Originally, the astrolabe was an astronomical instrument, also used for land surveying. An astrolabe designed to instantly calculate the motion of Jupiter and its satellites was invented by Gio Domenico Cassini , born in Perinaldo (Liguria) in 1625 and then called by Colbert to develop and manage the Paris Observatory, where he died in 1712.
The nautical astrolabe, derived from the astronomical one, was mainly used by Spanish and Portuguese navigators. The richest collection - nine specimens of about 80 in the world - is at the Maritime Museum in Lisbon.
The instrument, when designed for marine use, consisted essentially of a graduated ring, the inner part being empty, to avoid it to swing under the action of wind. It was stabilized in the vertical position by a suspended weight.
The observer rotates the alidade until he could sight the star through the viewfinders at its ends. The angle value was read on the graduated border, with an error that could be as high as one degree, corresponding to about 100 km.
Another drawback was that it required three workmen: one kept firmly the instrument, another one was pointing and the third one read the angle.