Surya Siddhanta
कोट्या दिक् साधनात् तिर्यक्सूत्रान्ते शृङ्गम् उन्नतम् । दर्शयेद् उन्नताम् कोटिम् कृत्वा चन्द्रस्य साकृतिः॥
After making a determination of directions by means of the perpendicular, point out the elevated (unnata) cusp at the extremity of the cross-line : having made the perpendicular (koti) to be erect (unnata), that is the appearance of the moon.
english translation