The sidereal zodiac

Two zodiacs share the same twelve sign names. Western astrology measures them from the March equinox, which makes it a tropical zodiac, while the Brihat Jataka, Varahamihira's book on horoscopes, starts its count at the first point of Aries and of the nakshatra Ashvini, a sidereal count tied to the stars. N. Chidambaram Iyer's 1885 translation calls the star-based count nirayana, the equinox-based one sayana, and the gap between them ayanamsa. To get a sidereal position, Iyer says, subtract the ayanamsa from the tropical one.

The gap grows because the equinox drifts against the stars by roughly 50 seconds of arc a year, though the digits in the scan of Iyer's introduction are damaged. For the zero point, the old books point to the junction of Revati and Ashvini, marked by the star zeta Piscium, and Iyer says the exact ayanamsa cannot be found by looking because that star has disappeared as a usable marker. The Surya-Siddhanta, an astronomy text in Ebenezer Burgess's 1860 translation, gives a rule of 600 revolutions in an Age of 4,320,000 years. Burgess reads it as a swing of 27 degrees to each side instead of a one-way drift, with the zero near AD 570 by his own calculation.

The Lahiri ayanamsa is the modern national standard. India's Calendar Reform Committee, set up by the CSIR in 1952 with Meghnad Saha as chair and N. C. Lahiri as secretary, reported in 1955, and a national calendar began on 21 March 1956. The Government of India's ephemeris defines the zero point as the equinox point of AD 285 and fixes its tropical longitude at 23 degrees 15 minutes at 0 hours on 21 March 1956, and the committee's divisions put the star Spica at 180 degrees. For other dates the ephemeris uses a precession polynomial anchored at 23 degrees 51 minutes 25.53 seconds in the year 2000, which gives 24 degrees 09 minutes 01.66 seconds at the start of 2021 and falls about 16 seconds of arc short of the fixed anchor at the 1956 instant.

No traditional value ever settled the matter. The six Indian almanacs that Iyer compared for 1 January 1883 differ from 18 degrees 14 minutes 20 seconds to 22 degrees 41 minutes 44 seconds, and his own mean was 20 degrees 24 minutes 15 seconds. The committee report itself notes that a Revati-based zero and a Spica-based zero differ by about 3 degrees 59 minutes. The calculator subtracts the Lahiri value from tropical longitude, and the sources read for this site do not cover other named ayanamsas.