Dhanur-yantra was an instrument used by early Indian astronomers.
It was one of Lalla's 12 instruments of astronomy.
It is essentially half of a cakra-yantra (i.e. it's a half-circle). It can help to measure nāḍis elapsed, and the degrees above the horizon of the Sun.
Ãryabhaṭa says it is used in conjunction with a flat circle on the ground and a gnomon shadow to calculate elapsed ghaṭīs in the day. 
Lalla describes a modification to it, called a Kartarī-yantra.
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