The mariner's astrolabe and Portuguese exploration

Image 1. Although the origins of the mariner’s astrolabe are obscure, its derivation is suggested by this unpreposessing but historically important instrument, the so-called Sodré astrolabe. The oldest mariner’s astrolabe yet discovered, it was found in 2014 on the wreck of the Portuguese ship Esmeralda, which sank off the coast of Oman in 1503, and has subsequently been dated to between 1496 and 1501. Its design marks a transition between the classic plane astrolabes and the much heavier open-wheel mariner’s astrolabes cast in brass about two decades later.

Image 2. The development of the mariner’s astrolabe was clearly stimulated by the Portuguese explorations of the Atlantic coast of Africa. The fact that these voyages were primarily in a north-south direction placed utmost importance on the capacity to keep track of the latitude of newly discovered islands and coastal locations.

Image 3. As the Portuguese navigators headed south, they could determine latitude by observing Pole Star, which coincides very closely with the celestial north pole. After they crossed the equator in 1473-4, however, the North Star disappeared from the sky. Since the southern hemisphere did not have a pole star, their best alternative was to observe the altitude of the sun and then to infer the latitude from declination tables. Nautical handbooks containing such tables and instructions for their use were known as regimentos. The earliest surviving regimento, known as the Regimento do astrolabio e do quadrante, was printed in Lisbon c.1509, although earlier books of this type were probably already in use in the 1480s.

Credits. Philipp Norhaft (May 2019) and Howard Hotson (May 2020)