Alhidade

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The Alhidade or alidade is the part of a theodolite that rotates around the vertical axis, and that bears the horizontal axis around which the telescope (or visor, in early telescope-less instruments) turns up or down.

In a sextant the alhidade is the turnable arm carrying a mirror and an index to a graduated circle in a vertical plane.

Alidade tables have also long been used in fire towers for sighting the bearing to a forest fire. A topographic map of the local area, with a suitable scale, is oriented, centered and permanently mounted on a leveled circular table surrounded by an arc calibrated to true north of the map and graduated in degrees (and fractions) of arc. Two vertical sight apertures are arranged opposite each other and can be rotated along the graduated arc of the horizontal table. To determine a bearing to a suspected fire, the user looks through the two sights and adjusts them until they are aligned with the source of the smoke (or an observed lightning strike to be monitored for smoke). See Osborne Fire Finder.

The word is Arabic (al-idhâdah, "ruler"), where it signifies the same thing. In Greek and Latin, it is respectively called ςδιοπτρα, "dioptra", and linea fiduciæ, "fiducial line".

This article incorporates content from the 1728 Cyclopaedia, a publication in the public domain.