Aynanamsha is a concept in Hindu astronomy. Modern astronomy recognizes and uses a moving zodiac, caused by the phenomenon of what is called ‘precession of the equinoxes’. But in Hindu astronomy a fixed zodiac is adopted and the positions of the planets are generally calculated with reference to the fixed groups of stars or constellations. This system is called nirayanapaddhati (sidereal system), while in the modern system it is referred to as Sayana paddhati (tropical system).
The Hindu nakshatra (stellar) system, as also sauramana (the
solar) year, is affected by the consideration of nirayanapaddhati. Since several
of the Hindu festivals are based on the solar and stellar reckoning, the ayanamsha
has a great influence on the precise dating and timing of such festivals.
While the first point of Mesha Rashi (Aries), the beginning
of the Hindu sidereal zodiac, is fixed in the Ashwini constellation, the moving
or tropical zodiac recedes along Kranti vritta (the ecliptic) at an annual rate
of about 50.26 degrees (modern value). The difference between the two first
points of two zodiacal signs goes on increasing at this annual rate.
To an observer on the earth, the sky appears like a huge
hollow sphere with an arbitrarily large radius of what is called ‘celestial
sphere’. The plane of the earth’s equator, when extended, cuts the celestial sphere
in a great circle called celestial equator. The apparent annual path of the
sun, as observed from the earth, is another great circle on the celestial
sphere, which is called ecliptic. The two great circles intersect at two points,
G and W that are called the vernal equinox and the autumnal equinox
respectively. In the course of its apparent annual revolution (as seen from the
earth), the sun comes to the vernal equinox around March 22 and to the autumnal
equinox by about September 23. On these two days, the durations of the day and
the night are equal everywhere (and hence the name equinox). The vernal equinox
is taken as the first point of Mesha on the ecliptic. Due to the gravitational
pull on the earth, the earth’s equator (hence the celestial equator) shifts is
position. This results in the ‘precession’ of equinoxes along the ecliptic.
In the course of the backward motion of vernal equinox, it
coincides with the first point of Mesha once in about 26,000 years as
calculated from the modern astronomical parameters. The amount of accumulated
precession of equinox from the time of the coincidence of the two first points
(called the ‘zero year’) is called ayanamsha. Different Hindu astronomers have
taken the zero year variously for the purpose of ayanamsha. The following are
some examples – Surya Siddhanta (modern version): 499 CE. Laghumanasa of
Manula: 527 CE., Grahalaghava of Ganesha Daivajna: 522 CE and Bhatatulya of
Damodara: 420 CE. The Calendar Reform committee appointed by the Government of
India fixed the year 285 CE in its report for the purpose of deciding the dates
of Hindu festivals and the computation of annual Indian Ephemeris as well as Rashtriya
Panchanga (National Almanac).
From the references available in the Vedic and post-Vedic
literature, it is evident that the phenomenon of the precession of equinoxes
and solstices was known to our antecedents. For example, in the list of lunar
asterisms, Ashwini – b-Arisetis is mentioned first now, while it used to start
with different asterisms during different periods in history.
At the time of Vedanga jyotisha (about 1500 BCE), the
starting asterism was Sravistha (or Dhanistha) identifies as b-Delphini).
Abhijit (Vega, star a-Lyrae) was the leading asterism in ancient Jaina
literature.
At the time of Satapatha Brahmana, the vernal equinox was
near Krittikas Ashwini during the Siddhantic period. Since the Hindu tradition
has adopted a fixed zodiac, there is now no change affected in respect of the
leading asterism of the zodiac. In Vasishtha Siddhanta, Vishnuchandra mentions
the ayanacalana (motion of equinoctial points), which works out at the rate of
57 degrees per year. Generally, the Hindu Siddhantic astronomers took the rate as
60 degrees (i.e. 1) per year although a couple of them gave rather a lower
rate. Varahamihira refers to the phenomenon and recommends verifications
through actual observations. Aryabhata I (476 CE) and Brahmagupta (c 598 CE)
seemed to have ignored the motion of equinoctial points mainly due to the small
amounts of ayanamsha during their times and hence could not be recognized by
crude observations.
Modern Hindu astronomers (and astrologers) have been
advancing various theories, explanations, and arguments to fix the zero year for
the ayanamsha and there is no unanimous acceptance in the matter. Manjula was the
first to mention the ayanamsha explicitly and he gave its value as six degrees
during his period.
Many Hindu astronomers argue that the modern Sayana (tropical)
system should be followed for the computation of panchanga, and religious observances.