Delporte's Boundaries and the Thirteenth Sign

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.