The name ALIDADA comes from Arabic.
Before Arab astronomy spread to late medieval Europe, the alidada was called DIOTTRA (from the Greek “dia” = “through” and “opteyo” = “to observe”, from which dioptra - instrument through which something is observed)
The term ALIDADA originally designated a revolving small rod with levels, centered in a goniometric scale drawn on the front or back flat face of many ancient astronomical and topographical instruments: dials, flat astrolabes, graphometers, and so on.
In THE ALIDADA there are two pinnules (or levels or sights), fixed perpendicularly on the circle of the alidada itself, which allows to aim at or watch an object that is a reference point on the coast, (e.g. a bell tower) through the holes made in them.
The ocular pinnula has a narrow slit to put the eye on and through which the observation is made
A PRISMA allows checking the value of the reading on the compass.
An index, often consisting of the very edge of the alidada, shows on a goniometric scale (azimutal circle), the angle (azimut) between the line of sight of the targeted object and a pre-fixed direction.
Some pieces of the compass are missing? Maybe.
Body to be repainted.
- Brass, Wood
- Manufacturer/ Brand
- U.S. Navy
- Estimated Period
- Country of Origin
- Fair condition - heavily used & with possibly minor parts missing
- 15×35×37 cm
- 8 kg