Science behind Vishu

The earth revolves around the sun.The  number of revolution  is counted   starting  from the fixed point known as Mesha sankranthi rekha   which is Alpha Aeries point. The movement  of the sun ( when viewed from the earth) from the sign of Meena to the sign mesha is the sankranthi.  It is infact  from the mesha sanskrathi starts  the   astronomical new  year day. i.e.  the  starting of  counting  the revolution of the  earth around the sun.  

 In Sanskrit it is known as mesha vishuvath  from the word vishuvath came vishu. There are mesha and tula vishuvath. The  time of  the change of the sign from meena to mesha (mesha sankranthi) is known as the punya kaalam. By assigning  the  Sankranti time as the right ascension (lagna) point   examining the position of all other  grahas the  vishu phala pravachana is done  for the next one year.

During Aryabhatta’s
period ,  AD 499 March 21st at 1.00 Noon was the  Mesha sankranthi
vishuvath, which slowly drifted every year  (ayanaachalanam) and  this
drifting for the last 1500 years  resulted in the change of the Mesha
sankranti from  March 31st to April 14th /15th . New year day of Kali
era  is considered as this day. How ever
astronomically the new year day is on April Vishuvath which we haved
used for spiritual, traditional  and social  customs and rituals as  in
the other Indian  traditions.