Surya Siddhanta
तन्मध्ये स्थापयेच् छङ्कुम् कल्पनाद्वादशाङ्गुलम् । तच्छायाग्रम् स्पृशेद् यत्र वृत्ते पूर्वापरार्धयोः॥
Place the vertical Gnomon of 12 digits at its centre and mark the two points where the shadow (of the Gnomon) before and after noon meets the circumference of the circle; these two points are called the west and the east points (respectively).
english translation
tanmadhye sthApayec chaGkum kalpanAdvAdazAGgulam । tacchAyAgram spRzed yatra vRtte pUrvAparArdhayoH॥
hk transliteration by Sanscript