What Is a Constellation? Not a Group of Stars but a Region of Sky

The International Astronomical Union established eighty-eight constellations in 1922 and fixed their boundaries in 1930, dividing the entire sky into regions with borders running along lines of constant right ascension and declination, so the borders are straight in those coordinates and appear as a rectilinear patchwork on a chart. That makes a constellation an address rather than a picture, and any object anywhere in the sky lies in exactly one of them, which is why a newly discovered comet or a distant galaxy can be assigned to a constellation regardless of whether it has anything to do with the traditional figure. The coordinate system used was that of 1875, and because the Earth's axis precesses the boundaries have since drifted slightly against the current coordinates, which is a small and occasionally awkward technicality.

The traditional constellations differ from the modern regions in several ways:

• •They were patterns rather than areas, so the sky between and around them belonged to nothing
• •Different cultures divided the same sky into entirely different figures, with no overlap in many cases
• •The Greek and Ptolemaic set covered only the sky visible from the northern Mediterranean
• •Southern constellations were added by European navigators from the sixteenth century onward and are named accordingly, with instruments and equipment rather than mythological figures
• •Proposed constellations were invented freely for centuries, honouring patrons and monarchs, and most were discarded in 1930
• •Asterisms, which are recognised patterns that are not constellations, survive informally

A constellation figure connects stars that have no physical relationship, since they lie at wildly different distances and are grouped only by appearing near each other from here. The stars of a familiar pattern may be separated by hundreds of light years, so the figure exists only from this vantage point and would be unrecognisable from anywhere else. The patterns also change, since stars move relative to each other, and over tens of thousands of years the familiar shapes distort beyond recognition, which can be computed and illustrated. There are exceptions where stars in a pattern are genuinely associated, since some belong to the same moving group formed together, and one familiar pattern contains several stars that are physically related while its two end stars are not, which is a good illustration that appearance and association are separate questions.

The practical use of these patterns is navigation around the sky, and the method is the same one used for finding anything in an unfamiliar place. A few conspicuous patterns are learned first and serve as anchors, with the arrangement in the northern sky pointing to the pole star and the southern cross serving the equivalent function below the equator. Lines drawn between bright stars in those anchors point towards others, which is the star hopping technique extended to naked-eye scale. Seasonal changes mean different constellations dominate at different times of year, so learning the sky takes a full cycle. Once the anchors are known the boundaries become useful, since knowing that an object is in a particular constellation narrows where to look to a definite region rather than to a vague direction.

The traditional figures encode a substantial amount of cultural history. The Greek set descends through Ptolemy from earlier Mesopotamian traditions, with several figures traceable back through cuneiform records, so the names carry material older than the Greek myths attached to them. The zodiac constellations acquired particular significance because the sun passes through them, which is an accident of the plane the planets orbit in and which supported astrology for millennia. Southern names record the period of their invention, with clocks, microscopes and air pumps in the sky. Other traditions divided the sky entirely differently, with Chinese astronomy using a far larger number of smaller groupings, and Aboriginal Australian traditions including figures defined by dark patches in the Milky Way rather than by stars, which is a different idea of what a constellation is.

Eighty-eight regions with borders fixed in 1930 divide the whole sky, so every object lies in exactly one and a constellation is an address rather than a picture. The stars in a figure are at wildly different distances and grouped only by appearance from here, and the patterns distort over tens of thousands of years. Southern names record the instruments of the age that invented them.