Mariner’s astrolabes are considered to be the rarest and most prized of artifacts to be found on ancient shipwrecks and only 104 examples are known to exist in the world.
These instruments are intimately associated with the earliest maritime explorations by the principal seaborne powers of Europe.
They were first used at sea on a Portuguese voyage down the west coast of Africa in 1481. Thereafter, they were relied on for navigation during the most important explorations of the late 15th century, including those led by Bartolomeu Dias, Christopher Columbus and Vasco da Gama.
Their extreme rarity, before divers greatly increased the total number recovered from shipwrecks, was such that each surviving specimen was added to a central register that recorded their vital measurements and characteristics and kept track of their location.
The astrolabe register was started by the UK’s National Maritime Museum at Greenwich by David Waters, the curator of navigation and astronomy, who produced the first published lists in 1957 and in 1966.
The Sodré astrolabe. Image credit: David Mearns.
The Sodré astrolabe was found in 2014 during archaeological excavations at the wreck site of a Portuguese Armada Ship that was part of Vasco da Gama’s second voyage to India in 1502-1503.
The 17.5-cm (7-inch) diameter disk weighing 344 grams was made between 1496 and 1501 and is unique in comparison to all other mariner’s astrolabes.
The artifact is the only solid disk type astrolabe with a verifiable provenance and the only specimen decorated with a national symbol: the royal coat of arms of Portugal.
As the earliest verifiable mariner’s astrolabe it fills a chronological gap in the development of these iconic instruments and is believed to be a transitional instrument between the classic planispheric astrolabe and the open-wheel type astrolabe that came into use sometime before 1517.
The Sodré astrolabe was analyzed by a team from WMG (Warwick Manufacturing Group) at the University of Warwick who traveled to Muscat, Oman in November 2016 to collect laser scans of a selection of the most important artifacts recovered from the wreck site.
Using a portable laser scanner, a 3D virtual model of the astrolabe was created.
An analysis of the results revealed a series of 18 scale marks spaced at uniform intervals along the limb of the disk.
Further analysis showed that the spacing of the scale marks was equivalent to 5-degree intervals. This was critical evidence that allowed the researchers to include the disk in their global inventory as the earliest known mariner’s astrolabe discovered to date.
David L. Mearns et al. An Early Portuguese Mariner’s Astrolabe from the Sodré Wreck-site, Al Hallaniyah, Oman. International Journal of Nautical Archaeology, published online March 16, 2019; doi: 10.1111/1095-9270.12353