History of constellations
Stars move across the sky on a regular schedule, much like the sun. At various times of year, different constellations appear at sunset. The rising constellations rotate based on the Earth's path through space, and so can be used to mark the seasons in regions when moderate weather may not convey the change between winter and spring.
Most of the constellations in the northern skies bear Greek and Roman names, but people were mapping the sky long before these empires took hold. The Greeks adopted their system from the Babylonians, whose origins may have stemmed from Sumerian traditions 3,000 years before. Even further back, scientists suspect that markings on the cave walls at Lascaux in southern France — created over 17,000 years ago — may chart the Pleiades and Hyades star clusters, making it the first known star map.
In 1929, the International Astronomical Union (IAU) officially defined 88 constellations across the sky. But these constellations aren't drawn to connect certain stars, they're actually more-or-less rectangular slices of the heavens holding the stars within them. Formally cementing these boundaries allows astronomers to communicate about the regions of the sky they study. Within these 88 regions are the individual groupings that people think of as constellations. For instance, the constellation of Ursa Major contains all of the stars around the shape known by the same name.
However, many of the groupings most people consider to be constellations aren't officially constellations. The Big Dipper, for example, which lays inside of Ursa Major, is not recognized as a constellation. Instead, it is an asterism, or a group of stars not officially designated but known by most nonastronomers.
Constellation names and zodiac signs
As the Earth rotates, the sun, the moon and planets travel on a set path through the sky known as the ecliptic. The list of 13 constellations they pass through are known as the stars of the zodiac. The zodiac constellations' names are:
Astrologers use 12 of these constellations as signs of the zodiac, omitting Ophiuchus, to make predictions. (Unlike astronomy, astrology is not a science.) Signs differ from constellations, bearing only a loose reference to one another. The sign of Pisces, for instance, corresponds to the rise of the constellation of Aquarius. Ironically, if you are born under a particular sign, the constellation it is named for is not visible at night. Instead, the sun is passing through it around that time of year, making it a daytime constellation that can't be seen. [How the Constellations Got Their Names]
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.
How far away are constellations?
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, with a bit of imagination, the constellations can provide a great source entertainment. They can also help the lost find their way, and aid skywatchers in the search for planets, comets, or other events, by a process called star hopping. And, as they surely did in ancient times, constellations can provoke a sense of timeless wonder.
- Guide to the Constellations (print)
- How the Night Sky Constellations Got Their Names
- Constellations: The Zodiac Constellation Names
- Orion Constellation: Facts About the Hunter
- Pegasus Constellation: Facts & Notable Features
- Draco Constellation: Facts About the Dragon
This article was updated Oct. 9, 2018, by Space.com Senior Writer, Meghan Bartels.