सिद्धान्तशिरोमणि: गणिताध्याय (भास्कराचार्य - ग्रहगणित, मध्यमाधिकार व स्पष्टाधिकार सटीक)

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This has to be converted into solar days to give us the
number of degrees of the error. The conversion is effected
by the rule of three as “If T tithis of a Kalpa constitute S
solar days of the Kalpa, what number of solar days corres-
ponds to (R' × 30) / S ?” The answer is ((R' × 30) / S) × (S / T) =
(R' × 30) / T = R' / (T/30) = R' / L where L is the number of
lunations in a Kalpa. This R' / L being the number of solar
days corresponding to the Adhimāsa-Sesha upto the day
concerned, the corresponding longitude of the Sun namely
R'° / L must be subtracted from the Sun’s longitude. Now the
question arises whether we have to multiply this R''° / L by 13
to be subtracted from the longitude of the Moon; not
necessary, enough to subtract R'° / L only, for, the Adhimāsa-
sesha extends from the preceeding Amāvāsya only when
the Moon’s longitude was equal to the Sun’s longitude.
We have thus got the mean positions of the Sun and the
Moon at the ending moment of the tithi on the previous
day. To get their mean positions at the Sun-rise of the
concerned day, we have to make amends for the time in
between, which is no other than Avama-Sesha as a fraction
of a mean solar day. Here Bhaskara makes an ingenious
approximation. The maximum Avama-Sesha could be a
tithi only and the Sun moves roughly by his daily mean
motion during a tithi. So, the rule of three adopted is
‘If for one tithi, the Sun’s daily motion is to be reckoned,
what for the Avama-Sesha? The answer is Avama-Sesha
multiplied by the Sun’s daily motion. The Avama-Sesha
being of the form R / T, (R / T) × m' = (R / T) × 59 (8' / 60) =