Following the relative position of the sun in relation to the movement of the earth is called Souramana. We know that the earth moves round the sun, but for the convenience of calculation, sun’s relative position is considered for the purpose. There are 27 stars on the orbit of the Sun and the Moon. The time taken for the movement in the orbit of the Sun and the Moon to cover the distance of two and a half stars is taken as on Rashi. accordingly there are twelve Rashi’s and the first on is Mesha.
The sun takes roughly thirteen days to cover the distance of one star. The star on which the sun is seen is called the Mahanakshatra. The sun takes 30 or 31 days to cover one Rashi. At the end of it the Sun crosses over to another star. That point of crossing is called Sankramana. In this way the sun takes 365.25 days to cover these twelve rashis. according to the solar way of calculation of the Sun’s movement (in relation to the earth’s position), there are 365.25 days in a year. The new year begins on the point when the sun cross the star Revathi to Ashwini, and the next day is considered the New Years Day.
Calculation of the days of the year based on the movement of the moon is called Chandramana. According this system the next day from one new moon day to the next is considered to be a month. Such twelve months constitute a year. In this system there are 354 days in a year. This result in the shortage of 11 days between the Souramana and Chandramana years. Therefore once in three years, the difference amounts to one month and we add one month and that month is considered Chandra Adhika Masa, (month) and it is added to the Souramana year. For this very reason, Soura Samvatsara is considered to be the Sthirasamvatsara and Chandra Samvatsara taken as Asthira Samvatsara by the Shastras.
Asthiraschandra maasah syat sthirah samvathsakaranam