What Is an Asterism? The Patterns That Are Not Constellations

Before the twentieth century a constellation was a figure, and where one ended was a matter of which chart you used, with boundaries varying between atlases and with numerous invented constellations competing for the same stars. The International Astronomical Union settled it in 1922 by fixing the list at eighty-eight and in 1928 to 1930 by adopting precise boundaries, drawn along lines of constant right ascension and declination, so that every point in the sky falls in exactly one constellation. That converted a constellation from a picture into a defined area, which is what astronomers need, since an object can then be located by naming the region it lies in. An asterism is any other recognisable grouping, whether within one constellation, spanning several, or forming part of a larger figure. The distinction is purely conventional, since nothing physical separates the two, and both are line-of-sight patterns among stars at very different distances with no connection to each other.

Several patterns better known than the constellations containing them:

• •The Plough or Big Dipper, seven stars within Ursa Major, used for finding north by extending the line of its outer two stars to the pole star
• •The Summer Triangle, spanning three constellations and joining the brightest star of each
• •Orion's Belt, three stars within the larger figure, which points towards Sirius in one direction and Aldebaran in the other
• •The Square of Pegasus, whose fourth corner actually belongs to Andromeda
• •The Teapot within Sagittarius, which is far easier to recognise than the archer it supposedly depicts
• •The False Cross, four stars in Carina and Vela frequently mistaken for the Southern Cross by navigators, with occasionally serious consequences

The 1930 settlement ended a long period of invention in which astronomers proposed constellations to honour patrons, monarchs, instruments and their own interests, and the discarded ones are an entertaining record of the practice. Proposals included a printing press, an air pump that survived, a cat, a reindeer, a hot air balloon, an owl, a Prussian eagle and several arrangements commemorating specific kings. Many were invented by cartographers filling southern skies that European astronomers had only recently charted, and the survivors from that period are noticeably different in character from the classical ones, since the southern sky is populated by instruments and animals named in the eighteenth century rather than by Greek figures. A few proposed groupings were absorbed into their neighbours and a few survive as asterisms. The eighty-eight retained include the classical Greek set inherited through Ptolemy, which is why the northern sky is full of mythological figures and the far southern sky is full of microscopes and clocks.

Nothing physical connects the stars in a pattern, and the arbitrariness runs deeper than most people realise:

• •The stars in a figure lie at wildly different distances, so the shape exists only from this particular viewpoint and would be unrecognisable from a nearby star
• •Brightness as seen from Earth combines true luminosity with distance, so a prominent star may be modest and close or enormous and far
• •Proper motion means the patterns change, slowly by human standards, with the Plough visibly distorting over tens of thousands of years
• •Five of the Plough's seven stars do share a common origin and motion, which is an exception rather than the rule
• •Precession shifts which constellations are visible from a given latitude and which star marks the pole, so the pole star of antiquity was not ours
• •The figures themselves rarely resemble what they are named for, which is why learning them depends on memorising shapes rather than on recognising pictures

The Western scheme is one organisation among many and others divide the same stars quite differently. Chinese astronomy uses a far larger number of much smaller groupings, several hundred of them, organised around an imperial court metaphor with officials, buildings and functions, and the system is ancient and independent. Indian astronomy divides the ecliptic into twenty-seven or twenty-eight lunar mansions tracking the moon rather than the sun. Aboriginal Australian traditions include dark constellations defined by the shapes of dust lanes in the Milky Way rather than by stars, with the Emu in the Sky the best known, which is a completely different way of reading the same view and one that Western astronomy has no equivalent for. Andean traditions likewise use dark cloud constellations. Polynesian navigation used star paths, meaning sequences of stars rising at the same point on the horizon, which is an organisation by function rather than by shape. Astronomical bodies have begun documenting these systems rather than treating the official list as the only description.

A constellation is now a defined area of sky rather than a figure, fixed at eighty-eight regions with precise boundaries between 1922 and 1930, and any other recognisable grouping is an asterism. The Plough sits inside Ursa Major and the Square of Pegasus borrows a corner from Andromeda. The settlement discarded proposals honouring patrons and instruments, and other traditions divide the same stars in entirely different ways.