Is Ophiuchus a real zodiac sign? A real constellation, yes. A sign, no.

Picture the sky as Babylonian astronomers did: a belt of stars the Sun, Moon and planets all move through. Early star lists from Mesopotamia name more than twelve figures along that belt. By around the 5th century BC, though, astronomers had settled on something tidier: 12 equal divisions of 30 degrees, close to the number of lunar months in a year, which made the arithmetic of the calendar much easier.

Greek astrology inherited that grid, and by Ptolemy's time in the 2nd century AD the signs were counted from the spring equinox, the tropical zodiac Western astrology still uses. Aries begins where the Sun crosses the equator in March, whatever stars happen to sit behind it. Because Earth's axis slowly wobbles (precession), those stars have drifted by almost a whole sign since then. The signs stayed with the seasons, by design.

Ophiuchus was well known to the same astronomers. Aratus describes the serpent-bearer in his poem Phaenomena in the 3rd century BC, and Ptolemy lists its stars. They knew its feet came down near the Sun's path. They simply did not count it, because the zodiac was a measuring tape, not a census.

The neat modern line that hands Ophiuchus 18.3 days of the year came much later. In 1930 the IAU published official constellation borders, drawn by the Belgian astronomer Eugène Delporte along lines of right ascension and declination. Those borders put a slice of the ecliptic inside Ophiuchus, and that slice is where every "13th sign" story begins.

The name is just Greek for what you see: ophioukhos, "the one holding a serpent." The constellation is drawn as a huge figure standing over Scorpius, gripping a snake that is its own constellation, Serpens, split into a head on one side and a tail on the other.

Classical writers were not unanimous about who he was. The most common identification, found in Hyginus's Astronomica and repeated by later star-lore collectors, is Asclepius (Aesculapius to the Romans), the healer whose staff with a single snake is still a symbol of medicine. In the story, his skill at raising the dead angered the gods, Zeus struck him down, and he was placed among the stars. Hyginus also records other candidates, including a Thracian king named Carnabon. That is a myth told in several versions, not a single canon.

If you want to see the real thing, look on a summer evening in the northern hemisphere, high above the reddish heart of Scorpius. Its brightest star, Rasalhague, sits at the figure's head. In 1604 a "new star" blazed up in Ophiuchus, the supernova now called Kepler's Supernova, the last one seen with the naked eye in our galaxy. And Barnard's Star, one of the Sun's nearest neighbours, sits inside its borders too.

By December, when the Sun is in front of Ophiuchus, the constellation is lost in the glare. You are standing on the wrong side of the Sun to see it, which is a nice way to think about your birthday month.