1) In a → planispheric astrolabe,
the small revolving rod fixed to the center of
the goniometric scale plotted on the → front
or → back of the instrument.
Unlike the → rule,
the alidade has little vanes with holes or slots at each
end, called → pinnules, which are used as sights,
through which the observer can aim at a particular object. An index, often consisting of
the edge of the alidade itself, shows on the goniometric scales the
angle between the line of sight of the targeted object and a
predetermined axis, i.e. the vertical of the observation locality
(online museo galileo, VirtualMuseum).
M.E. allidatha, alhidade, from L. alhidada, from Ar.
Ezâdé, from Ar. al-'izâda.