Ayanamsa
Ayanamsa is the angle between the tropical zodiac, tied to the equinox, and the sidereal zodiac, tied to the stars. It is about 24° today and grows by about 50 arcseconds a year. Indian astrology uses the sidereal frame.
Why two zodiacs
Western astronomy measures longitudes from the vernal equinox, which slides backwards through the stars once every 25,800 years. Indian astronomy fixes its zero point among the stars instead. The difference between the two is the ayanamsa. Every sidereal quantity in the panchang, the nakshatra, the rashi of a planet, the sankranti, depends on the ayanamsa chosen; the tithi, being an angle between Sun and Moon, does not.
The common ayanamsas
Lahiri (Chitrapaksha) is the official value of the Indian national calendar and the default on Hastro; it places Spica (Chitra) at 180° in the mean. True Chitrapaksha holds Spica at exactly 180° at every instant and differs from Lahiri by a fraction of an arcminute. Raman is about 1°26′ smaller and Krishnamurti (KP) about 6′ smaller than Lahiri. The choice shifts nakshatra and rashi boundaries by minutes to hours, which is why two calendars can disagree on a sankranti time.
FAQ
Which ayanamsa should I use?
Lahiri, which Hastro uses for the calendar itself (months, adhik maas, sankranti, festivals and muhurat), as every published almanac does. For a birth chart you can choose Raman, Krishnamurti or True Chitrapaksha in the Kundali tab; the value in use is printed on the chart.
Does the ayanamsa affect tithi or festival dates?
Tithi, yoga and karana are unaffected. Nakshatra-based and sankranti-based dates (Onam, Makar Sankranti, Kharmas) can shift by a day in rare borderline cases.
Related: Nakshatra Kundali (birth chart) Sankranti