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

xliv सिद्धान्त दर्पण
Siromani, the standard Siddhantas at present in use,
give a rule to test the amount of ayanamsa by
observation. It consists in subtracting from 12 signs
the longitude of the Sun computed after the
Siddhantas for the moment when the Sun crosses
the vernal equinoctial point. Thus, on the 13th April
1894, the Sun’s longitude for Greenwich mean noon
was 23°32′7″, while an almanac gives us 1°21′25″
as the Sun’s longitude for the same instant, making
the ayanamsa 22°10′37″. It will be seen therefore
that not withstanding the higher precessional rate
of the Surya-Siddhanta, the calculated amount
becomes less than the actual by nearly 1°16′, while
the Grahalaghava makes it greater by about 39″.
To come to the second method, viz. to find the
true longitude of the initial point as defined by stars.
All the modern Siddhantas agree in the statement
that the longitudes of all heavenly bodies are to be
measured from the Star Revati situated on the
ecliptic. The star has been identified with Picicum.
Now, this is a star of the fifth magnitude and is
barely visible to the naked eye. The question is,
why was this particular star out of the many situated
on the ecliptic, - some of which are larger, - chosen
for marking the beginning of the zodiac? The answer
has been - and there is no doubt about the accuracy
of the answer - that the vernal equinox happened
near the star when the present system of
astronomical measurement came into vogue.
So far it is plant sailing. For, we can easily find
the longitude of the star from the vernal equinoctial
point and at once get the required ayanamsa. From
the Right Ascension and Declination of the star,
ζ Piscium, on the 13th April, 1894, we get 18°23′49″