Jai Prakash Yantra Jantar Mantar Observatory

The design and purpose of this instrument is similar to the Kapali Yantras discussed earlier. As before, a map of the visible heavens is inscribed on the inner surface of the bowls, with additional scales (including the zodiacal divisions of the year) around the rim. A small ring is suspended from cross wires. Its shadow projects the position of the sun onto the inscribed celestial map.

The ring is visible (barely) in this photograph, hovering in the shadow at center top just between the two square projections. The ring's small shadow lies somewhat above the midpoint on the second projection from the left.

The instrument is made more useful by cutting out sections in alternation (like the Ram Yantra), to allow an observer to make measurements from the inside. At night, an observer could sight through the ring to discover the position of any star or planet of interest.