What Are Constellations, Really?
We draw lions, hunters, bears and mythological figures across the night sky. But the stars forming those patterns are often separated by enormous distances and may have no physical connection at all.
The entire sky is divided into 88 constellations
In modern astronomy, every position in the sky belongs to a constellation. Their official boundaries work rather like borders on a map, allowing astronomers to describe where an object lies on the celestial sphere.
Official constellations
Together they divide the whole celestial sphere into recognised regions.
Modern standardisation
The International Astronomical Union formally adopted the modern constellation system in the early twentieth century.
A complete celestial map
No part of the sky lies outside a constellation boundary.
Constellation or asterism?
Constellation
An officially defined region of the celestial sphere. Ursa Major, Orion and Cassiopeia are constellations.
Example: Ursa MajorAsterism
A recognisable pattern of stars that is not itself an official constellation. An asterism can form part of one constellation or span several.
Example: the Big DipperThe stars are not really sitting next to each other
When we look upward, stars appear projected onto a two-dimensional celestial dome. That hides the third dimension: distance. Two stars that look side-by-side can actually be hundreds of light-years apart.
Change the viewpoint
From Earth the stars line up into a familiar pattern. Move away from Earth and the apparent shape changes because each star sits at a different distance in three-dimensional space.
Use the Big Dipper to find Polaris
One of the most useful patterns in the northern sky is the Big Dipper. Two stars on the outer edge of its bowl — Merak and Dubhe — act as pointer stars toward Polaris, the North Star.
Follow the pointer stars
Imagine a line running from Merak through Dubhe and continue it roughly five times the separation between those two stars.
You arrive close to Polaris, the North Star. Because Polaris lies very close to the north celestial pole, it appears almost fixed while the rest of the northern sky turns around it.
Six useful constellations to learn first
Orion's Belt is one of the easiest star patterns to recognise and provides several useful routes to other bright stars.
Contains the Big Dipper asterism and its famous pointer stars toward Polaris.
Its distinctive W-shaped pattern makes it particularly easy to identify from northern latitudes.
The Swan lies along the Milky Way and includes the prominent Northern Cross asterism.
Leo is recognised by its curved Sickle pattern and the bright star Regulus.
Its curved shape and reddish star Antares make Scorpius one of the most distinctive southern-sky patterns.
Why do we see different constellations during the year?
Earth travels around the Sun once each year. As our nighttime side points in different directions through space, different regions of the celestial sphere become visible during different seasons.
Why can't everybody see the same constellations?
Your latitude determines which parts of the celestial sphere can rise above your horizon. Observers in Britain see a very different selection of southern constellations from observers in Australia or South Africa.
Some constellations near the celestial pole are circumpolar: from sufficiently high latitudes they never set below the horizon and can be seen throughout the year.
What makes the zodiac constellations special?
The Sun, Moon and planets appear to move through a narrow band of the sky centred on the ecliptic — the apparent path of the Sun across the celestial sphere.
The constellations intersected by this region are known as zodiac constellations. Astronomy uses the actual constellation boundaries; this should not be confused with the equal symbolic divisions used in astrology.
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