Amanta and Pournimanta

Amanta and Pournimanta
For a solar month, the sun's ingress to the sign alone matters. Therefore, there is only one its kind. But as the lunar month is either reckoned with the end of amavasya or Pournami there are two kinds of them. These are namely the New moon ending and Full moon ending. Those months which end with an Amavasya or New moon are called Amanta and those which end with Pournima or Full moon are called Pournimanta. Both the modes of reckoning were in vogue since the Vedic times. Even now, Pournimanta lunar months are followed in some parts of India. Only the Amanta system is taken for calendaric calculations for determining Inter-calary month or Adhikamasa and decayed month or Kshayamasa. These Amanta lunar months commerce just at the moment of a New moon and end at the instant of the next New moon.

Twelve solar months based on the twelve Samkrantis or solar ingresses into each of the twelve signs of the Zodiac constitute one solar year. These solar months have varying periods of 30 to 32 days due to the vagaries of the movement of the sun, in relatio to the earth. It was found that the twelve lunar months, varying from 20 to 30 days, based on the occurence and re-occurence of either the Full moons or the New moons did not converge exactly with this solar year. The length of an solar year on an average is 365 days, 5 hours, 49 minutes (Surya Siddhanta takes this as 365 days, 6 hours and 13 minutes) and of a lunar month 29 days, 12 hours, 44 minutes. So 12 lunar months add to 354 days, 9 hours and lag behind the solar year by about 11 days (10.896 days) per year. It is evident then that in three years this difference itself will be more than thirty days resulting in a slide of a month in relation to the solar reckoning.