सिद्धान्तदर्पण (महामहोपाध्याय सामन्त चन्द्रशेखर सिंह - नग्न-नेत्र खगोल वेध एवं गणित महाकोश)

xlii सिद्धान्त दर्पण
The exact amount of the ayanamsa may be
apparently determined in different ways. 1. The
Siddhantas furnish a rule for computing it, which
is in principle the same as the method of finding
the longitude of a star at any given date by applying
the amount of precession to its longitude at some
other date. 2. Defining the initial with the help of
other data, such as the recorded longitudes of stars,
its present longitude from the equinoctial point may
be ascertained. 3. Knowing the exact year when the
initial point was fixed, its present longitude
(ayanamsa) may be calculated from the known rate
of precession. But it so happens that the results
obtained by these three methods do not agree.
To begin with the first method, it will be seen
that the different Siddhantas do not agree, either
in the nature of precessional movement, or in its
annual rate. According to some, the equinoxes have
an oscillatory motion, turning to the right and to
the left of the initial point within certain limits, and
extending over a large interval of time; while others
maintain their continuous motion backwards.
Colebrooke compared the views of the libration and
revolution theorists, and gave the rate of precession
accordign to each. They are as follows:-
Liberation Theory | Annual Rate
Surya-Siddhanta | 54″
Soma¹ Siddhanta | 54″
Sakalya Siddhanta | 54″
Laghu-Vasishtha Siddhanta | 54″
Parasara-Siddhanta | 52″.35

1. Ranganatha, in his commentary to the Surya-Siddhanta,
   quotes Soma-Siddhanta in support of the reading.