What is a constellation?

Constellation

When you ask someone in Australia to name a constellation, they usually name the Southern Cross, because it’s on our national flag. But getting them to name another leads to either a shrug of the shoulders or a garbled reference to asterisms using the stars of Orion, such as the three stars of Orion’s belt, or the Saucepan. Very few people, including people actually interested in the night skies, take the time to learn the difference between a constellation and an asterism, or even the history of the constellations and why we use the ones we do. I think this is a great shame, for this is our cultural heritage from ancient times, before there was written language. But let us address the matter in light of the present day and make clear which is which.

Constellations once did mean a pattern of stars in the sky, and sometimes even just a single star was representative of a figure, as Sirius once did, representing the Dog of Heaven in its entirety before the Romans furnished him with a body and limbs. Different cultures had their own constellations, and in western civilisation astronomers began making up their own constellations to fill in the perceived gaps from the late 1500’s. While some of the more modern constellations were accepted by their creator’s peers, others just never caught on – for which we can probably be grateful for in the majority; I just can’t see anyone appreciating the nobility of Limax the Slug, as proposed by John Hill in the mid 1700’s.

UraniasMirrorHydra

Sadly Felis the Cat and Noctua the Owl didn’t make the final list of 88 constellations used today.

Towards the end of the 1800’s it was realised that a standardisation of the sky was needed, a global language for astronomers, no matter what country they came from. First a list of constellations to be retained was agreed upon. Then the newly formed International Astronomical Union (IAU) commissioned the Belgian astronomer Eugene Delporte to draw the boundaries between them. The finished work was presented in 1930, a mere 86 years ago as I write this. How quickly this history has been forgotten! But these boundaries are important to astronomers for descriptive purposes, for when they say a object is “in” a constellation they mean it is within these invisible, but fixed boundaries.

Asterism

On the other hand, asterisms can be fluid and changeable, belong to no one and you’re even allowed to make up your own! We can describe an asterism as a prominent pattern of stars that often has a common name associated with them. The shapes of the brighter and more well-known constellations are really asterisms within the constellation borders. Examples of asterisms such as these would be the Teapot of Sagittarius, the Pointers to the Southern Cross (alpha and beta Centauri), the Sickle of Leo (the Lion’s question-mark shaped head, since we rarely use sickles in our increasingly urbanised lives) and the Big and Little Dippers of the northern polar Bears. Asterisms can even cross constellation boundaries, such as the Summer Triangle, a large asterism in the northern sky made up from the three bright stars Deneb in Cygnus, Altair in Aquila and Vega in Lyra.

Saucepan asterism

The Saucepan in Orion, as seen from the southern hemisphere.

I’m sure that we will still continue to look up and search for those familiar asterisms to guide us to the constellations, and there is nothing wrong with that. It’s kind of like making up nick-names and short-cuts to help us get around. But I think its also good to remember there is a proper formality of address that was set in place to help us communicate in a universal language about the sky around the world, so there was no confusion, the same way there is a Universal Time system.

Do you have a favourite asterism? Perhaps you’d like to let us know which one in the comments.

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