Ophiuchus

Ophiuchus is treated elsewhere on this site chiefly through the recurring, overstated claim of a thirteenth zodiac sign. Set that argument aside, and the constellation holds a genuinely remarkable object with nothing to do with astrology at all: the star with the largest known proper motion of any star in the sky.

Barnard's Star, discovered by the American astronomer E. E. Barnard in 1916, moves across the sky faster, relative to the background stars, than any other star known, a motion large enough to be measured across a human lifetime by careful observation rather than requiring instruments sensitive to changes over centuries. It is also the second-closest stellar system to the Sun after the Alpha Centauri system, close enough and fast-moving enough that its position noticeably shifts against more distant background stars over just a few decades, an effect essentially imperceptible for almost every other star in the sky.

Proper motion of this kind reflects a star's actual velocity through space relative to the Sun combined with its relative closeness, since a star moving at any given real speed appears to shift faster across the sky the nearer it is to an observer, the same reason a passing car appears to move faster than a distant plane travelling at a similar true speed.

The serpent-bearer, invoked every few years in stories about a missing thirteenth sign, quietly holds the fastest-moving star anyone has ever measured.

Every few years a version of this appears: astronomers have announced a thirteenth zodiac sign, Ophiuchus, and everyone's sign has changed. It is reliably presented as news and it is reliably not news.

The underlying fact is real. The ecliptic passes through the constellation Ophiuchus. Under the boundaries adopted in 1930, the Sun is inside Ophiuchus for around eighteen days each year, roughly the end of November and the start of December, and inside Scorpius for only about seven. If you sort people by which constellation the Sun was actually in, there are thirteen groups, not twelve, and the groups are of very unequal size.

It has been known throughout. Ophiuchus' contact with the ecliptic was noted in antiquity, and the modern boundaries that make it precise date from 1930. Nothing has been discovered. What happens periodically is that the fact is restated, usually by an observatory or a science communicator, and reported as a discovery.

Why it does not land where it is aimed. The argument only works against an astrology that claims the signs are the constellations. Tropical Western astrology does not claim that and has not since Ptolemy: the signs are twelve equal divisions of the year measured from the equinox, and the number twelve comes from the division, not from a count of star patterns. Adding a thirteenth constellation to the sky does not add a thirteenth thirty-degree arc to a circle. To make the objection bite you have to first establish that the zodiac ought to be sidereal, which is a separate argument and one the sidereal astrologers have not won either.

Where a real objection does sit. Precession does undercut a specific and very common piece of astrological reasoning, the habit of explaining a sign's meaning by reference to the creature in the sky, the Ram's aggression or the Scorpion's sting. That reasoning connects the sign to a constellation the sign no longer coincides with. An astrologer who defends tropical astrology on the grounds that it measures the season cannot then explain Aries by pointing at a ram. Some do both, and that is a genuine inconsistency rather than a misunderstanding by outsiders.

A handful of astrologers have tried to work with a thirteen-sign zodiac. It has not been taken up, in either the tropical or the sidereal camp.