Ayana Chalanam - Axial Precession

The earth revolves around itself
resulting in day and night for us.

However, the revolving is not smooth.
It wobbles a bit.

The wobble is something like the diagram below.

Inline image 1

The wobble you see repeating every
few seconds in the diagram above
for the earth, actually repeats around
every 25,772 years ! [+]

The reason for the wobble is
attributed to the gravitational forces
of the sun and the moon. [*]

Due to this wobble, we observe
Ayana Chalanam
- the axial precession.

The start of Uttarayana happens
20 minutes earlier than that of the previous year.

After about 72 years, this shift adds up to
one day. So, after about 72 years,
Uttarayana will start a day earlier.

This shift is called Ayana Chalanam
(axial precession)

Ayana Chalanam explains why :

the Bharata war but starts much earlier now

Uttarayana a few centuries back but it is no longer the case

But now the Vernal Equinox happens about 23 days earlier.
Varahamihira (around 500 AD) explains
Ayana Chalanam in his works. [#]

There is a calendar system called Sayana Sauramana,
which takes into account the Ayana Chalanam.
However, this calendar system has not got
into popular practice.

The traditional Solar calendar is the Nirayana Sauramana.
However, this calendar does not take into account Ayana Chalanam.

Why is Ayana Chalanam important ?
Because the start of Uttarayana or Dakshinayana
defines the seasons.

So a shift in Uttarayana start means shift of seasons.
And seasons are our most important annual physical phenomena.

[+] http://en.wikipedia.org/wiki/Axial_precession_(astronomy)#Effects
[*] http://en.wikipedia.org/wiki/Axial_precession_%28astronomy%29#Cause
[#] Varahamihira's Pancha-sidhhantika and Brihat-Samhita

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