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

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pratipat, Dwitīyā etc. upto the 30th i.e. Amāvāsyā or New
Moon. Thus pratipat starts when ☾ = ☉ i.e. when the
longitude of the Moon is equal to that of the Sun i.e.
from the moment of New Moon when the Moon is in
conjunction with the Sun. अमा सह = वसतः सूर्य्याचन्द्रमसावस्या
मित्यमावस्या i.e. Amāvāsyā is that point of time when
the Sun and the Moon are together. Pratipat lasts
till ☾ = ☉ + 12°; then Dwitīyā begins and lasts till
☾ = ☉ + 24; Thus purnima begins from the moment
when ☾ = ☉ + 168 and lasts till ☾ = ☉ + 180° and
Amāvasyā begins when ☾ = ☉ + 348 and lasts till ☾ is
again equal to ☉ Thus a tithi is the time that is taken
by the Moon to overcome the Sun by 12° beginning from
the moment of New Moon. In other words the tithi is a
measure of the phase of the Moon with a particular
convention.
(2) In modern astronomy the phase of the Moon or
that of a planet is measured by the formula (1 + cos EPS) / 2
where P is the planet or the Moon. Since ES is almost
Fig. 29
parallel to PS (Fig. 29) ÊPS may be taken to be nearly
equal to P̂ES which is the elogation of the planet or the
Moon so that phase = (1 - cos EPS) / 2 approximately. So