What an ayanamsha is
Ayanamsha is a Sanskrit term for the angular gap between the tropical zodiac and the sidereal zodiac. The tropical zodiac starts at the March equinox. The sidereal zodiac starts from a point fixed against the stars. Because the axis of the Earth wobbles slowly, an effect called precession, the two starting points drift apart by about one degree every 72 years. The ayanamsha is the current size of that gap.
To find a sidereal position, an astrologer takes the tropical longitude of a planet and subtracts the ayanamsha. A Moon at 40 degrees tropical, which is 10 degrees of Taurus, becomes a Moon at about 16 degrees of Aries when the ayanamsha is about 24 degrees. For the wider picture see sidereal vs tropical.
Lahiri ayanamsha
The Lahiri ayanamsha is named after Nirmala Chandra Lahiri, an Indian astronomer who worked on it in the 1950s. It was adopted by the Calendar Reform Committee of the Government of India in 1955 and is now the official ayanamsha of the Indian national almanac. It is also called the Chitrapaksha ayanamsha because it is built on the star Spica, called Chitra in Sanskrit. The system places Spica at exactly 0 degrees of Libra in the sidereal zodiac, which is the middle of the Nakshatra Chitra.
The two zodiacs match at a moment the Lahiri system sets in the year 285 of the common era. Since then precession has added the gap. In 2026 the value is about 24 degrees 13 minutes, and it grows by about 50 arcseconds each year, so it reaches 25 degrees around the year 2082.
Most Vedic software, including free online tools and the Swiss Ephemeris library, uses Lahiri as its default. This is the reason that a chart made in one tool usually agrees with a chart made in another.
Why the value matters
A change of one degree in the ayanamsha has several effects.
The Moon. The Moon moves about half a degree an hour, so one degree shifts the Moon position by about two hours of time. If your Moon was close to a sign or Nakshatra boundary, your Moon sign or Nakshatra may differ between systems. Because the Vimshottari Dasha starts from the Moon, the whole timeline can shift by months.
The Lagna. The rising sign moves about one degree every four minutes, so a one degree gap is small in time. It matters only if the Lagna is near the edge of a sign.
Divisional charts. Charts such as the Navamsa divide each sign into small parts, so small differences in degree can change them more often.
For most people, nearly all placements stay the same in sign between nearby ayanamshas, and only one or two planets near a boundary differ.
Other ayanamshas
Lahiri is the most used, but several others exist.
Raman. Proposed by the Indian astrologer B. V. Raman. It is about one and a half degrees smaller than Lahiri and is still used by some astrologers in southern India.
Krishnamurti, or KP. Used in the KP system. It is very close to Lahiri, about six arcminutes less.
Fagan Bradley. Used by Western sidereal astrologers. It is about one degree larger than Lahiri.
True Chitra Paksha. A version that follows the real position of Spica, which makes the value change slightly from year to year.
Yukteshwar and others. Used by smaller groups, and they differ by larger amounts.
The differences exist because the sidereal zodiac needs a fixed reference point and astrologers disagree about where exactly it should be. There is no scientific test that picks one value as correct, because astrology itself has not been shown to have predictive power. Within the Vedic tradition, though, Lahiri has become the standard.
How to check which one a chart uses
A good chart report states the ayanamsha. Look for a line such as Ayanamsha: Lahiri, followed by a value in degrees, minutes and seconds. If the report does not say, compare the position of the Sun with a tool you trust. For a Sun at about 17 degrees tropical Libra on 10 October, a Lahiri chart shows the sidereal Sun at about 23 degrees Virgo, because the gap is about 24 degrees. If two charts for the same person give signs that differ by one full sign for many planets, one of them is probably tropical.
Which one should I choose?
If you follow Indian Vedic astrology, choose Lahiri. It is the standard in India, it matches printed almanacs and most teachers use it. If you read a Western sidereal book, check which ayanamsha the author used. Do not mix values in the same reading, and do not switch only because a result looks better.
Ayanamsha in the Panchang
The Panchang depends on the same value. The Nakshatra of the day, the Yoga and the Tithi are found from sidereal positions, so printed almanacs state the ayanamsha. Tithi and Karana, which use the angle between the Sun and Moon, are the same in all systems because the shift cancels out. See What is Panchang?.