An astrolabe recovered from a sunken Portuguese armada ship has been verified – twice – as the oldest of its type ever discovered.
Astrolabes are instruments that were widely used from the classical period through to the late Middle Ages. They were deployed by astronomers and navigators as a reliable method of calculating position in relation to the sun, other stars, and the horizon.
So useful were they that in a recent interview Loiuse Devoy, curator of England’s Royal Observatory in Greenwich described them as the medieval equivalent of a smartphone.
Given their ubiquity and value it is hardly surprising they were present on ships commanded by the famous Portuguese explorer (or vanguard of European imperialism, take your pick), Vasco da Gama, who died in 1524.
The astrolabe that has been subject to the latest scrutiny is one such, recovered from the sunken wreckage of a ship that was part of da Gama’s second voyage to India, which took place in 1502 and 1503.
The ship, called the Esmeralda, was discovered in waters off the coast of Oman in 2016. It was under the command of da Gama’s cousin, Vincente Sodré, and sank in a storm while Sodré and his colleagues were enjoying a spot of pillaging and freebooting near the Gulf of Aden.
The instrument, known as a solid disk astrolabe, represents an intermediate technology, with a design that sits between “planispheric” models that were used during antiquity and “open-wheel” types that were introduced around 1507.
Its function and provenance were established by a team comprising UK salvage expert David Mearns, and Jason Warnett and Mark Williams from England’s University of Warwick.
Using a portable 7-axis Nikon laser scanner, the researchers established that, when new, the copper alloy disk of the astrolabe was 175 millimetres in diameter and weighed 344 grams. A series of 18 regularly spaced marks running across one section were identified as scale measures, each equivalent to a five-degree interval.
Provenance was easy: the astrolabe is embossed with the Portuguese royal coat of arms.
“Without the laser scanning work … we would never have known that the scale marks, which were invisible to the naked eye, existed,” says Mearns.
“Their analysis proved beyond doubt that the disk was a mariner’s astrolabe. This has allowed us to confidently place the Sodré astrolabe in its correct chronological position and propose it to be an important transitional instrument.”
The research is published in the International Journal of Nautical Archaeology.
And just to be doubly sure of the facts of the matter, researchers contracted by Guinness World Records have also verified the findings. Vasco da Gama’s astrolabe will duly appear in the publishers’ famous annual book.