The Difference Between Asterisms And Constellations

By Debra Kelly on Tuesday, December 23, 2014
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“The stars are golden fruit upon a tree / All out of reach.” —George Eliot, The Spanish Gypsy

In A Nutshell

Even amateur astronomers will be able to find the Big Dipper, the Little Dipper and Orion in the night sky, but most of those amateur astronomers will think that they’re finding constellations. Orion is one of only 88 constellations that were established in ancient Greece and later between the middle of the 16th century and the middle of the 18th century. The dippers, along with other star formations, are asterisms, which are star patterns that make up small parts of the larger constellations.

The Whole Bushel

Ask anyone about the first constellations they learned to recognize in the night sky, and chances are good that their responses will include a couple familiar ones—Orion, the Big Dipper, the Little Dipper. Only, they’re not all constellations.

A constellation isn’t just the name of a pattern of stars, it’s a very specific group of them. There are 88 constellations, a mix of mythical heroes, animals, insects, even serpents and dragons. The first written reference to the constellations in a series of images that might look familiar to us today came in 270 B.C. with a poem by the Greek writer Aratus. In A.D. 150, Ptolemy wrote a rather definitive guide to the Greek knowledge of the stars, and included 48 constellations that we still consider part of our 88.

The rest were added centuries later, mostly between 1550 and 1750. For the next hundred years or so after that, it was something of a fad for astronomers to create constellations based on whose good graces they wanted to get into, but most of these never caught on. They show up in contemporary maps, but just as quickly drop off the next set of astronomical charts.

The only constellations are these 88 sets of stars that include the signs of the zodiac and familiar nighttime sights like Orion. It doesn’t include other star patterns like the Dippers, though.

The Big Dipper and the Little Dipper are called asterisms. They’re star patterns in the same way constellations are, but they’re smaller patterns that make up the bigger constellations. The dippers are parts of the bear constellations; the Big Dipper makes up part of the tail and rear half of Ursa Major, while the Little Dipper is part of Ursa Minor.

Just as the constellations have undergone some changes while remaining a part of the same pattern, so have asterisms. The Big Dipper is called a Plough in Britain and a Saucepan in France, but the stars remain the same, notable because they’re always visible in the sky in the northern hemisphere in the same pattern that rotates throughout the seasons. It makes them an invaluable navigational tool, even today.

In 600 B.C., we hear stories about the Little Dipper being a part of the constellation Draco the Dragon rather than Ursa Minor; taking the wings off the dragon created the “new” constellation we’re more familiar with today.

The dipper and the bear constellations aren’t the only pairing of constellations and asterisms we have, they’re just the most popular. All constellations are made up of these smaller asterisms; for example, the constellation Sagittarius has three asterisms: the HSL Chain, the Teapot, and the Teaspoon. Aquarius has the DNA Strand and the Water Jar, while Virgo has Jaws and the Skip Loader.

Many constellations and asterisms are fixed stars. They’re not technically, precisely, absolutely fixed in space, but they’re close enough that we’ll see no difference in our lifetimes and even thousands of years from now, they’ll still look pretty much the same. The dippers were in the sky 100,000 years ago and will still be there 100,000 years in the future, with only some fairly slight variations. That’s because the grouping has enough gravitational pull that it’s remaining together and moving in the same direction—a surreal realization about how many generations have looked up at the same night sky.

Show Me The Proof

EarthSky: Big and Little Dippers: Everything you need to know
Constellations: FAQ
Asterisms: The Night Sky Atlas
Doorstep Astronomy: See the Big Dipper