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

212 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