Cakra-yantra, or circle instrument was an instrument used by Vedic astronomers. 
Lalla described it as a 360° graduated circular board which had a central upright needle. The whole thing was tilted at a certain angle. It indicated nāḍis elapsed since sunrise, or nāḍis to-elapse before sunset. Subbarayappa and Sarma note problems with the calculation method described by Lalla. They also describe a criticism made by Brahmagupta on the matter of calculating nāḍis.