Jaya Prakasa and Kapala A Yantra 
Written by Yogesh Soman 
Sunday, 26 March 2006 00:55 
These are both multipurpose instruments consisting of hemispherical surfaces of concave shape and inscribed with a number of arcs. These arcs indicate the local time, and they measure various astronomical parameters, such as the coordinates of a celestial body. One difference between the two instruments is that the Kapala A indicates the ascendants, while the Jaya Prakasa reveals the sign on the meridian. Another is that the Jaya Prakasa is built in two complementary halves, giving it the capacity for night observations.
The engravings for coordinate measurement are identical on the surfaces of both Jaya Prakasa and Kapala A. They are inverted images of two spherical coordinate systems, namely, the azimuthaltitude, or horizon system, and the equatorial. In the azimuthaltitude system, the rim of each hemispherical surface represents the local horizon and the bottommost point, the zenith. Cardinal points are marked on the rim, and cross wires are stretched between them. A great circle drawn between the north and the south points and passing through the zenith on the instrument's surface represents the meridian. From the zenith point a number of equal azimuth lines are drawn up to the rim or horizon. Next, a number of equallyspaced circles with their centers on the vertical axis passing through the zenith are inscribed on the surface. These circles are parallel to the rim and intersect the equal azimuth lines at right angles. For the equatorial system, a second set of coordinates is inscribed. For these coordinates, a point on the meridian, at an appropriate distance below the south point on the rim, represents the north celestial pole. At a distance of 90 deg further down, a great circle intersecting the meridian at right angles represents the equator. On both sides of the equator, a number of diurnal circles are drawn. From the pole, hour circles radiate out in all directions up to the very rim of the instrument. On a clear day, the shadow of the crosswire falling on the concave surface below indicates the coordinates of the sun. These coordinates may be read either in the horizon or in the equator system as desired. The time is read by the shadow's angular distance from the meridian measured along a diurnal circle. In this regard the two instruments work as sundials. For determining the ascendants, the instrument Kapala A has a set of 12 curves inscribed on its surface. The curves represent the 12 ascendants. On a clear day, the shadow of the crosswire falling on one of the curves indicates the ascendant or sign emerging at the horizon at that very moment. The Jaya Prakasa, on the other hand, indicates the sign approaching the meridian. A number of curves are engraved on the instrument's surface for this purpose. As the shadow of the crosswire passes over one of these curves the sign designated by the curve is on the meridian at that very moment.

Last Updated on Sunday, 26 March 2006 01:03 