Astronomical Sextant

Qing Dynasty, 1673
Ancient Observatory, Beijing

The astronomical sextant uses line-of-sight to measure the angular distance of an object above the horizon, or between two objects, or the angular diameter of an extended object such as the moon. A geared wheel is used to tilt the instrument around its sighting axis. See detailed photos on Chinese Astronomy, and compare: the terrestrial sextant on PBS.