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

THE SECTION KNOWN AS ADHIMĀSĀDI-
NIRNAYA IN Ch. I
Verse 1. A special feature of the computation of
Ahargaṇa. If the Ahargaṇa is to be increased or decreased
by unity to adjust it with the week-day, the tithis also are
to be increased or decreased by unity to be adjusted with
the week-day. Then in that context of increasing or
decreasing the Ahargaṇa by a day the Adhimāsa-Sesha is
to be increased or decreased by the number of Adhimāsās
of a Kalpa and the Avama-Sesha is to be inoreased or
decreased by the Avamās or Kshayāhās of a Kalpa.
Comm. After having computed the Ahargaṇa as per
the directions given under verses 1-3 of the section of
Grahānayana, we have to test its correctness on the basis
of the week-day taking it that the Kalpa began with
Sunday. In other words, since the number of the elapsed
week-days accord with the number of Sun rises, the week-
day on the day on which the Ahargaṇa is computed should
accord with the Ahargaṇa ie dividing the Ahargaṇa by
seven, the remainder must give us the week-day of the day
in question. But it so happens that we may have to add
or subtract one from the Ahargaṇa arrived at to adjust
the Ahargaṇa with the week-day. Why does this happen ?
The reason is as follows. When we find the Ahargaṇa
for a particular number of elapsed tithis, we are unwit-
tingly taking the number of elapsed true tithis in the place
of the average tithis. The fact that we are given in the
table of constants, the number of mean units of time in
a Kalpa, for example mean solar days, mean tithis etc, and
the fact that computation proceeds only on the basis of
these mean units, means that we have taken into account
only the number of mean tithis elapsed. Hence this is
to be corrected by us. If we are computing the Ahargaṇa