← Encyclopedia of Astrology · lesson 8/235
Sidereal zodiac
Listen to this article
What it is
The sidereal zodiac — from the Latin sidus, "star" — divides the ecliptic into the same twelve equal signs of 30° as the tropical zodiac, with the same names, but fixes them to the stars instead of to the seasons. Its 0° Aries is not the March equinox but a point defined by one or more fixed stars, and it stays put against the starry background century after century.
It is the zodiac of Vedic astrology, Jyotish, and was almost certainly the zodiac of the Babylonian astrologers who first drew up the twelve signs. A smaller school of Western astrologers, the siderealists, use it too.
Equal signs are still signs, not constellations: a sidereal sign of 30° does not follow the uneven outlines of the star groups any more than a tropical one does.
The ayanamsa
Because of the precession of the equinoxes, the equinox point slides backwards against the stars by about 1° every 72 years, so the two zodiacs move steadily apart. The gap between them at any date is called the ayanamsa (Sanskrit, "portion of movement"). To turn a tropical position into a sidereal one, you subtract it.
Astrologers do not agree on exactly where the sidereal zodiac starts, so there are several ayanamsas, differing by up to a degree or so. The best known:
- Lahiri (Chitrapaksha) — places the star Spica at 0° Libra; the official standard in India and the most used in Jyotish. In 2026 it is about 24°13'.
- Fagan–Bradley — devised by Cyril Fagan and Donald Bradley for Western sidereal astrology, about 25° today.
- Raman and Krishnamurti — two other Indian variants, slightly different from Lahiri.
A degree of difference matters when a planet or the Ascendant sits near the edge of a sign, or near the boundary of a nakshatra — the Moon's 27 mansions, which only make sense in a sidereal frame.
What changes in a chart
Converting a chart from tropical to sidereal moves every planet and angle back by the ayanamsa. Roughly speaking, with a gap of about 24°, a placement in the last six degrees or so of a tropical sign stays in the same sign sidereally, and everything earlier in the sign moves back one sign: a tropical Taurus Sun born mid-sign is a sidereal Aries Sun. The planets' relationships to each other — the aspects — do not change at all, nor, in quadrant house systems, do the houses the planets fall in, because the cusps and the planets move back together. In whole-sign houses, where a house is a whole sign, a planet can change house when it changes sign.
What changes is the sign each placement falls in, and with it rulers, dignities and sign-based interpretations.
Which is right?
The question has been argued for centuries and is not settled. Siderealists hold that the signs draw their meaning from the stars and that a zodiac drifting away from them has lost its anchor. Tropical astrologers hold that the signs are defined by the seasons and the Sun's relation to the Earth, which precession does not alter. Many astrologers treat the two as distinct, coherent systems, each with its own body of technique — Vedic astrology, for instance, is far more than a tropical chart moved back by 24°. Whichever one uses, it is used consistently.
In a reading here
Charts here are cast in the tropical zodiac; a reading's astrological information says which zodiac its chart used.