The sea astrolabe was a brass tool constructed and used for the purpose of determining one’s latitude. Most commonly, it was used on long marine voyages that required knowledge of the ship’s exact whereabouts in order to continue in the correct direction. The astrolabe measures the altitude of the sun at noontime or the meridian altitude of a star that has a known declination (or altitude). It could withstand the boat’s rocking in rough seas and intense winds, which a previous model called, simply, “the astrolabe”, was not able to endure. The instrument was beneficial to marine exploration, although it did have some limitations; wind was still an issue and until later improvements were made, the alidade (see photo) was not long enough to provide a high angular accuracy.
The Sea Astrolabe (view 2)