The sky — astronomy

The 88 Constellations: A Map of the Whole Sky

There are 88 official constellations, and together they cover the entire sky with no gaps. The International Astronomical Union (IAU) adopted the list in 1922 and the boundaries were published in 1930. Hydra is the largest by area and Crux, the Southern Cross, is the smallest.

How many constellations are there?

There are 88 constellations. That is the official number set by the International Astronomical Union (IAU). Every point on the celestial sphere belongs to exactly one constellation, so together they form a complete map of the sky, much like countries on a political map.

It helps to understand what the word means in science. A constellation today is not just a pattern of stars but a region of the sky with precisely defined borders. When astronomers say a comet is “in Leo,” they mean it lies within that patch of sky, not necessarily close to the stars that draw the lion. The stars of a constellation are usually at very different distances from us and are not physically connected. The pattern is only a matter of our viewpoint.

Who defined them?

The IAU was founded in 1919. In 1922 it adopted the list of 88 constellations along with three-letter abbreviations, such as Ori for Orion. The Belgian astronomer Eugène Delporte later drew the exact boundaries, which were approved by the IAU in 1928 and published in 1930. The borders follow lines of celestial coordinates (right ascension and declination) fixed for a particular epoch, which is why they look like stepped, angular outlines on charts rather than smooth curves around the figures.

Ancient and newer constellations

Most of the names have a long history.

• The ancient constellations. The Greek astronomer Claudius Ptolemy described 48 constellations in his Almagest in the 2nd century CE. Many, such as Orion, Ursa Major, Cassiopeia and Scorpius, are still in use. Their roots go back even further, to Babylonian and Greek traditions.
• Southern additions. The far southern sky is not visible from the Mediterranean, so Ptolemy could not describe it fully. In the late 16th century, Dutch navigators made observations in southern seas and introduced new figures, many named after animals and instruments.
• Lacaille’s constellations. In the 1750s the French astronomer Nicolas-Louis de Lacaille mapped the southern sky from South Africa and introduced 14 constellations, many named after scientific instruments, such as Fornax, Horologium and Microscopium. He also divided the huge ancient ship Argo Navis into three: Carina, Puppis and Vela.
• Standardisation in the 20th century. In the early 1900s, different star atlases showed different figures and borders. The adoption of one list of 88 ended the confusion and gave astronomers worldwide a common language.

Some old figures did not survive. Argo Navis is the best-known example: it is no longer a constellation, having been split into three.

Asterisms: patterns that are not constellations

An asterism (from the Greek for “group of stars”) is an easily recognised pattern of stars that is not an official constellation. Asterisms may be part of one constellation or span several.

• The Big Dipper (the Plough) is the most famous asterism. It is made of seven bright stars in Ursa Major, which is a much larger constellation.
• The Summer Triangle links bright stars from three different constellations: Lyra, Aquila and Cygnus.
• Orion’s Belt is a row of three stars of similar brightness inside Orion.
• The Pleiades are an open star cluster in Taurus, often seen as a small hazy cluster of stars.

The difference is simple: constellations are officially defined regions covering the whole sky, while asterisms are informal patterns used for finding your way around.

The largest and the smallest

By area, the largest constellation is Hydra, the water snake, covering roughly 1,300 square degrees. It stretches a very long way across the southern sky. The smallest is Crux, the Southern Cross, at about 68 square degrees. Though tiny, it is one of the best-known landmarks of the southern hemisphere and has long been used for finding direction. Read more in the article on the Southern Cross.

Constellations and the zodiac

Many people first meet constellations through the zodiac. The zodiacal constellations are those the Sun appears to pass through during the year, and they are only a small share of the 88. See Zodiac constellations for details. It is worth remembering that astrological signs and astronomical constellations are not the same thing. The difference is explained in Why the zodiac signs do not match the constellations and The difference between astronomy and astrology.

Getting started with finding them

The easiest way is to begin with one strong landmark, such as Orion in winter or the Big Dipper year-round at northern latitudes, and hop from there to neighbouring figures. A dark site away from city lights and a few minutes for your eyes to adapt make a big difference. You will find practical tips in How to watch the sky.

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