Constellations
The 88 official sky regions and their patterns of stars.
Overview
A constellation is, formally, one of the 88 regions into which the International Astronomical Union divides the celestial sphere — a complete, non-overlapping partition of the sky. Informally it also means the star pattern within that region, which is what most people picture.
- The 88 constellations tile the entire sky with no gaps and no overlaps; every celestial object lies in exactly one.
- The stars in a constellation are almost never physically associated — they are a line-of-sight coincidence.
- Boundaries were fixed to the coordinate grid of 1875, so precession has since carried them slightly askew.
- An asterism, such as the Big Dipper or the Summer Triangle, is a recognised pattern that is not itself a constellation.
From pattern to region
For most of history a constellation was a picture: a group of stars imagined as a figure. Different cultures drew entirely different figures from the same stars, and the boundaries between them were vague. That is workable for storytelling and unworkable for cataloguing.
In 1922 the International Astronomical Union adopted a definitive list of 88 constellations, and in 1930 Eugène Delporte published precise boundaries running along lines of constant right ascension and declination. Since then a constellation is an area of sky, not a picture, and every object has an unambiguous constellation assignment.
Why the boundaries look slightly crooked
Delporte drew the boundaries along the coordinate grid as it stood at the standard epoch B1875.0. Because Earth's rotation axis precesses, completing a circuit in roughly 26,000 years, the coordinate grid has since rotated relative to the stars.
The boundaries move with the stars, so the assignments remain correct — but plotted on a modern coordinate grid the once-rectangular borders now appear tilted. This is why a modern star atlas shows constellation edges that step diagonally rather than following today's lines of right ascension and declination.
Depth: the illusion behind every figure
A constellation is a projection. The stars composing a figure are typically at wildly different distances and have no physical relationship to one another. In Orion, Betelgeuse and Rigel are hundreds of light-years away but not near each other; the belt stars are further still. Viewed from another star, the pattern would be unrecognisable.
There are exceptions where stars genuinely are associated — parts of the Big Dipper belong to a common moving group, and the Pleiades is a true physical cluster — but these are the minority and are identified by shared motion and distance rather than by apparent proximity.
Northern, southern, and the history of the sky map
The classical constellations descend from Ptolemy's list of 48, itself inheriting Mesopotamian and Greek figures. That catalogue covered only the sky visible from Mediterranean latitudes, so the far southern sky was blank to European astronomy until the southern voyages that began in the 1590s. Keyser and de Houtman's observations on the 1595–97 Dutch expedition supplied twelve new constellations, engraved on Plancius's globe of 1598 and printed in Bayer's Uranometria of 1603; Lacaille added fourteen more from the Cape of Good Hope in 1751–52.
The southern constellations were therefore invented across the age of exploration and instrumentation, which is why they are named for equipment and animals encountered abroad — Telescopium, Microscopium, Octans, Tucana — rather than for mythological figures. Later attempts to add political or commemorative constellations were not retained in the 1922 list.
Constellations versus zodiac signs
The zodiac constellations are astronomical regions of very unequal size that the Sun's apparent path crosses. The zodiac signs of Western astrology are twelve exactly equal 30-degree divisions measured from the March equinox. These are different systems that share names.
Two consequences follow. The Sun spends different amounts of time in each zodiac constellation because they differ in width — and it also passes through Ophiuchus, which is not one of the twelve signs. And precession has shifted the signs relative to the constellations by roughly one sign over the past two thousand years, so the astrological sign for a date generally does not match the constellation the Sun is actually in.
Continue in the data
Catalogues, hubs, and reference pages that hold the underlying records for this topic.
Frequently asked
- How many constellations are there?
- Eighty-eight, as fixed by the International Astronomical Union in 1922 with precise boundaries published in 1930. They tile the whole celestial sphere with no gaps or overlaps, so every star, galaxy and comet position falls in exactly one of them.
- Are the stars in a constellation close to each other?
- Almost never. A constellation is a line-of-sight projection, and its stars typically lie at very different distances with no physical connection. Viewed from a different vantage point in the Galaxy the patterns would not exist. A few genuine exceptions — such as the Pleiades — are real physical clusters, identified by shared distance and motion.
- Why is Ophiuchus not a zodiac sign if the Sun passes through it?
- Because zodiac signs and zodiac constellations are different systems. The signs are twelve equal 30-degree divisions of the ecliptic used in astrology; the constellations are irregular sky regions of unequal width. The Sun's path does cross Ophiuchus, which is an astronomical fact, but the twelvefold sign scheme was fixed long before modern constellation boundaries and was never intended to track them.
- What is the difference between a constellation and an asterism?
- A constellation is one of the 88 official IAU sky regions. An asterism is any recognisable star pattern that is not itself one of those regions — the Big Dipper is an asterism within Ursa Major, and the Summer Triangle is an asterism spanning three separate constellations.