Bharatiya Jyotihshastra (History of Indian Astronomy), translated by R. V. Vaidya v2.c12.s5.32

v2.c12.s5.32

Draw a circle with any radius on an even horizontal ground. Mark off principal directions on it and then draw arcs showing ‘agrā’ (amplitude of the day), both in the East and the West. Draw another concentric circle smaller in size and having a radius equal to “dyujyā” (sky-radius). Mark off points on it to show 60 ghaṭis. Take a stick equal in length to the radius of the outer-circle and hold it pointing to the Sun, with its one end placed at the centre of the circle, so that the stick will cast no shadow. Place in the smaller circle a rod which is equal in length to the distance of the other end of the stick and the end of the amplitude marked in the east. The arc showing ghaṭis intercepted between the ends of the stick will show the time elapsed (in ghaṭis). If the sun is in the West, the distance between the ends will give the number of ghaṭis showing the time left for the day to end. The methods of finding a number of things like ‘palabhā’, with the help of the ‘Yaṣṭiyantra’ have been described. Brahmagupta and Lalla have described the method of finding the distance between the Sun and the Moon, and from that the tithi, with the help of a similar but somewhat different type of a ‘yaṣṭiyantra’.