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Unnatamsa is an instrument for measuring:
altitude - the angular height of an object in the sky.

The large graduated brass circle hung from the supporting beam is the measuring instrument of the Unnatamsa. The brass circle is pivoted to rotate freely around a vertical axis. The ring has two cross beams in the vertical and horizontal directions. A sighting tube is pivoted at the centre of the circle. It can be moved in the vertical direction to align it towards any celestial object. This pivoting of the Unnatamsa is analogous to the Alt-Az mounting of a modern telescope.

The vertical movement of the sighting tube and the horizontal rotation of the brass ring can be adjusted to sight the celestial object and read the altitude against the graduations on the rim of the circle. The rim of the brass circle has graduations marked in such a way that the smallest division is one tenth of a degree. Larger divisions of 1 degree and 6 degrees are also marked on the circle.