The thirteenth star sign: what there is to Ophiuchus

The headline resurfaces at intervals, and it works every time: there is supposedly a thirteenth star sign, Ophiuchus, and anyone born between late November and mid-December is not really a Sagittarius at all. The all-clear is short: it only takes telling apart two words that seem to mean the same thing in everyday use.

Ophiuchus is nothing new

The constellation Ophiuchus, the serpent bearer, has been in the catalogues since antiquity. Ptolemy lists it in the second century among the forty-eight constellations he compiled. That the Sun's apparent yearly path grazes this constellation was therefore never a secret, and it is no discovery. What was reported each time is something that can be looked up in any star atlas.

A constellation and a zodiac sign are two different things

A constellation is a piece of sky. People joined bright points into a figure and gave it a name. The areas that come out of this differ in size and are cut to fairly irregular shapes.

A zodiac sign, by contrast, is a measure. The circle the Sun describes over the course of a year is divided into twelve equal sections of 30 degrees. Twelve times thirty makes three hundred and sixty — the division comes out exactly, because it was built that way. The signs do carry the names of constellations, but they are not constellations. They are the marks on a ruler.

Why constellations make a poor division

Since 1930 the boundaries of the constellations have been fixed uniformly the world over; before that, every atlas drew them a little differently. Along the Sun's path these sections come out very unequal. The Sun needs more than six weeks for the constellation Virgo, barely a week for the constellation Scorpio, and it crosses Ophiuchus in about two and a half weeks.

A zodiac made of constellations would therefore have sections of six days next to sections of over forty days — and it would shift as soon as someone redrew the boundary lines. That is exactly why astrology has worked with sections of equal size since antiquity, and not with pictures.

What the zodiac actually hangs on

The zero point of the zodiac in western use is not a star but a moment in time: the spring equinox. That is where Aries begins, and the other eleven sections follow it. What a sign shows is therefore the position of the Sun over the course of the year — at bottom, the season you were born into.

How many constellations lie behind this scale changes nothing about the scale. There is also an older dispute about whether the zodiac is better anchored to the stars than to the equinox. That is a subject of its own and has nothing to do with the story of a thirteenth sign.

Nothing changes for your birth chart

Your range of dates stays as it was. And even if you wanted to move it: the sun sign is only one building block among many anyway. The Moon, the ascendant and the other planets stand in your chart regardless of how many figures anyone counts in the sky. You can read what stands in each of the twelve sections under zodiac signs.

Why the story keeps coming back all the same is quickly explained: in a single sentence it promises a personal upheaval, and that makes a good headline. It only becomes a nuisance when nobody adds that a picture and a measure are simply not the same thing.