alidade
noun
al·i·dade ˈa-lə-ˌdād 
: a rule equipped with simple or telescopic sights and used for determination of direction: such as
a
: a part of an astrolabe
b
: a part of a surveying instrument consisting of the telescope and its attachments
Example Sentences
Recent Examples on the Web Lookout Jodi King shows correspondent Conor Knighton the Osborne Fire Finder, an alidade with two sighting apertures used to pinpoint the location of a fire. 
CBS News, 15 Aug. 2021 Dalton showed the couple around the lookout, explaining the old telephone system and the fire finder, a type of alidade used by fire lookouts. BostonGlobe.com, 29 Sep. 2019
CBS News, 15 Aug. 2021 Dalton showed the couple around the lookout, explaining the old telephone system and the fire finder, a type of alidade used by fire lookouts. BostonGlobe.com, 29 Sep. 2019 Word History
Etymology
Middle English allidatha, from Medieval Latin alhidada, from Arabic al-ʽiḍāda the revolving radius of a circle
First Known Use
15th century, in the meaning defined above