What is Ayanamsa?

Ayanamsa is the angle separating the two zodiacs in use today: the sayana (tropical) zodiac measured from the equinox, and the nirayana (sidereal) zodiac measured against the fixed stars. It is the one number that converts a Western chart into an Indian one, and it is why the two disagree about which sign anything is in.

Precession is what creates it

The Earth’s axis wobbles like a slowing top, carrying the equinox point backwards along the ecliptic by about 50.3 arcseconds a year — a full circuit in roughly 25,800 years. The stars stay put; the equinox moves. The two frames coincided in the early centuries CE and have been drifting apart ever since, at close to one degree every 72 years.

Lahiri, and why the choice is stated

Because the frames coincided at a moment nobody recorded exactly, the ayanamsa’s zero epoch is a convention, and several exist — Lahiri (Chitrapaksha), Raman, Krishnamurti and others, differing by up to about a degree. Lahiri is the value adopted by India’s Calendar Reform Committee and used in the Rashtriya Panchang, and it is the default here. A degree is a thirtieth of a rashi, so any tool that does not name its ayanamsa has left something out.

What it changes and what it does not

Applying an ayanamsa shifts every longitude by the same amount, so it changes which rashi a graha occupies, and with it the sankranti dates — currently about 24° worth, roughly 24 days of solar motion. It does not change any angle between grahas, so tithi, yoga, karana, eclipses and conjunctions are untouched by the choice.

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