Surya Siddhanta
सूर्योनशीतगोर् लिप्ताः शुक्लम् नवशतोद्धृताः। चन्द्रबिम्बाङ्गुलाभ्यस्तम् हृतम् द्वादशभिः स्फुटम्॥
The difference in longitude of sun and moon, when converted in minutes (kaläs) and divided by 900, gives the part of waxing phase of the moon (çuklängça). This multiplied by the measure of moon’s disc (vimbänguli) in digits (aìgula) and divided by 12, gives us the corrected part of waxing phase of the moon (sphuta çuklängça).
english translation