What Are Constellations?



   The patterns of stars seen in the sky are usually called constellations. Astronomers use the term constellation to refer to an area of the sky, which contains all the stars and other celestial objects within that area. The International Astronomical Union (IAU) divides the sky into 88 official constellations with exact boundaries, so that every place in the sky belongs within a constellation.

   Today, it can be difficult to make out some of the shapes of the constellations because city lights obscure many of the dim stars. But they are still used as an important organizational tool to help identify and locate objects in space.

   Throughout the centuries, people have looked to the stars to help them navigate across open oceans or featureless  deserts, know when to plant and harvest, and preserve their myths and folklore. Ancient peoples used the appearance or disappearance of certain stars over the course of each year to mark the changing seasons.



   Constellations were seen by ancient people as pictures in the stars. Their origins date back hundreds of years into our past. These grouped alignments of stars, or constellations are also called asterisms. Ancient cultures around the world have assigned different pictures to these star groupings. Many of the images seen by ancient people find their roots shrouded in mythology. Many of the oldest known constellations were imagined by the ancient Greek and Roman civilizations.   

   Our modern constellation system comes to us from the ancient Greeks.  The oldest description of the constellations as we know them comes from a poem, called Phaenomena, written about 270 B.C. by the Greek poet Aratus.  However, it is clear from the poem that the constellations mentioned originated long before Aratus’ time. No one is sure exactly where, when, or by whom they were invented.  And yet a little detective work reveals a plausible origin.   

   The first clue is that Aratus’ constellations did not include any near the south celestial pole (the point on the celestial sphere directly above the Earth’s south pole) because that area of the sky was always below the horizon of the ancient constellation-makers. From the size of this uncharted area of the sky, we can determine that the people responsible for the original constellations lived near a latitude of 36° north — south of Greece, north of Egypt, but similar to the latitude of the ancient Babylonians and Sumerians.   

   In addition, the constellation-free zone is not centered exactly on the south celestial pole. Because of a “wobble” of the Earth’s axis of rotation, the position of the celestial poles changes slowly with time, a phenomenon known as precession. The uncharted area is centered on the place in the sky where the south celestial pole would have been around the year 2000 B.C. This date matches the time of the Babylonians and Sumerians.   

   Thus, it seems likely the Greek constellations originated with the Sumerians and Babylonians.  From there, knowledge of the constellations somehow made its way to Egypt (perhaps through the Minoans on Crete who had contact with the Babylonians and settled in Egypt after an explosive volcanic eruption destroyed their civilization), where early Greek scholars first heard about the constellations and wrote about them.   

   Over the centuries, some astronomers have attempted to name constellations after themselves or to flatter a patron or king.  This reached a peak during the heyday of celestial mapping in the seventeenth and eighteenth centuries.  Few of these survived longer than the astronomers who named  them, although they sometimes can be seen in antique star charts.   

   At its first meeting in 1922, the International Astronomical  Union (IAU), astronomy’s governing body which is responsible, among other things, for assigning names to celestial objects and features on those objects, officially adopted the list of 88 constellations that we use today.  Definitive boundaries between constellations, which extend out beyond the star figures, were set in 1930, so that every star, nebula, or galaxy, no matter how faint, now lies within the limits of one constellation Today, constellations refer not so much to the patterns of stars, but  to precisely defined areas of the sky.



   The constellations appear to form shapes across the sky, but the stars themselves don’t make up patterns in space. The distance from our world to the individual stars in a constellation varies, often by tens of light-years, scattering the stars randomly across the galaxy.

   The pictures we see at night are formed because we only see two dimensions on the night sky, missing the depth that is also present.    Still, the constellations can provide entertainment and a source of imagination. They can help the lost to find their way. They aid sky watchers in the search for planets, comets, or other events, by a process called star hopping. And, as they surely did with the ancients, they can provoke a sense of timeless wonder.



   Ancient astronomers often spoke of the “fixed stars,” which maintained permanent positions in the sky.  And, indeed, the stars do seem almost fixed in place; the patterns they form look much the same today as they did when the constellations were first named nearly 3000 years ago.  But the stars are all moving relative to the Sun, most with speeds of many kilometers per second. Because they are so very far away, it will take thousands of lifetimes to see significant changes in the star patterns. But, over time, they will change.  Because of the motions of the stars within it, for example, the handle of the Big Dipper will, in about 50,000 years, appear significantly more bent than it is today.  We will, no doubt, keep the same names for the constellations, even if the stars change their positions. Constellations are, after all, products of human imagination, not nature.


One thought on “What Are Constellations?

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