What Are Constellations? Patterns People Drew on a Random Sky
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The stars in Orion's belt are not near each other. Alnitak is about 1,260 light years away, Alnilam roughly 2,000 and Mintaka around 1,200, and they appear in a line only because of where the Earth happens to sit. Every figure in the sky is like this: an accident of perspective, turned into a hunter, a bear or a set of scales by people who needed a way to organise what they were looking at. The patterns are cultural, the boundaries are bureaucratic, and both are more useful than the arbitrariness suggests.
Where the familiar ones came from
The figures used in Western astronomy are an inheritance with a traceable path. Many originate in Mesopotamia, where Babylonian records from the second millennium BC already name a bull, a lion, scales and a scorpion along the path of the sun, and they passed to the Greeks, who added their own mythology and whose catalogue was fixed by Ptolemy in the second century AD in a list of forty-eight. That list travelled through the Islamic world, where the star names were largely set, which is why so many begin with the Arabic definite article, Aldebaran, Altair, Algol, Betelgeuse, and returned to Europe in translation. The southern sky was missing from it entirely, because Ptolemy's Alexandria could not see it, and it was filled in by European navigators and astronomers in the sixteenth to eighteenth centuries, which is why the far south is populated not by heroes and monsters but by a clock, a microscope, a telescope, a chisel, an air pump and a painter's easel.
The official eighty-eight
The traditional figures had no agreed edges, different atlases drew them differently, and there was a long tail of inventions proposed by astronomers hoping to flatter a patron, including Charles's Oak, the Brandenburg Sceptre, Frederick's Glory and a cat. In 1922 the International Astronomical Union settled on a list of eighty-eight, and in 1930 the Belgian astronomer Eugene Delporte drew formal boundaries along lines of right ascension and declination, so that the sky is now divided exhaustively into eighty-eight regions with no gaps and no overlaps. This changes what the word means: a constellation is now an area of sky rather than a pattern of stars, and every object in that area belongs to it, which is why a galaxy millions of light years beyond the figure is still said to be in it. The boundaries were drawn for the sky as it stood in 1875, and because the Earth's axis wobbles, they have drifted slightly askew since.
Asterisms and other cultures
Many of the best-known patterns are not constellations at all in the official sense. A recognisable shape that is not one of the eighty-eight is called an asterism, and several of them are far more famous than the regions containing them:
- •The Plough, or Big Dipper, which is seven stars within the much larger Ursa Major
- •The Summer Triangle, made of the brightest stars of three separate constellations
- •The Square of Pegasus, which borrows one of its corners from Andromeda
- •The False Cross and the Diamond Cross in the south, both of which are mistaken for the real Southern Cross by navigators
- •The Teapot, which is the bright core of Sagittarius and points at the centre of our galaxy
Why people bothered
The patterns were not decoration. Before clocks and calendars, the sky was the only reliable annual timetable, and the stars visible at dusk told a farming society where it stood in the year. Hesiod tells Greek farmers to begin the harvest when the Pleiades rise before the sun and to plough when they set; the Egyptians timed the Nile flood by the reappearance of Sirius after its weeks of invisibility; Polynesian navigators crossed thousands of kilometres of open Pacific using a star compass, holding a course by the rising and setting points of known stars in sequence through the night. The band of figures along the sun's apparent path, the zodiac, exists because the sun, moon and planets are always found near that line, so it is the part of the sky worth dividing carefully. Different cultures divided the same stars quite differently: Chinese astronomy uses a few hundred small groupings rather than a few dozen large ones, and several Aboriginal Australian traditions name dark shapes in the Milky Way, including an emu formed by dust clouds rather than by stars, whose appearance signals the season for collecting emu eggs.
Nothing stays still
The figures are temporary in three ways. The stars have their own motions across the sky, which are slight over a human lifetime and substantial over a hundred thousand years, by which time the Plough will have lost its shape as its end stars drift in opposite directions. The Earth's axis precesses on a cycle of about 26,000 years, so the pole star changes, and Polaris has held the position only for the last millennium or so, with Vega taking a turn in about 12,000 years. And the same precession has moved the sun's position against the zodiac by roughly one constellation since the signs were assigned two thousand years ago, so that the sign a horoscope assigns to a birthday no longer matches the constellation the sun was actually in, a discrepancy astronomers point out and astrologers, who define signs by season rather than by stars, do not regard as an error.
The takeaway
The figures are chance alignments of stars at wildly different distances, inherited from Mesopotamia through Greece and the Islamic world, fixed at forty-eight by Ptolemy and completed in the southern sky by early modern navigators with instruments instead of heroes. Since 1930 the sky has been divided into eighty-eight bounded regions, so the word now names an area rather than a pattern, and famous shapes such as the Plough are asterisms within them. They were built as a calendar and a compass, and stellar motion and the precession of the Earth's axis slowly take them apart.