Azimuth Theodolite

Qing Dynasty, 1715
Ancient Observatory, Beijing

This instrument is used to find the altitude (elevation) and azimuth (compass direction) of an object in the sky, that is, the object's alt-azimuth coordinates. It operates on line-of-sight; first you line up the pointer on the object, then you read off the azimuth on the horizontal circle and the altitude on the vertical arc.