Bharatiya Jyotihshastra (History of Indian Astronomy), translated by R. V. Vaidya v2.c12.s7.5

v2.c12.s7.5

Time is determined from (the length and direction of) shadow cast by the gnomon. The shadow is, however, mainly useful in setting up the ‘Nalikā’ sighting tube before observation. Following is the main method of setting up the ‘Nalikā’ for observation :—To determine, beforehand, by calculation from the astronomical works the length and direction of the shadow likely to be cast by the gnomon when set up in the light of the sun or any planet at a particular moment and then to set the sighting tube in that position for observing the planet. If the planet be observed in that position, then the position calculated from the book would be considered correct.