Surya Siddhanta
चतुरस्रम् बहिः कुर्यात् सूत्रैर् मध्याद् विनिर्गतैः । भुजसूत्राङ्गुलैस् तत्र दत्तैर् इष्टप्रभा स्मृता ॥
(In order to find the direction of a given shadow of the Gnomon at a given time, describe a circle in the plane of the Horizon with a radius whose length is equal to that of the given shadow and at its circumference determine the points of the Horizon, the Meridian and east and west lines as mentioned before:) Then describe a square about the said circle through the lines drawn from the centre (of the circle to the points of the Horizon, in such a manner that the square shall touch the circle at the cardinal points) and in this circle (towards the western or eastern part of it according as the given time is before or after noon), draw a line (as a sine,) equal to Bhuja[1] (of the given shadow and perpendicular to the east and west line towards the north or south according as the Bhuja is north or south. To the end of this perpendicular, draw a line from the centre). This (line) will denote the direction of the given shadow (at the given time).
english translation
caturasram bahiH kuryAt sUtrair madhyAd vinirgataiH । bhujasUtrAGgulais tatra dattair iSTaprabhA smRtA ॥
hk transliteration by Sanscript