Getting the Chart Right

Two Zodiacs, One Sky

Sayana and nirayana, and why the gap between them keeps growing.

You've met this. Now the mechanics.

Foundations told you Jyotish uses the sidereal zodiac and Western astrology the tropical one, about 24° apart. Here's what that actually means.

Both zodiacs are twelve signs of 30°. They differ in exactly one thing: where 0° Aries is.

• Tropical, sayana: 0° Aries is the point where the Sun crosses the celestial equator heading north, the March equinox. It is tied to the seasons.
• Sidereal, nirayana: 0° Aries is a point fixed against the stars.

The Sanskrit names describe the difference. Ayana here is the slow movement of the equinox: sa-ayana moves with it, nir-ayana does not.

Why they drift apart

Earth's axis wobbles very slowly, like a spinning top. That wobble, precession, drags the equinox point backwards against the stars by about 50 arcseconds a year. That's roughly 1° every 72 years, and a full circle in about 26,000 years.

So the gap between the two zodiacs, the ayanamsa, is not a constant. It grows every year. Computed by this app's engine on Pushya-paksha:

| Date | Ayanamsa |
|---|---|
| 1 Jan 1950 | 22°02′ |
| 1 Jan 2000 | 22°44′ |
| Sept 2026 | 23°05′ |

That's why software computes the ayanamsa for the birth date. It never uses one fixed number.

Neither zodiac is a mistake

It's tempting to say one zodiac is right. They answer different questions.

The tropical zodiac measures the seasons. Its Aries always begins at the equinox. The sidereal zodiac measures the stars. Its Aries stays where the stars are.

Jyotish needs the second one because it is built on the nakshatras, mansions named after real stars. If the grid followed the equinox instead, Ashwini would slide off its own stars by a degree every lifetime. The Western astrologer and the Jyotishi aren't disagreeing about the sky. They're measuring different things in it.

Your Western sign, translated

A tropical Sun at 10° Aries, born in 1990. The Pushya-paksha ayanamsa that year is about 22°35′.

Sidereal longitude = tropical longitude − ayanamsa.

10° Aries − 22°35′ goes below zero, so we borrow a sign: 40° Pisces − 22°35′ = 17°25′ Pisces.

Now take a tropical Sun at 25° Aries. 25° − 22°35′ = 2°25′ Aries. It stays in Aries.

The rule: if the tropical degree is less than the ayanamsa, the sidereal sign is the previous sign. With an ayanamsa of 22–23°, that happens about three times in four. That's why most people's Western sign and Vedic Sun sign differ.

Remember it

Sayana follows the seasons; nirayana follows the stars. The gap between them is the ayanamsa, and it grows about a degree every 72 years.

Terms introduced here

Sayana and nirayana

The tropical (sayana) zodiac starts at the March equinox; the sidereal (nirayana) zodiac starts at a point fixed against the stars.

Precession and the drift

The equinox slides backwards against the stars about 1° every 72 years, so the gap between the zodiacs grows every year.