Your sign is not
where the Sun is.

The wobble

The Earth spins on a tilted axis, and that axis does not point in a fixed direction. It traces a slow cone, the way a spinning top does as it loses speed, completing one circuit in about 25,800 years. Hipparchus noticed it in the second century BCE by comparing his own star positions with records a century and a half older. It is called the precession of the equinoxes.

The practical consequence: the point where the Sun crosses the celestial equator each March — the March equinox, the zero point of the whole zodiac — slides backwards along the ecliptic at about 50 arcseconds a year. That is one degree every 72 years, and one full sign every 2,150 years.

What has actually moved

When the Greeks fixed the zodiac, the March equinox sat in the constellation Aries. That is why the zero point is still formally called the First Point of Aries in navigation and astronomy today, and why Aries is the first sign.

The point left the constellation Aries around the second century BCE. It has been in Pisces ever since, and it will move into Aquarius at some point in the next few hundred years — a transition that has a name everybody knows, the Age of Aquarius, and a date nobody agrees on, because the constellation boundaries are a modern administrative decision rather than a natural edge.

The same has happened at every hinge of the year. The June solstice was in Cancer and is now in Taurus; the December solstice was in Capricorn and is now in Sagittarius. The Tropic of Cancer and the Tropic of Capricorn are still printed on every world map, named for constellations the Sun no longer reaches them in.

Why this is not the gotcha it is presented as

Every few years a story goes round that “your star sign is wrong” and that there is a secret thirteenth sign. Both halves are muddled, and it is worth being precise about why.

Western astrology has used the tropical zodiac since antiquity: a sign is a thirty-degree division of the Sun's path measured from the March equinox. It was never a claim about which stars are behind the Sun. It is a seasonal framework, anchored to the solstices and equinoxes, and precession does not invalidate it any more than it invalidates the first day of spring.

Indian astrology mostly uses the sidereal zodiac instead, which is anchored to the stars and therefore does drift against the seasons. Both systems are internally consistent. They give different answers, they have both been in continuous use for two thousand years, and the difference between them is currently about 24 degrees — which is why a Westerner reading an Indian chart usually finds their Sun has moved back a sign.

As for the thirteenth: Ophiuchus, the serpent-bearer, is a real constellation, the Sun really does pass through a corner of it for about eighteen days, and it has never been a sign in either system. It is a fact about IAU constellation boundaries drawn in 1930, not a discovery about astrology.

Sign against constellation, month by month

Here is the whole disagreement in one table. Note the widths: the signs are all exactly thirty days because they are all exactly thirty degrees. The constellations are wildly unequal, because they are pictures with borders drawn round them — Virgo gets forty-five days and Scorpius gets seven.

What follows from this

Three things worth keeping.

• Your tropical sign is not wrong. It is measured from the equinox and always was. The Sun really is 25 degrees past the March equinox on 15 April, and that really is the middle of Aries in the system Western astrology uses.
• Your tropical sign is also not where the Sun is against the stars. Both statements are true at once. They are answers to two different questions.
• The constellation is worth looking at anyway. It is the older half of the story and almost nobody has seen it. Every sign page here carries the star pattern it was named after, with the ecliptic ruled across it.