Rare, 16th-century navigational aid confirmed as oldest known example

The astrolabe, a forerunner of the sextant, was found at the site of a Portuguese shipwreck off the coast of Oman

A navigational astrolabe discovered at a Portuguese shipwreck site off Oman has now been confirmed as the oldest in the world, dating from 1496.

A rare navigational instrument found at a 16th-century shipwreck off the coast of Oman has been confirmed as being the oldest known example.

Academics said extensive testing of the astrolabe, a forerunner of the sextant, had dated the object to the late 15th century.

The metal disc, decorated with the coat of arms of Manuel I, King of Portugal, who reigned until 1521, was uncovered by marine archaeologists in 2014.

Divers were searching for the remains of the Esmeralda, a Portuguese ship which was wrecked with great loss of life on Al Hallaniyah island in May 1503.

“Because of its size and shape and other factors, we were fairly certain that this was an astrolabe,” said David Mearns, whose company Blue Water Recoveries discovered the wreck site in 1998.

“We had a good idea of the date of the artefact as we knew the ship departed Lisbon in February 1502, and sank in Oman in May 1503.”

An astrolabe was an instrument used to determine latitude by measuring the angle between the horizon and a known heavenly body, such as the sun.

Mr Mearns discovered the 175millimetre -wide disc buried in the sand on May 8, 2014, more than 500 years since it was last touched by human hand.

Subsequent laser scanning carried out by the UK’s University of Warwick revealed the edge of the disc was etched with fine lines at five-degree intervals.

This “proved beyond doubt that the disk was a mariner’s astrolabe,” said Mr Mearns.

“It has allowed us to confidently place the astrolabe in its correct chronological position.

“But without the laser scanning work we would never have known that the scale marks, which were invisible to the naked eye, existed.”

The story of the astrolabe's last voyage, discovery and identification is told in a paper published this week in The International Journal of Archaeology.

The Esmeralda and the Sao Pedro - another ship lost in the same storm in May 1503 - were part of the fourth Portuguese armada to India, led by the legendary explorer Vasco da Gama.

The commanders of the two ships, Vicente and Bras Sodre, defied da Gama’s orders to protect newly established Portuguese trading outposts along the Indian coast and instead led their five vessels on a brutal campaign of piracy against Arab shipping in the Gulf of Aden.

Their greed would be their undoing, however, when after ignoring advice from local Arab fishermen and seeking shelter from the south‐west monsoon in a bay on the north‐east coast of Al Hallaniyah, the Esmeralda and the Sao Pedro were torn from their moorings by a “sudden and furious wind” that drove them onto the rocky shore.

Between 2013 and 2015, Blue Water Recoveries, working with Oman’s Ministry of Heritage and Culture, mapped and recovered 2,810 artefacts from the wreck site, including ceramics, cannonballs made of stone and iron, gold and silver coins and even ship’s supplies, such as pepper and cloves.

All the objects have been kept by Omani authorities and some pieces, including the Sodre astrolabe, can be seen on display at the National Museum in Muscat.