What does alidade mean?
An alidade is a sighting instrument used to measure angles, consisting of a straight bar fitted with vanes or sights at either end and mounted so that it can rotate over a graduated circular scale. In plane-table surveying, the surveyor sights a distant object through the alidade's vanes and draws a line along its edge, allowing positions to be fixed by intersecting such rays from two stations. On historical instruments such as the astrolabe, the alidade served as the rotating index arm used to sight stars and read their altitudes. The word entered English in the early seventeenth century from French, ultimately deriving from an Arabic term meaning 'the little arm' or 'the ruler'. Today the alidade survives mainly in technical surveying contexts and among collectors of antique instruments, though telescopic versions remain in use for precise angular measurement.
nounA sighting device used in surveying and navigation, typically a straight rule with vanes or sights at each end, that rotates over a graduated circle to measure angles or take bearings.
- A pivoting sighting bar with vanes or sights, rotated over a graduated circle, used to measure horizontal angles or bearings in surveying.
- The rotating index arm of an astrolabe or similar astronomical instrument, used to sight stars and read their altitude on the instrument's scale.
"The surveyor aligned the alidade with the distant benchmark and read the angle off the graduated circle."
"Using the plane table's alidade, she drew a ray toward the church spire and plotted its position by intersection."
"On the brass astrolabe, the astronomer turned the alidade until the star's light passed through both pinholes, then read its altitude from the rim."
Standard plural; used mainly when referring to multiple instruments in a collection or inventory.
"The museum's collection includes several nineteenth-century alidades."
The word 'alidade' comes from Arabic — it literally means 'the little arm' — and without this humble sighting bar, mapmakers couldn't have drawn the world.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of alidade
The word 'alidade' was borrowed into English in the early seventeenth century from French, which took it from Arabic. The Arabic source, al-ʿidāda, means 'the little arm' or 'the ruler', formed with the definite article al- attached to a word for a limb or staff. It belongs to the same family of scientific terms that English absorbed from Arabic astronomy, alongside words like azimuth, zenith, and nadir.
How alidade is actually used
Technical vocabulary belonging chiefly to surveying, navigation, and the history of astronomy; rarely encountered outside these fields. In modern usage it most often appears in discussions of plane-table surveying and antique scientific instruments.
Easily confused with alidade
A sextant measures the angular altitude of celestial bodies using mirrors and an arc of sixty degrees, whereas an alidade is a straight sighting bar rotated over a graduated plate to measure horizontal (or vertical) angles.