Surya Siddhanta
कोटिकर्णयुताद् बिन्दोर् बिम्बम् तात्कालिकम् लिखेत् । कर्णसूत्रेण दिक्सिद्धिम् प्रथमम् परिकल्पयेत्॥
from the point of intersection of the perpendicular and the hypothenuse describe the moon's disk, according to its dimensions at the given time. Then, by means of the hypothenuse, first make a determination of directions.
english translation