Ayanacalana, also known as the precession of the equinoxes, holds significant importance in Hindu astronomy for the computation of the Hindu panchanga (almanac) and the precise timing of domestic rituals and religious festivals.
In this astronomical context, the ecliptic, a celestial
circle housing 27 constellations, and the celestial equator, another celestial
great circle, are inclined relative to each other and intersect at the
equinoxes. While the ecliptic remains fixed, the celestial equator exhibits
continuous backward motion, causing the equinoxes to gradually shift backward
at a rate of about 50.2 seconds per year. This phenomenon is termed ayanacalana
or "precession of the equinoxes," as it results in the equinoxes
moving from one constellation to the preceding one.
The concept of ayanacalana has been observed in India since
Vedic times, evidenced by Vedic priests adjusting the start of the year to
determine timings for sacrifices in different eras. Hindu astronomy considers
ayanacalana as a to-and-fro oscillatory motion. The inception of the Kali Era
in 3102 BCE, coinciding with the sun and planets being in the first point of
Aries (Meshadi), marked the beginning of a zero-precession year. A full
oscillation, moving from zero to 27 degrees and back, was defined as 7200
years. Consequently, during the initial 1800 years from the zero-Kali year, the
equinox regressed by 27 degrees. In the subsequent 1800 years (reaching 3600
Kali, around 499 CE), during the time of the astronomer Aryabhatta I (476 CE),
the equinox returned to the zero position. It will continue regressing by 27
degrees for another 1800 years until 2299 CE when it will regress by 27 degrees
for another 1800 years to complete one full cycle of 108 degrees.
Currently, the equinox's regression, the accumulated precession, stands at about 23 degrees. Hindu astronomy measures time by the sidereal year, in contrast to Western astronomy, which employs the tropical year. Consequently, Meshadi remains the starting point of the year, despite the first point of Aries shifting due to precession. To obtain the actual positions of the sun and planets as per Hindu astronomical texts, it is necessary to add the ayanamsha (degrees of precession), which measures the accumulated ayanacalana from the zero-precession point to the current date (approximately 23 minutes), to the calculated times. These "ayana" (added times) are commonly used to determine the timing of Hindu domestic rituals and religious festivals.