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

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for, say, the pratipat of phālguna, we automatically
take that the elapsed true tithis from the beginning of
the luni-Solar year is 11 × 30 = 330 tithis. But it may
so happen that had we taken only the number of mean
tithis elapsed, either 329 tithis or 331 tithis might have
elapsed and not 330 as construed which therefore works
an error amounting to unity in the Ahargaṇa arrived at.
At the same time the error does not exceed unity, for,
during the course of every lunation, the longer tithis are
almost compensated by tithis of shorter duration in the
same lunation. The variance in the length of a tithi being
wrought by the variable motion of the Moon, which again
depends on the distance of the Moon from his apogee, is
rounded off in every lunation, as the Moon completes a
circle with respect to the apogee in what is called an
anomalistic month.
Thus it is that the Ahargaṇa is to be rectified on the
basis of the week day. When this is done and the
Ahargaṇa happens to be increased or decreased by unity,
automatically it goes without saying, that the tithis are also
increased or decreased by one. In this context there is a
still more deeper significance as detailed below. The
Adhimāsās are computed from the following formula viz.
(A × a) / S = I + (F / S) × 30 (1) where A = Ahargaṇa, a = adhi-
masās in a Kalpa, S = solar days in a Kalpa, I = Integral
quotient obtained by division and (F / S) × 30 the Adhimasa-
Sesha-tithis. If now A is to be increased or decreased by 1
((A ± 1) a) / S = (Aa / S) ± (a / s) = I + (F / S) × 30 ± a/S from (1) = I +
(F × 30 ± a) / S ; hence the adhimāsa-sesha namely F × 30 is
increased or decreased by a ie the number or adhimasās.
Similarly the Avama-Sesha or the Kshayāha-Sesha is
to be increased or decreased by the Kshayāhas in a Kalpa.