Also 4 allidatha, 6–7 alhidada, 7–9 alhidad(e, 8– alidad(e. [In mod. form, a. Fr. alidade, in earlier, a. med.L. alhidada (cf. Sp. alhidada, alidada), ad. Arab. al-siḑādah, the revolving radius of a graduated circle; f. saḑd, saḑid, saḑud, the humerus or upper arm (which revolves in its socket).]
The index of an astrolabe, quadrant, or other graduated instrument, carrying the sights or telescope, and showing the degrees cut off on the arc of the instrument. In the astrolabe it revolved at the back, and was called by Chaucer the Rule.
c. 1450. Insertion in MS. L of Chaucer’s Astrolabe (Skeat 81), ed. 1561, 164/1. Ley thy rewle of thy astrolabye, that is to sey, the allydatha [ed. 1561 Allidatha], ypon þe day in the Kalendre of the Astrolabye, & he schall shewe the thy degree of the sonne.
1571. Digges, Geom. Pract., I. xxix. Note bothe what degrees the Alhidada cutteth of the circle, and the perpendiculare of the semicircle.
1611. Cotgr., Alidade, Th’ Alhidada of an Astrolabe; the rule which turneth on the back thereof.
1679. Moxon, Math. Dict., 5. Alhadida … a word seldom used by English authors … signifies only the Label or Index that moves upon the centre-pin of an Astrolabe.
1762. Parsons, in Phil. Trans., LIV. 162. Moved in the limb by a vertical motion in either direction by the alidad alone.
1834. U. K. S., Nat. Philos., III. xiii. 66/1. Morin went so far as … to attach a telescope to the alhidade of what he calls a planisphere.
1837. Whewell, Induct. Sc. (1857), I. 178. The alidad of an instrument is its index, which possesses an angular motion.
1878. Newcomb, Pop. Astron., 579. Alidade, a movable frame carrying the microscopes or verniers of a graduated circle.
