Have you ever wondered why many festivals across India like Vishu in Kerala, Bihu in Assam, Puthandu in Tamil Nadu and Baishaaki in north India are celebrated almost on the same day (April 14th or 15th) ? What makes this date so particular that we find a unity in fixing festivals on this day? Well to understand this, one should know the calendar system that developed in ancient India, whose modified versions are being used even now in different parts of India ( eg: Saka Varsh in the North and Kolla Varsh system in Kerala). Before going into a little history and details of the above mentioned calendar, let me proudly mention here that the calendar system that our ancestors had devised was much better than any other in the world (even better than the widely accepted Gregorian Calender).
A quick peep into the history of our civilisation would bring into our attention the story of our calendar system. The necessity of a calendar, which could precisely give a foresight about the seasons to come, was felt as soon as the nomadic human beings started to settle down into tiny groups and started to cultivate food items (rather than collecting them from the woods). A good harvest depended greatly on the time of the sowing & harvesting, at the right conditions of rain or sun as predicted by a calendar. The best physical phenomena to which our ancestors could correlate the seasons were the travelling of sun and the moon with respect to the background star field. It should be noted that they possessed exceptional observation skills which enabled them to note the slightest changes in position of sun and the moon in the celestial sphere.
The collective observational wisdom of some generations was capable enough to prove to the ancient astronomers that the sun travelled eastward by some angle every day, completing one cycle in around 365 days, while the moon does the same in about 27 days. Also the path followed by the sun and the moon are fixed and does not shift. These observations formed the basis of the first calendar whose modified version is being used by us. The circle in the sky through which the sun seems to move was divided into 12 parts (namely Raashis), each identified by the name given to the imaginary animal or object formed by joining the stars in that portion of the sky. Thus we have 12 months, each named after the constellation which comes in the background of the sun in that particular month i.e during Makar month (makaram in malayalam) the sun will have Makar Raashi (or Capricorn constellation) in its background and so on. Thus your sun sign actually represents the position of the sun in the sky at the time of your birth.
The days on the other hand are named after the star or constellation that appears nearest to the moon on that particular day. Thus we have 27 star signs (from Aswathi to Revathi in malayalam). The star sign in your horoscope actually points to the position of the moon on the day of your birth. (Kindly note that the science part of your horoscope ends here and that the star sign has nothing to do with your future). The calendar system in the northern parts of India was slightly different. In north India the days where named as per the waxing and waning of the moon (Krishna paksham and sukla paksham) rather than the stars adjacent to it. The calculation of day was thus less precise in northern parts of India.
Now it was time to decide as to when to start the year. Again the astronomers came to rescue and announced that there exists two peculiar days in a year when the sun would rise perfectly from east. These days called as the vernal equinoxes (Vishuvam in malayalam) are characterised by perfect time distribution between day and night (i.e 12hours each). The tilt in the earth’s axis would, on all other days, cause the sun to rise from North-East or South-East directions. It was decided to use Poorva Vishuvam or the equinox that coincides with the entry of sun to Medam Raashi (Aries constellation) as the start of the year. Thus it is in fact the new year that is being celebrated around India as Vishu, Bihu etc.
This calendar system is very precise because it takes into account both the rotation and revolution of earth. This seldom requires corrections, unlike the Gregorian calendar which requires one day to be added every leap year as correction. However the precession of earth axis is a third motion that affects the system of calendars and hence seasons. This went unnoticed because of the large time it takes to complete one cycle (29,000 years). There are evidences to prove that even this was corrected in a periodic manner in ancient times by advancing the new year day by one day or two in a couple of decades. The now fixed date of Meda Sankranthi (i.e movement to sun to Aries constellation) was fixed at the time when the last correction was done.
The above discussion indicates that the Vishu day or the equinox day should have shifted by some days and will not be exactly on April 14th as of now. The present Vishu day is in fact on 21st of March. Readers may please note that seasons would also shift by this amount. This in fact leads to a phenomenon which is much discussed and misinterpreted as an indication of global warming and stuff. The phenomenon I am referring to is the early blossoming of Kanikonna, the state flower of Kerala. Kanikonna is known to be a tree which flowers during Vishu, which as per the calendar followed now is on April 14th. It can be noted that in these days kanikonna would flower around the third week of March rather than in April. Isn’t it great to notice that the kanikonna knows the shift in our calendars but we are still ignorant to make relevant corrections in our calendar system and instead blame the nature for making kanikonna bloom “earlier”……….…!