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alidad(e|ælɪˈdæd, ˈælɪdeɪd|
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-ﻋiḑādah, the revolving radius of a graduated circle; f. ﻋaḑd, ﻋaḑid, ﻋaḑud, 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.
c1450Insertion in MS. L of Chaucer's Astrol. (Skeat 81), ed. 1561, 164/1 Ley thy rewle of thy astrolabye, that is to sey, the allydatha [ed. 1561 Allidatha], vpon þe day in the Kalendre off the Astrolabye, & he schall shewe the thy degree of the sonne. 1571Digges Geom. Pract. i. xxix, Note bothe what degrees the Alhidada cutteth of the circle, and the perpendiculare of the semicircle. 1611Cotgr., Alidade, Th' Alhidada of an Astrolabe; the rule which turneth on the back thereof. 1679Moxon 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. 1762Parsons in Phil. Trans. LIV. 162 Moved in the limb by a vertical motion in either direction by the alidad alone. 1834Nat. Philos. III. xiii. 66/1 (U.K.S.) Morin went so far as..to attach a telescope to the alhidade of what he calls a planisphere. 1837Whewell Induct. Sc. (1857) I. 178 The alidad of an instrument is its index, which possesses an angular motion. 1878Newcomb Pop. Astron. 579 Alidade, a movable frame carrying the microscopes or verniers of a graduated circle. |