Gnomonic Shadow

Abstract

Now, the method to identify the (four) directions. First prepare a level surface. It should be such that if water falls at its centre, the water should spread in a circle and flow forth on all the sides uniformly. That is the indication for a level surface. On this surface draw a circle (in the following manner): Take a rod slightly bent at both ends and, with one end of the rod fixed at the centre, rotate the other end on all sides (so that a circle will result). The point where the end (of the rod) is fixed is known by the terms kendra and nābhi (centre). The line resulting from the rotation of the other end is called nemi (circumference). Fix (vertically) at the centre a uniformly rounded gnomon (śaṅku). On any morning, observe the point on the circumference where the tip of the shadow of the gnomon graces and enters into the circle and, in the same manner, also the point where the tip of the shadow graces the circumference and goes out of the circle in the afternoon. Mark these two points on the circle with dots. These two points, between themselves, will be almost along the east-west. For this reason, these are termed east and west points. These would have been the exact east and west points if they were the shadow-points of the stars which do not have any north-south motion. The Sun has a north-south motion on account of (its motion between) the solstices, and during the interval from the moment, when the western shadow-point gets marked, to the moment when the eastern shadow-point is formed, if the Sun has moved north due to the change in its declination, then to that extent the tip of the shadow would have moved to the south.