Unique to the Jantar Mantar Observatory, Jaipur, are the Kapala instruments which have a functionality for conversions between the different celestial co-ordinate systems. These are thus unique to the Jantar Mantar group of instruments in terms of having these theoretical conversions built in their construction. It is only one of these units, the eastern Kapala, which is meant for theoretical conversions of celestial co-ordinates.
The twin Kapala Yantra instruments (sunken hemisphherical bowls) on either side of the vertical Chakra Yantra at the Jaipur Jantar Mantar Observatory
The Kapala are built as two hemispherical units, each hemisphere being a complete reflection of the sky overhead.
The western Kapala unit is built for observations while the eastern segment is meant for theoretical conversions of co-ordinates from one system to another. The western Kapala unit is analogous to the Jaiprakas – a hemispherical bowl on which every point is a reflection of a point in the sky.
By looking at the shadow of a cross wire stretched over its surface, the co-ordinates of the Sun in the sky, can be determined with the western Kapala.
The Kapala does not have gaps cut into it, unlike the Jaiprakas, and thus, cannot be used for night time observing.
(To be edited and completed)