What are constellations?

While we know that the stars are many light years away, and all at different distances, astronomers still put them into groupings called constellations. These are named after the characters, animals and objects they resemble – some of which are from the pages of mythology.

The constellations help us make sense of the night sky. Without them, it would be a chaotic sprinkling of stars. We could still use celestial coordinate systems – which are similar methods to how we use latitude and longitude on Earth – to locate objects in the night sky, but constellations give astronomers a quick and visual way of getting their bearings in the cosmos.

There are 88 official constellations (unofficial constellations, such as the Big Dipper, are called asterisms) and these provide the catalogue names of the stars. For example, Deneb is the brightest star in the constellation Cygnus, so this is known as Alpha Cygni. The brightest star in Boötes is Arcturus, so it is also called Alpha Boötis. The constellations also help us to locate deep sky objects. The Andromeda galaxy is in the constellation of Andromeda, so seasoned stargazers will know straight away the rough area of the sky in which to look.

Distances to the stars of Orion – originally appeared in HIW Issue 80

The entire sky is mapped by constellations – there’s not one bit of celestial real estate that isn’t included. The modern constellation borders were drawn up in 1930 by the International Astronomical Union (IAU), and extend beyond the traditional star patterns to include the regions of space around them.

You’ll probably have heard of the Zodiac constellations – Capricorn, Aquarius, Pisces and so on. They are plotted along the path that the Sun appears to move through each year. Your sign is the constellation that the Sun was in at the time of your birth.

Origins of constellations

Civilisations going back to ancient times are thought to have charted the constellations. At first, these patterns of stars were used for astrological predictions and navigation, as well as for communication among astronomers. However, as the modern field of astronomy developed, it was soon discovered that different culturally nominated constellations made communication tricky. To solve the problem, the IAU divided the sky into 88 constellations between the Northern and Southern Hemisphere and gave them universally accepted names.

The constellation of Orion depicts a hunter from Greek mythology

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