Children and adults around the world love gazing at the stars, seeking out groupings they have been told about or recreating their own. But these same collections that can provide amusement and joy likely originated as instruments to help people mark the time of year. Today, these star constellations continue to stand as tools for astronomers and stargazers.
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 we know of today bear Greek and Roman names, but people mapped the sky before these empires took hold. The Greeks adopted their system from the Babylonians, whose origins in turn may have stemmed from Sumerian traditions 3,000 years before. Even further back, scientists suspect that markings on a 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 span of the sky. Formally cementing these boundaries allows astronomers to communicate about the regions of the sky they study.
The official constellations are in fact rectangular slices of the heavens holding the stars within it. Included are the individual groupings. 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 actually officially aren't. The Big Dipper, which lays inside of Ursa Major, is not recognized as a constellation. Instead, it is an asterism, a group of stars not officially designated but known by most nonastronomers.
Constellations names: What's your Zodiac sign?
The sun, the moon, and the planets travel on a set path through the sky known as the ecliptic as the Earth rotates. The list of 13 constellations they pass through are known as the stars of the Zodiac. The Zodiac constellations' names:
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, that 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]
How far are constellations?
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 skywatchers 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.