Astrolabe

The astrolabe is an altitude-measuring instrument of medieval Arab and European origin, adapted into the simplified mariner’s astrolabe by the late fifteenth century. A graduated brass ring with a pivoting alidade let a navigator measure the angular height of the sun or a star above the horizon, from which latitude could be derived. Portuguese pilots used it on the African and Indian voyages of the Age of Discovery before the cross-staff and backstaff, and eventually the sextant, superseded it. Surviving sea astrolabes are prized maritime-archaeology finds.