Surya Siddhanta
क्रान्त्योर् ज्ये त्रिज्ययाभ्यस्ते परक्रान्तिज्ययोद्धृते । तच्चापान्तरम् अर्धम् वा योज्यम् भाविनि शीतगौ॥
Multiply the sines of the two declinations 'by radius, and divide by the sine of greatest declination : the difference of the arcs corresponding to the results, or half that difference, is to be added to the moon's longitude when the aspect (pdta) is to come; . -
english translation