Sidereal vs Tropical Zodiac and the Lahiri Ayanamsa

The tropical zodiac used in Western astrology starts Aries at the spring equinox, while the sidereal zodiac used in Vedic astrology fixes the signs against the stars. Because the equinox slowly drifts against the stars, the two zodiacs now sit about 24 degrees apart, which is why the same planet can appear in different signs in a Western horoscope and an Indian kundli. The size of that gap is called the ayanamsa, and the Lahiri ayanamsa, the official standard in India, is about 24°14′ in 2026.

Neither zodiac is "wrong". They measure from different starting points, and each tradition is internally consistent.

Two ways to start the zodiac

Both zodiacs divide the Sun's apparent path into twelve signs of 30 degrees. They differ only in where they place 0° Aries.

• Tropical: 0° Aries is the point where the Sun stands at the March equinox. The signs are tied to the seasons, so the Sun always enters Aries around 20 or 21 March.
• Sidereal: 0° Aries is tied to the background stars. In the Indian tradition the anchor is the star Chitra (Spica), which is placed at 0° Libra, exactly opposite the start of Aries. This version is called Chitrapaksha.

Why the two zodiacs drift apart

The Earth's axis wobbles slowly, like a spinning top, completing one turn in roughly 25,800 years. As it does, the equinox points move backwards against the stars by about 50 seconds of arc a year, which adds up to one degree in about 72 years. This is the precession of the equinoxes.

The two zodiacs coincided roughly seventeen centuries ago. Since then the tropical starting point has slid backwards, and the gap keeps growing: about 23°10′ in 1950, 23°51′ in 2000 and 24°13′ at the start of 2026, according to the same Swiss Ephemeris calculation used on this site.

What an ayanamsa is, and why there are several

The ayanamsa is the number of degrees to subtract from a tropical position to get the sidereal one. Because the sidereal zodiac depends on which star is chosen as the anchor and when the two zodiacs are taken to have coincided, astrologers have proposed several ayanamsas. Besides Lahiri, the best known in India are the Raman and Krishnamurti (KP) ayanamsas, and Western sidereal astrologers often use Fagan-Bradley. They differ from Lahiri by amounts ranging from a few minutes of arc to more than a degree.

Those differences are small, but not trivial. A Moon within a degree of a nakshatra boundary can land in a different nakshatra under a different ayanamsa, which is one reason two reports occasionally disagree.

Why Lahiri is the Indian standard

After independence the Government of India set up a Calendar Reform Committee, chaired by the astrophysicist Meghnad Saha, to bring order to the many regional calendars in use. Its recommendations, published in 1955, included a standard ayanamsa based on the Chitrapaksha principle. It came to be known by the name of N. C. Lahiri, a member of the committee, and it is used in India's official astronomical almanac and by most Indian astrologers and Panchang makers today.

This site uses Lahiri for every Vedic calculation, so its Moon signs and nakshatras line up with the great majority of Indian almanacs.

What the gap means for your signs

Because the zodiacs are about 24° apart, many positions move back by one sign when converted from tropical to sidereal. For the Sun, the dates shift by around three and a half weeks:

The sidereal dates can vary by a day from year to year. The Capricorn date is Makar Sankranti, and the Aries date is the solar new year celebrated in several regions of India.

The Moon shifts in the same way. With a gap of about 24° out of each 30° sign, roughly four births in five have a sidereal Moon sign one sign earlier than the tropical one. The girl in Example 1 on this site, a fictional birth on 27 May 1997, has her Moon at 7°32′ Aquarius in the tropical zodiac but 13°43′ Capricorn in the sidereal zodiac, because the ayanamsa that day was about 23°49′.

How this site uses each zodiac

To be transparent about it, the site uses both:

• Kundli matching uses the sidereal zodiac with the Lahiri ayanamsa, as Vedic tradition requires.
• The Hindu calendar (tithi, nakshatra, yoga) is also sidereal, as every Panchang is.
• The sign horoscopes are organised by the familiar Western Sun-sign dates, and their daily text is based on the Moon's position in the tropical zodiac. If you know your Rashi from your kundli, you can read the sign that matches it instead.

How the Moon's sidereal position becomes a sign and a nakshatra is shown step by step in Moon sign and nakshatra: how they are calculated. For the role the Moon plays in matching, see what is Kundli matching, and for why exact birth details matter even more than the choice of ayanamsa, see why birth time and birthplace matter.