In Search of an Ayanamsa

Ever since I first read Cyril Fagan’s Primer of Sidereal Astrology (1971), I have been puzzled by the various ayanamsas available in the literature, that is, which one to use? Fagan’s study of ancient texts led him to believe that the “zero year” (when the tropical and sidereal zodiacs aligned) was 221 AD. By Fagan’s estimate, the ayanamsa for January 1, 2000, would be about 24d 44m (calculated on astro-seek.com).

Indian astrologers commonly use the ayanamsa proposed by Lahiri, a scientist who was appointed by the Indian Government to come up with an Ayanamsha that would be the basis for a common and widely used ephemeris (panchanga). According to Lahiri, the ayanamsa for January 1, 2000, would be about 23d 51m (calculated on astro-seek.com) or 23d 51m 23s (calculated on astrosage.com).

Another popular version is the so-called KP ayanamsa, proposed by Professor Krishnamurti, the founder of KP astrology who believed that the “zero year” occured in 291 AD. some 70 year after the date suggested by Cyril Fagan. There are two versions, the Old and the New KP ayanamsa. According to New version, the KP ayanamsa for January 1, 2000, would be about 23d 46m (calculated on astro-seek.com). This is quite close to the Lahiri value.

Another version used by KP astrologers is that of S.P. Khullar, who proposed the formula: AyanKP = (50.2388475/3600)*((YR-292)+(261/365)+...)). Using this formula, I calculated the Khullar ayanamsa for January 1, 2000 to be 23d 50m 44s or rounded, 23d 51m. which is very close to the Lahiri value.

Image from https://www.researchgate.net/figure/Precession-and-nutations-of-the-spin-axis-about-the-orbit-pole-Adapted-from-Lowrie-2011_fig1_345007076

Different astrologers use different ayanamsas, apparently depending on their experience and personal preferences. Mathematically, an ayanmsa is not easy to determine because the Earth doesn’t travel at an exactly uniform rate around the Sun, and the Earth is also subject to nutation, that is, a periodic swaying or oscillation of its axis that causes the precession of the Earth’s poles to follow a wavy rather than a circular path. A good discussion of this issue can be found in the U.S. Naval Observatory Circular #179 (Sep 1, 2005).

There is also an excellent book on this topic: Study of KP Ayanamsa with Modern Precession Theories by D. Senthilathiban, in which the author incorporates modern models of the Earth’s motion to offer revised values for the KP and Lahiri ayanamsas. He writes: “Hence for our astrological calculation it is recommended to adopt IAU2006 Precession Model along with latest Nutation Model IAU2000A to obtain True Ayanamsa and True Obliquity, so that more accurate Nirayana position of Cusps, Planets can be obtained (Both for KP System and Hindu Vedic Astrology).” According to his calculations, the mean ayanamsa for April 15th at 00:00 UT are:

Lahiri, 2000: 23:51:55New KP, 2000: 23:47:01Difference: 00:04:54—
Lahiri, 1950: 23:10:01New KP, 1950: 23:05:07Difference: 00:04:54—
Lahiri, 1900: 22:28:08NewKP, 1900: 22:23:13Difference: 00:04:55—


Thus, using up-to-date models and observations, the difference between the 2000 and 1900 values of the ayanamsas are 1d 23m 47′, which is equivalent to a rate of change of about 0.013963888 degrees per year, or 50.27 seconds of arc annually.