Believed to date from 1495-1500, the artefact was recovered from the wreck of the Esmeralda, which sank in a storm in 1503.

The ship was part of a 20-vessel fleet bound for India under Portuguese explorer Vasco da Gama. Thought to have been commanded by his uncle Vicente Sodre, the vessel was the earliest from Europe’s Age of Discovery ever to be found.

When divers from UK-based Blue Water Recovery recovered the 17.5cm-diameter disc among many other artefacts from the shallow wreck in 2014 they believed that it could be an astrolabe, but were unable to make out any navigational markings on it.

It was engraved with the Portuguese coat of arms and the emblem of King Don Manuel I, who reigned from 1495-1521.

Professor Mark Williams from WMG, University of Warwick, has now applied the latest laser-scanning techniques to establish the object as a late 15th-century astrolabe. His analysis revealed etched marks at 5-degree intervals around the edge of the object, and enabled a high-resolution 3D model to be produced.

The marks would have allowed mariners to calculate latitude by measuring the  angle above the horizon of the sun or a star.

“Usually we are working on engineering-related challenges, so to be able to take our expertise and transfer that to something totally different and so historically significant was a really interesting opportunity,” said Prof Williams.

David Mearns of  Blue Water Recovery, who led the excavation, commented: “It's a great privilege to find something so rare, something so historically important, something that will be studied by the archaeological community and fills in a gap. It was like nothing else we had seen.”

Initial claims by the University of Warwick that the object was the earliest-known astrolabe have been questioned by the Smithsonian National Museum of American History.

“It’s certainly one of the earliest examples of this particular marine navigational tool,” said the museum's Catherine Eagleton. “There’s another one from probably around the same date… what’s interesting about this instrument isn’t only the date it was made, but the underwater archaeological context in which it was found.”

Divernet - The Biggest Online Resource for Scuba Divers