Two different zodiacs, both called “the zodiac”
Every few years, a story does the rounds claiming that everyone’s zodiac sign has shifted, or that there’s secretly a 13th sign nobody told you about. Neither claim is about an error being corrected. The first is about two different systems that use the same twelve sign names; the second is about constellations, which neither system uses.
The tropical zodiac
This is the system used throughout the rest of this site, and the one behind virtually every Western newspaper horoscope and birthday-sign lookup. The tropical zodiac anchors its twelve 30° divisions to the seasons — specifically, to the March equinox, defined as 0° Aries. It’s fixed to the Earth’s relationship with the sun, not to the backdrop of stars behind it, and it’s the system covered in detail in why zodiac dates shift from year to year and in the equinox and the old start of the year.
The sidereal zodiac
The sidereal zodiac, used in Vedic (Jyotisha) astrology and some other traditions, also has twelve equal signs of 30°, but fixes them against the stars rather than the equinox. The two would be the same except for a slow wobble in the Earth’s rotational axis called axial precession, which moves the equinox backwards against the stars by about one degree every 72 years, completing a circuit in roughly 26,000 years. The tropical zodiac moves with the equinox; the sidereal one stays with the stars. Sidereal astrology measures the gap with a correction called the ayanamsa, and different schools set it slightly differently.
On the Lahiri ayanamsa, the one most widely used in India, the gap is about 24° in 2026, and the two zodiacs last coincided in the third century CE. That puts the start of sidereal Aries at about 24° of tropical Aries, so sidereal Aries runs from 24° tropical Aries to 24° tropical Taurus: it is mostly what a tropical chart calls Taurus. In dates, the sun was in sidereal Aries from about 14 April to 14 May in 2026, while tropical Aries ran from 20 March to 20 April. Seen the other way round, tropical Aries is mostly sidereal Pisces, which is why someone who has always been an Aries is usually told by a sidereal chart that they are a Pisces.
Who noticed the drift in the first place
The gap between the two zodiacs isn’t a modern discovery. Precession was identified in the 2nd century BCE by the Greek astronomer Hipparchus, who noticed that the positions of stars relative to the equinoxes had shifted measurably compared with older observations available to him. It is an old piece of astronomical detective work, done centuries before anyone had a telescope, and the same slow wobble has been widening the gap between the two zodiacs ever since.
Where “the 13th sign” comes from
The 13th sign story comes from the constellations, not from either zodiac. The International Astronomical Union fixed the boundaries of the 88 modern constellations in 1930, and the sun’s path crosses thirteen of them. The thirteenth is Ophiuchus, between Scorpius and Sagittarius: in 2026 the sun is inside its boundary for about eighteen days, from the end of November to the middle of December. The constellations along the path are also very unequal in width — the sun spends about a week in Scorpius and more than six weeks in Virgo.
That is true and uncontroversial as a statement about constellation boundaries. But both astrological zodiacs, tropical and sidereal, divide the path into twelve equal signs of 30°, and neither was ever built from the constellations’ drawn outlines. Only a zodiac drawn from the constellations’ real, unequal widths, which is a minority practice, has a place for Ophiuchus. For the tropical and sidereal zodiacs there is no thirteenth sign to add, because they were never counting constellations to begin with.
Which one is “right”?
Neither, in the sense of one being astronomically correct and the other mistaken — they’re two different, internally consistent systems answering two different questions: “where is the sun relative to the equinox” (tropical) and “where is the sun relative to the fixed stars” (sidereal). Choosing between them is a matter of which astrological tradition you’re working within, not a factual dispute. This site uses the tropical system throughout, consistent with the Western astrological tradition it reports on, and says so plainly rather than presenting it as the only possible zodiac.
Why this connects to sun sign versus birth chart
Whichever zodiac is used, a sun sign — tropical or sidereal — is one placement among many in a full astrological chart, not the whole picture. See why a sun sign is not a birth chart for that broader distinction.