International Astronomical Union

The Union was founded at Brussels in 1919, in the round of international scientific organisation that followed the First World War, and its membership is of two kinds: national members, which are countries adhering through an academy or a national committee, and individual members, who are professional astronomers. Its business is done in divisions, commissions and working groups, and its General Assembly meets every three years.

Naming is the part of its work that reaches the public. The Union is the recognised authority for the designation of celestial bodies and of surface features on them, so the names of craters, of asteroids, of features on Pluto and of exoplanets are settled through its working groups against published rules about what sorts of names are admissible for what sorts of objects. The Working Group for Planetary System Nomenclature maintains the gazetteer.

Its most publicly argued decision was the resolution of 2006 defining a planet in the solar system, which established three criteria and, in consequence, moved Pluto into a new category of dwarf planet. The reaction to that was a useful demonstration of something the Union does not otherwise have to think about: a naming convention agreed by specialists can turn out to be a matter of general feeling, because the public had learned the old list at school and regarded it as a fact rather than as a definition.

Before 1922 there was no agreed list of constellations and no agreed edges to them. Atlases drew different figures, invented new ones to flatter patrons, and left the gaps between figures undefined, so that a faint star could belong to two constellations or to none. The Union adopted a list of eighty-eight in 1922, with standard three-letter abbreviations, and commissioned Eugene Delporte to fix the boundaries. His survey, published in 1930, drew them as segments of constant right ascension and declination for the epoch B1875, which made every constellation a rectilinear region that could be looked up rather than a picture that had to be recognised. Precession has since carried the sky slightly past those lines, so the boundaries now sit at a small angle to the current coordinate grid.

This bears on astrology, and not in the way the newspapers say. The tropical zodiac used in western astrology divides the ecliptic into twelve equal arcs of thirty degrees measured from the vernal equinox. It is a division of a circle, not a list of star patterns, so the Union's constellation boundaries do not govern it and were never meant to. The ecliptic does pass through thirteen constellations, Ophiuchus among them, and it always did. The recurring story that astronomers have added a sign and changed everyone's birth sign is the result of treating two different systems as one.