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Ayanamsa and the sidereal zodiac

Western astrology uses the tropical zodiac, which starts where the Sun crosses the equator in March (the spring equinox). Jyotish uses the sidereal zodiac, fixed to the stars. The gap between them is the ayanamsa (अयनांश).

Why the two drift apart

The Earth's axis wobbles like a spinning top, a motion called precession. It makes the equinox slide backwards against the stars by about 50.3 arc-seconds a year, a full circle in roughly 25,800 years. The two zodiacs coincided some 1,700 years ago; today the gap is about 24°, and it grows by about one degree every 72 years.

What it changes

Almost everything shifts back by about 24°. Someone born on 1 April is an Aries in Western astrology but has the Sun in Meena (Pisces) in Jyotish: the Sun enters sidereal Mesha only around 14 April, which is why Mesha Sankranti, Baisakhi, Vishu and the Tamil New Year fall then. Nakshatras are defined in the sidereal zodiac, so they need an ayanamsa too.

Lahiri, the Indian standard

Several ayanamsas exist (Raman, Krishnamurti, Fagan-Bradley among them) and differ by up to a degree or two. India's Calendar Reform Committee of the 1950s, with the astronomer N. C. Lahiri, adopted one that places the star Chitra (Spica) at the start of Tula; it is used for the national calendar and by most Indian astrologers, and is called the Lahiri or Chitrapaksha ayanamsa.

The other ayanamsas sit close by: Krishnamurti's (used in KP astrology) is about 6 arc-minutes smaller than Lahiri, Raman's about 1½° smaller, and Fagan-Bradley's, used by Western sidereal astrologers, almost a degree larger. A degree matters only for planets near a sign or nakshatra boundary, but there it decides the answer.

Solar months and Sankranti

The Sun's entry into each sidereal sign is a Sankranti and starts a solar month. Makar Sankranti, the Sun entering Makara, falls around 14 January now; in the tropical zodiac the Sun enters Capricorn at the December solstice, about 24 days earlier. The gap between the two dates is the ayanamsa, measured in days of the Sun's motion.

How we compute it

Planet positions come from NASA's JPL DE421 ephemeris. We subtract a linear Lahiri model, 23.853° at the start of 2000 growing by 50.24″ a year. Swiss Ephemeris, the engine inside most astrology software, uses 23.857° for 2000; across 2,000 test charts our planet and ascendant positions agreed with it to within about 0.004° on average, and every position fell in the same sign except one sitting exactly on a boundary.

Why it matters for testing

Many claims depend on sign boundaries, for example a planet entering a new sign. Using the same, documented ayanamsa everywhere means a test can be repeated by anyone with the same inputs. See how we test.

For learning and curiosity, not investment advice.