovered a new asteroid. Someone who discovers a new asteroid the first may assign an official name to it through the IAU.
  3. Your friend must become an important person in the arts or sciences and must then die. Newly discovered craters on other planets or the Moon are sometimes named for such people by the IAU, but only long after they are dead. Scientific satellites are also sometimes named for such people (by their makers), but satellites usually last for only a few years.

None of these methods is easy.

There are no legal obstacles to assigning a name of your own to a celestial body, so there are people who offer, for a fee, to give a star a name selected by you. In exchange for your money, you receive an official-looking document that solemnly lists the name (as given by you) and the location of the star. However, that document only has value as a souvenir, and the new name of that star is not recognized or used by anybody except perhaps by you and the seller. And that same person or someone else could sell the right to name that very same star to twenty more people. You could also select your own star, create such a document yourself, and keep the money in your pocket.

Should you still be interested in such a souvenir, then you can try using your favorite search engine on the internet to look for "buy a star".


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Last updated: 2016−02−07