A replica of a mariner's astrolobe made by the Vancouver Maritime Museum in 1963. The alidade is attached by a screw, with a tiny groove in each end. The rim of the circle is divided into four 90 degree scales, with only one marked at 1 degree intervals.
Invented in the 2nd century BCE in Egypt, astrolobes were refined by Islamic astronomers in the middle ages. A simpler version of the astrolobe was developed for use at sea to measure the height of celestial bodies. They were primarily used in the fifteenth, sixteenth and early seventeenth centuries.