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How did mariners use astrolabes for celestial navigation?

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Astronomical map Star trails over banksia trees, in Gippsland, Vic., Austl. The south celestial pole, located in the constellation Octans, is at the centre of the trails.

Mariners used the astrolabe—specifically the mariner’s astrolabe—as a tool for celestial navigation to determine their latitude at sea.12

While the traditional planispheric astrolabe was a complex instrument used by astronomers to calculate the positions of celestial bodies, the mariner’s version was adapted for use on the unstable deck of a ship. To determine latitude, a navigator used the instrument to measure the elevation angle of a celestial body, such as the polestar or the noonday Sun.12

By measuring the angle between the celestial object and the vertical (the reference direction for the instrument), the navigator could calculate their north-south position. When using the Sun, navigators relied on almanacs to perform the necessary calculations to account for the Sun's declination. Although these instruments were essential for early navigation, they were eventually supplanted by more precise tools like the cross-staff and, later, the sextant.231