Digamsa Yantra

The instrument is used to determine the azimuth of a celestial object by sighting the object from the outer edge of the inner wall through the center of a crosswire suspended from the outer wall, and adjusting a weighted string attached to the central pillar and suspended over the outer wall until it also lines up with the sighted object and center of the crosswire. The weighted string indicates a vertical plane through the intersection point, and the azimuth may be read by looking at the point on the outer wall where the string goes over the edge. Jai Singh built Digamsa Yantras at Jaipur, Ujjain, and Varanasi, and although they are approximately the same in overall size, the height of the inner wall varies significantly. At Jaipur and Varanasi, the inner wall and pillar are about a meter in height, and the outer wall is twice that. At Ujjain the inner wall and outer wall are the same height. Although the original design called for the outer wall to be twice the height of the inner wall, in practice it only matters that the outer wall be higher than
the inner wall.