 Alidade

A sighting rule used to measure a direction or angle. On a conventional astrolabe, it rotates over the back's angular scale; it is distinct from the front rule used to read the projected sky geometry.

An alidade is a precision optical instrument used primarily in surveying and navigation to take accurate angular measurements. It typically consists of a sighting device, such as a telescope or a straightedge, mounted on a base or a transit. The instrument allows the user to align the alidade with a distant object, enabling the measurement of the angle relative to a reference line.