The Constellations

by International Astronomical Union

Origin of the Constellations

Ever since people first wandered the Earth, great significance has been given to the celestial objects seen in the sky. Throughout human history and across many different cultures, names and mythical stories have been attributed to the star patterns in the night sky, thus giving birth to what we know as constellations.

Over half of the 88 constellations the IAU recognizes today are attributed to ancient Greek, which consolidated the earlier works by the ancient Babylonian, Egyptian and Assyrian. Forty eight of the constellations we know were recorded in the seventh and eighth books of Claudius Ptolemy’s Almagest, although the exact origin of these constellations still remains uncertain. Ptolemy’s descriptions are probably strongly influenced by the work of Eudoxus of Knidos in around 350 BC. Between the 16th and 17th century AD, European astronomers and celestial cartographers added new constellations to the 48 previously described by Ptolemy; these new constellations were mainly “new discoveries” made by the Europeans who first explored the southern hemisphere. Those who made particular contributions to the new constellations include the Polish-born, German astronomer Johannes Hevelius; three Dutch cartographers, Frederick de Houtman, Pieter Dirksz Keyser and Gerard Mercator; the French astronomer Nicolas Louis de Lacaille; the Flemish mapmaker Petrus Plancius and the Italian navigator Amerigo Vespucci.

IAU and the 88 Constellations

Originally the constellations were defined informally by the shapes made by their star patterns, but, as the pace of celestial discoveries quickened in the early 20th century, astronomers decided it would be helpful to have an official set of constellation boundaries. One reason was to aid in the naming of new variable stars, which brighten and fade rather than shine steadily. Such stars are named for the constellation in which they reside, so it is important to agree where one constellation ends and the next begins.

Eugène Delporte originally listed the 88 modern constellations on behalf of the IAU Commission 3 (Astronomical Notations), in Délimitation scientifique des constellations. (Delporte, 1930)

Constellation Figures

In star maps it is common to mark line “patterns” that represent the shapes that give the name to the constellations. However, the IAU defines a constellation by its boundary (indicated by sky coordinates) and not by its pattern and the same constellation may have several variants in its representation.

The constellations should be differentiated from asterisms. Asterisms are patterns or shapes of stars that are not related to the known constellations, but nonetheless are widely recognised by laypeople or in the amateur astronomy community. Examples of asterisms include the seven bright stars in Ursa Major known as “the Plough” in Europe or “the Big Dipper” in America, as well as “the Summer Triangle”, a large triangle, seen in the summer night sky in the northern hemisphere and composed of the bright stars Altair, Deneb and Vega. Whilst a grouping of stars may be officially designated a constellation by the IAU, this does not mean that the stars in that constellation are necessarily grouped together in space. Sometimes stars will be physically close to each other, like the Pleiades, but constellations are generally really a matter of perspective. They are simply our Earth-based interpretation of two dimensional star patterns on the sky made up of stars of many differing brightnesses and distances from Earth.

Constellation Names

Each Latin constellation name has two forms: the nominative, for use when talking about the constellation itself, and the genitive, or possessive, which is used in star names. For instance, Hamal, the brightest star in the constellation Aries (nominative form), is also called Alpha Arietis (genitive form), meaning literally “the alpha of Aries”.

The Latin names of all the constellations they are a mix of the ancient Greek patterns recorded by Ptolemy as well as some more modern patterns observed later by more modern astronomers.

The IAU adopted three-letter abbreviations of the constellation names at its inaugural General Assembly in Rome in 1922. So, for instance, Andromeda is abbreviated to AND whilst Draco is abbreviated to DRA.