Alidade
The alidade ( Arabic العضادة, DMG al-ʿiḍāda , the ruler ') is a rotatable measuring device for angle display or angle adjustment on instruments such as astrolabe or sextant , on theodolite and occasionally on dragonflies .
In the simplest case, with the astrolabe, the alidade is a rotatable arm that carries a hole sight at both ends . You hang the astrolabe vertically, aim at a star through the two holes and read the angle of elevation of the star directly on the dial (see picture on the left).
In the marine sextant (picture on the right) the alidade is the rotatable arm on which the index mirror is attached and the angle can be read at the lower end under the index line on the angle scale of the fixed arc. Higher reading accuracy is achieved either by a vernier between the degree arc and alidade or with the drum sextant on the minute drum or its vernier scale.
On the theodolite (picture below) the entire rotatable middle part is called the alidade. It carries the two supports for the telescope, the reading optics for the horizontal circle below, the height circle in the left support , the clamps and fine movements of both circles and the alidad vial for leveling the instrument. The substructure with the three foot screws on which the alidade rotates contains the vertical axis, the horizontal circle, its adjustment drive and the associated lighting mirror.
