Draft. This lesson is being reviewed and may change before publication.
Lesson 2 of 20 · Foundations
The sidereal zodiac and the ayanamsa
Why Vedic and Western signs differ by 24 degrees, what the Lahiri ayanamsa is, how it changes each year, and why your sign may move. Lesson 2.
Written by Khushal ThakkarReviewed by Review pendingUpdated October 29, 2026
The sidereal zodiac measures a planet's position against the fixed stars, while the tropical zodiac measures it from the spring equinox, and because the equinox slowly drifts backwards through the stars, the two zodiacs have separated by about 24 degrees since the year they coincided. That gap is the ayanamsa, and subtracting it from a tropical position gives the sidereal one Jyotish uses.
Precession in one paragraph
The Earth's axis wobbles like a slowing top, completing one circle in about 25,800 years. As a result the point where the Sun crosses the celestial equator in spring, the equinox, moves backwards along the zodiac by about 50 arcseconds a year. Western astrology keeps 0° Aries pinned to that moving point; Jyotish keeps 0° Mesha pinned to the stars. Around 285 CE the two zero points were together. Today they are 24° apart, and the gap grows a degree every 72 years.
Which ayanamsa?
Several scholars have fixed the sidereal zero point slightly differently. The most used is Lahiri (Chitrapaksha), defined by India's Calendar Reform Committee: 23°15′ on 21 March 1956, carried to other dates by precession. This site uses Lahiri and matches the Swiss Ephemeris value to within half an arcsecond. The alternatives differ by up to about a degree, which matters only for a planet within a degree of a sign boundary.
| Date | Lahiri ayanamsa |
|---|---|
| 1 January 1950 | 23°10′ |
| 14 August 1990 | 23°44′ |
| 1 January 2000 | 23°51′ |
| 1 January 2026 | 24°13′ |
From tropical to sidereal
- Compute the planet's tropical longitude for the instant of birth.
- Compute the ayanamsa for that date.
- Subtract. If the result is negative, add 360°.
- Divide by 30° for the sign; the remainder is the degree within it.
Worked example
On 14 August 1990 at 12:00 IST the Sun's tropical longitude is 141°13′, which is Leo in the Western zodiac. The ayanamsa that day is 23°44′. Subtracting gives 117°29′, which is 27°29′ of Karka in the sidereal zodiac. The Western Leo is a Vedic Karka, as most Western Leos born after about 1950 are.
Why this matters for everything that follows
Every sign, nakshatra, house and dasha in this course is sidereal. If a result here disagrees with a Western chart, the ayanamsa is almost always the reason, and the result card shows the exact value used so the difference can be checked.
Calculate yours
Try the calculator this lesson explains.