Amhaspati is a term used in reconciling differences in the solar and lunar Hindu calendars. In Hindu religion, both the sidereal year and the lunar year have been in vogue. The annual difference between them is of 10.89258 days, the sidereal year being greater. For the observation of religious rites, reckoning seasons and even for civil purposes, the two had to be systematized and synchronized. The annual difference (10.8958 days) gradually adds up to a lunar month in about thirty-three months. Hence an extra lunar month is added to that lunar year. This extra month is called adhika masa.
A lunar month in which there is no solar ingress or
sankramana is called an adhika masa. It takes the name of the succeeding month
with the prefix adhika. An adhika masa occurs roughly once in 33 months (32
months, 16 days and 4 ghatikas to be precise). The cycle repeats once in nineteen
years. There are some other points also like the Magha masa never being an
adhika masa.
If during a lunar month, two solar entries (i.e. two
sankramans or Sankranti) occur into nirayana rashi, such a lunar month is
called kshaya masa. It occurs in one of the three lunar months – Kartika,
Margasira or Pushya.
Since the sun is usually around the perigree, it has a ‘faster
motion’, so there is the possibility of two sankrantis in this period of the
year.
If, in a lunar year, a kshaya masa occurs, there will be two
adhika masas, one before and one after it. The former, also called amahaspati, occurs
generally once in 141 years; though this may also occur after a gap of 19
years, after 122 years according to Bhaskara II in this Siddhanta Siromani.
Both in the adhika and kshaya masas auspicious functions, vratas, upavasas,
consecration of temples and yajnas are generally avoided.
Amhaspati literally means a pati (Lord) of papas (sins), and
so is considered inauspicious. Kshaya masa means a month deleted.