What Is a Constellation?

A constellation is a pattern of stars in the night sky that people have grouped together and given a name. Most of the names we use in Europe today come from ancient Greek and Roman myths, but other cultures have seen completely different shapes in exactly the same stars. Today astronomers officially recognise 88 constellations, and together they cover every patch of sky.

  • Official constellations88Agreed by the International Astronomical Union in 1922
  • LargestHydraThe water snake, covers approx. 3% of the sky
  • SmallestCruxThe Southern Cross, only 0.16% of the sky
  • From ancient Greece48Catalogued by Ptolemy approx. 150 AD
  • Zodiac constellations12Sit along the Sun's path through the sky
  • Brightest star (any)SiriusIn the constellation Canis Major (Big Dog)

The 88 official constellations

In 1922, an organisation called the International Astronomical Union (the IAU) divided the entire night sky into exactly 88 constellations, each with a fixed boundary like a country on a map. Every star you can see, no matter how faint, sits inside one of those 88 areas. So when astronomers say a new star or galaxy has been discovered "in Orion", they mean it lies inside the official boundary of the constellation Orion, not just somewhere near the pattern of stars itself.

Constellations vs asterisms

Not every star pattern is a constellation. A smaller, well-known pattern of stars that is part of a constellation (or stretches across more than one) is called an asterism. Some famous examples:

  • The Big Dipper (or "the Plough" in the UK) is an asterism that forms part of the constellation Ursa Major.
  • Orion's Belt is an asterism inside the constellation Orion.
  • The Summer Triangle is an asterism that links three bright stars from three different constellations.

Asterisms are great for finding your way around the sky, but they do not have official boundaries the way constellations do.

How were constellations named?

The constellations Europeans know best come from ancient Greek and Roman mythology. Around 150 AD, an astronomer called Ptolemy wrote down 48 constellations in a book called the Almagest, and these are still in use today. They include all the famous ones: Orion the Hunter, Ursa Major the Great Bear, Leo the Lion, Cassiopeia the Queen, and the 12 zodiac signs.

The other 40 constellations were added much later, mostly by European astronomers in the 1600s and 1700s who sailed south for the first time and could see the southern sky. That is why southern constellations have names like Microscopium (the microscope) and Telescopium (the telescope), which sound oddly modern. They were named after the instruments scientists were using to study the sky at the time.

Fact Many cultures saw very different things in the same stars. To the ancient Greeks, the seven bright stars of the Plough were the tail and back of a great bear. To traditional Chinese astronomers the same stars were a giant government official's carriage. Some Australian Aboriginal peoples saw the dark patches between the stars of the Milky Way as a giant emu in the sky. None of these is "wrong"; constellations are pictures we draw with our imagination.

The zodiac constellations

Twelve of the 88 constellations are special because they sit along the path the Sun appears to follow through the sky over the course of a year. That path is called the ecliptic, and the constellations along it are the zodiac: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces. The Sun appears to pass through each one in turn, spending about a month in each.

(Astronomers sometimes count a 13th: Ophiuchus, the Serpent Bearer. The Sun does pass through a small part of Ophiuchus every December, but astrologers usually leave it out of the zodiac.)

How do you find a constellation?

You do not need a telescope to spot the brightest constellations. With clear, dark skies (away from city lights) and a bit of practice, you can navigate the whole sky just using your eyes. Some easy ones to start with:

  • Ursa Major (Big Dipper / Plough): the easiest northern constellation. Once you find it you can use it to point at Polaris, the North Star.
  • Orion: easy in northern winter (or southern summer). Look for the three stars in a row that make up Orion's Belt.
  • Cassiopeia: a clear W shape almost opposite the Big Dipper across the North Star.
  • Crux (Southern Cross): the easiest constellation in the southern sky. The two pointer stars of Centaurus point straight at it.
Did you know? The stars in a constellation are not actually close to each other in space. They just happen to lie in the same direction as we look out from Earth. The seven stars of the Big Dipper sit at different distances, from about 58 to 124 light years away. If you flew a spaceship to one of them and looked back, the familiar pattern would disappear completely.
Deeper dive: why every star has an official constellation

Before 1922, astronomers around the world had been using slightly different constellation boundaries for centuries. This caused real problems, especially with the discovery of new objects: an astronomer in Paris might say a new comet was "in Pisces" while one in Berlin called the same patch of sky "in Cetus".

To fix the confusion, the International Astronomical Union (IAU) was set up in 1919 and one of its first big jobs was to standardise the constellations. A Belgian astronomer called Eugene Delporte spent the early 1920s drawing precise boundaries around all 88 constellations, using straight lines that ran along the celestial coordinate grid (lines of right ascension and declination, the sky's equivalent of latitude and longitude). His boundaries were officially adopted in 1930 and are still used today.

Because of this neat division, every single point in the sky now belongs to exactly one constellation. When astronomers discover a new star, galaxy, planet or comet, the first thing they record is which of the 88 constellations it sits in. Even the most distant galaxies, billions of light years away, get an entry in the "Hydra" or "Coma Berenices" lists depending on where they appear in our sky.

Want to explore individual constellations? See Orion, Ursa Major, Cassiopeia, Leo or the Southern Cross.