As we all know that is Sun is at the center of our solar system and Earth is revolving around the Sun. Suppose Earth is not tilted then it follow the ecliptic path as shown in the figure below.
Since there is axial tilt of 23.26 degrees, Earth follows the ecliptic path as shown below.
Basically these two paths form a virtual plane around the Sun. The intersection line of the two planes results in two diametrically opposite intersection points, known as the equinoxes. The name “equinox” is derived from the Latin aequus (equal) and nox (night), because around the equinox, the night and day are approximately equally long. There will be two equinoxes: the vernal (Spring) equinox and the autumnal equinox.
The vernal equinox is marked as the beginning of the year. The Tamil New Year (also called as Puthandu) celebration follows the vernal equinox (also known as Nirayanam). However there is an issue in marking the vernal equinox as the celebration. Because the Earth orbits around the Sun by rotating itself and by that it experiences a slow down, similar to a spinning top which takes larger angular rotation towards the end before stopping.
Hence, there will be slight shift in the vernal equinox point every year. For example this year (2009) vernal equinox occurred around 20th March. After few years it will happen little bit early. In a way, it logically does not account for a complete revolution by the earth around the Sun.
So our ancient people, few thousand years ago, identified the vernal equinox point which occurred on 14th of April and from that point divided plane into 12 divisions of Rashi or (Veedu which means House) as shown below.
A Rashi indicates the group of stars (or Nasktra, which means ‘not in motion’) governing the Earth as it rotates.
The Tamil New Year celebration marks the occasion of Earth entering into Mesha (Aries) Rashi followed by the vernal equinox point. (Now , the vernal equinox occurred in 20th March and entering into Mesha rashi is happening on 14th April). The starting month is called Cittirai, a period of mild sun with much light and less humid wind known to be very soothing and refreshing. (Now it is very hot, since the vernal equinox point is receded to March). In some parts of India, like Maharastra, Andra Pradesh, Karnataka, etc. celebrate the new year (Gudhi Padwa, Ugadi) based on the Lunar calendar which also mark the occasion of following the vernal equinox and on the new-moon day (Amavasya). But for your information, there is no astronomical significance in celebrating the 1st January as the new year, it just signifies a logical end.
Now lets see few things about Time. We measure time based on the planetary motion. But what is time at subtle level? According to the Surya Siddhanta, a treatise of Indian astronomy, describes the concept of time as,
‘That which begins with respirations (prAna) is called real; that which begins with atoms (truti) is unreal.’
‘Six respirations make a vinAdi, sixty of these a nAdi. And sixty nAdis make a sidereal day and night. Of thirty of these sidereal days is composed a month; twelve months make a year.’
Basically according to the Surya Siddhanta; 6 respirations x 60 x 60 = 21600 respirations will make a day.
Each proper respiration takes 4 seconds ; 21600 x 4 = 86400 seconds makes a day.
[Hence breathe properly.]
Fundamentally, breathing process is called ‘Ajapa’, an unconscious repetition but constantly. The breath of every person in entering makes the sound of ‘sah’ and in coming out that of ‘ham’. These two sounds make ‘Soham’ which means ‘I am He’. Sage Vyasa, author of Mahabharata, quotes Time is not a continuous entity rather it is discrete. When you concentrated i.e. your cognition flows towards a particular thing, then the notion of time disappears. It reminds me the Einstein’s quote, ‘Put your hand on a hot stove for a minute, and it seems like an hour. Sit with a pretty girl for an hour, and it seems like a minute. That’s relativity.’ Understanding the concept of Time makes you to understand about life.
However, every change happens within you is the real [new year] occasion.