What Are Constellations?
Ancient cultures traced star patterns into the images, or constellations, that modern astronomers use as the basis for celestial maps.
Our desire to chart the night sky led people to imagine connections between star locations.
Making Sense of the Stars
At the very birth of astronomy thousands of years ago, people gazed up at the darkness and imagined lines stretching from star to star to create images drawn from their mythology.
In our current scientific era, astronomers work with a different concept of constellations. Modern researchers divide the night sky into 88 sectors in a process that is a bit like dividing a map of a land mass into states.
Each of these sectors is an area of the sky with borders that are defined by celestial coordinates: numbers based on the celestial equivalents of longitude and latitude on Earth.
See constellations in your night sky
All 88 sectors were created to include a constellation. But there is more to a modern constellation than just one pattern of stars. Everything inside an area’s borders—deep sky objects, galaxies, passing asteroids or comets, and so on—is considered to be a part of that constellation.
Of course, these modern constellation borders aren’t visible, but on a clear night you can get a good view of some traditionally well-known star groupings.
The Big Dipper (also known as the Plough) is an asterism; a star formation that can be part of a larger constellation.
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Asterisms are often confused with constellations, but while any distinct grouping of stars can be thought of as an asterism, these are usually smaller patterns that are not recognized as a constellation. The Big Dipper (also known as the Plough in the UK and Ireland) is possibly the best-known asterism, made up of a group of stars in the larger constellation of Ursa Major.
Who Decides About Constellations?
Since 1919, the International Astronomical Union (IAU) has been defining the constants of the night sky, including constellations. At the 1928 IAU Congress, the group officially recognized 88 constellations, most of them drawn from ancient Greek astronomy.
The IAU is also responsible for naming celestial bodies. With members in more than a hundred countries across the globe, the organization hosts conferences, promotes research, and works to raise the profile of astronomy with the public.
Twelve Signs of the Zodiac
Zodiac, an ancient Greek word for cycle or circle of little animals, has become shorthand for a well-known set of 12 constellations:
- Capricorn (The Goat)
- Aquarius (The Water Bearer)
- Pisces (The Fish)
- Aries (The Ram)
- Taurus (The Bull)
- Gemini (The Twins)
- Cancer (The Crab)
- Leo (The Lion)
- Virgo (The Virgin)
- Libra (The Scales)
- Scorpio (The Scorpion)
- Sagittarius (The Archer)
During the Earth’s year-long orbit, the Sun passes through these star groupings in a band of sky that stretches about 8° north and south of the ecliptic—an imaginary line that traces the Sun’s annual path.
Many people don’t realize that the Sun also moves through a 13th constellation known as Ophiuchus, the Serpent Bearer, sitting between Sagittarius and Scorpius, but this sign isn’t included in the Zodiac because astronomers preferred to divide the ecliptic into twelve equal sections of 30° each.
The Moon and some planets pass through these constellations too. And for most of us, this celestial movement is best known as the basis for predictions made by horoscopes.
Even though they are not evidence-based, horoscopes are very popular and, ironically, have probably had the effect of boosting the profile of the science of astronomy by spreading knowledge about the constellations and inspiring star-gazing in general.
Deep Roots in History
The tradition of night sky observation that created the basis of our modern constellation system stretches far back in time. Well before the Greeks, as long ago as 1800 BCE, Babylonian astronomers were probably the first to divide the ecliptic into twelve sections. Other ancient cultures in China, Sumeria, and Egypt also developed their own star maps around this time.
Identifying constellations has an even longer history. Possibly the oldest star map found is carving on a mammoth tusk that resembles the constellation of Orion, thought to have been created some 32,000 years ago.
Also, some of the famed cave paintings of Lascaux, France, dating in the range of 33,000 to 10,000 years old, appear to depict Pleiades and the Northern Crown, among other star formations.