One of the mysteries of the English language finally explained.
A sighting device or pointer for determining directions or measuring angles, used in surveying and (formerly) astronomy.
- ‘In the circle an index on the alidades gives the angle that they form with the vertical diameter.’
- ‘A wide variety of alidades and plane tables are available, both in manual and electronic versions.’
- ‘Both the alidades and the straight edges of the quadrant are fitted with viewers.’
- ‘A pointer, called an alidade, was pivoted at the centre of the disk.’
- ‘Telescopic alidades came into use in Europe in the early 1800s, and were soon introduced to American practice.’
Late Middle English: directly or (in modern use) via French and Spanish from Arabic al-‘iḍāda ‘the revolving radius’, probably based on aḍud ‘upper arm’.
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