Ophiuchus and the 13th sign, explained in 3D

Every few years a story goes round that there's a 13th zodiac sign, Ophiuchus, and that everyone's sign has changed. Your sign hasn't changed. The story mixes up two different things: the zodiac signs, which are twelve equal slices of the year, and the constellations, which are patterns of stars of all different sizes. Here they are side by side.

Signs and constellations are different things

The zodiac signs are a calendar. They divide the Sun's yearly path into twelve equal parts of 30 degrees, starting from the spring equinox, the moment the Sun crosses the celestial equator in March. Aries is always the first 30 degrees after the equinox, Taurus the next 30, and so on. That's the gold ring above.

The constellations are star patterns behind that path. They were never equal: Virgo stretches across more than 40 degrees of the Sun's path and Scorpius covers less than 7. Their official borders were only drawn in 1930, by the International Astronomical Union, and those borders put a stretch of Ophiuchus, the Serpent Bearer, across the Sun's path between Scorpius and Sagittarius. That's the outer ring, with Ophiuchus in violet.

Why they don't line up any more

When the system was set down around two thousand years ago, the signs and the constellations they were named after roughly lined up. Since then the Earth's axis has slowly wobbled, a motion called precession, and the equinox has drifted backwards against the stars by about one degree every 72 years. The signs moved with the equinox; the constellations stayed with the stars. Today they're about 24 degrees apart, so in late March the Sun is in the sign of Aries but in front of the stars of Pisces. Pick a date above and you can watch it happen.

So is Ophiuchus a sign?

Not in Western astrology, which ties the signs to the seasons rather than the stars, and has always had twelve. The Sun really does pass in front of Ophiuchus for about eighteen days a year, from around 30 November to 17 December, but that makes it a constellation on the Sun's path, not a sign.

Sidereal astrology, used in India's Jyotish tradition, does tie the signs to the stars, so its signs sit about 24 degrees behind ours. Even sidereal astrology keeps twelve equal signs. A 13-sign zodiac exists, but it's a modern proposal followed by very few astrologers.

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