Mariner's astrolabe

technology

A stripped-down instrument for one job: measuring the altitude of the sun or a star from the deck of a moving ship. Unlike the elaborate planispheric astrolabe of Islamic and medieval European astronomy, which is a computing device carrying star maps and multiple plates, the mariner's version is a heavy cast-metal ring with a graduated scale and a pivoted sighting rule, deliberately pierced and weighted so that wind cannot swing it and it hangs vertical. Accuracy was poor by later standards — often several degrees in a seaway, which is tens of nautical miles of latitude — and navigators preferred to take sights ashore when they could. It competed with the quadrant and, later, the cross-staff and back-staff, each trading precision against usability in a swell. Very few genuine examples survive, and most known specimens come from wreck excavations rather than from collections, which is itself a comment on the risk of the trade.

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