HAPPY VISHU!
It's spring time and it's festive season as many states in India are celebrating the traditional New Year around this time of the year. So why around this time? Our forefathers had great knowledge on Astronomy and they knew that Equinox (that occurred on 20 March) has a physical significance as the Sun is directly overhead at the equator and the day and night are nearly equal that day. So they wanted to consider this day as the start of a new year. The world celebrates 1 Jan as the New Year, but it has no real significance other than change of a date.
Ok, Equinox is on 20 March, but our traditional New Year is few days after that. Why is it so? That is because of an astronomical phenomenon called Precession of the Equinox (see the section below that explains it). But why Ugadi (also known as "Yugadi", which is also a new year for certain states) is celebrated much earlier to Baisakhi, Puthandu, Vishu, etc.? That is because Ugadi on 2 April is based on Lunar calendar (This is called "Chandramana Ugadi") and is celebrated on the first New Moon after the Equinox. Whereas, 14/15 April is considered as "Suryamana Ugadi" (solar based New Year), because this is the day on which the Sun appears to enter a new Zodiacal constellation ("Pisces" or "Meena" or "Meenam) as viewed from Earth.
By the way "Vishu" means "Equal". Probably this name was used to indicate the nearly equal day and night on the day of the Equinox.
Precession of the Equinoxes
Now, there is more to this. When all these were observed by our forefathers, they realised that as Earth spins at the rate of 1000 miles per hour, it wobbles like a top (see the first picture below) due to the torque exerted by other bodies like the Moon and the Sun. This wobble is so slow, it completes one full circle of the wobble across 25,772 years. That means, in which date the Sun appears to enter Pisces constellation keeps shifting to the right. So our festivals that are based on Sun's movement (like Uttarayan, Makar Sankranti, Ugadi, etc.) will continue to come 1 day late every 71 years. It is amazing to note that our traditional calendars already considers these and make necessary adjustments to the date to account for this shift.
Illustration below shows how over the years (in red text) the constellation on which the Spring Equinox shifts due to the precession of the Equinoxes (or Earth's wobble).
Dbachmann, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
Next time, let's celebrate the Equinox (that always occurs around 20 March, give or take 1 day) and our traditional New Year festivals, by being fully aware of these physical and astronomical significance.
#CandleInTheDark is my attempt to explain our traditional celebrations based on scientific/astronomical facts
Have a blessed year ahead!