What may be the world’s oldest marine navigation instrument has been found on a wreck. A 500-year-old copper disk, discovered in the remains of a shipwreck in 2014, has been found to be an astrolabe. 3D scanning technology has revealed measurements etched into the disk, confirming that it is indeed an astrolabe – the precursor of the marine sextant.
The team of excavators, led by marine scientist David L Mearns and his company Bluewater Discoveries, removed the astrolabe from sand covering hundreds of other relics on the sea floor off the coast of Oman.
It is thought to be the earliest such find from a period known as the ‘European Age of Exploration’ and was located on the ship ‘Esmeralda’, which belonged to the 20-vessel fleet led by Portuguese explorer Vasco da Gama during his search for a route from Europe to India in 1502-03 AD. Previously, the earliest known example of a mariner’s astrolabe was on a 1554 Spanish shipwreck off south Texas.
The 3D scans revealed 18 hidden lines arranged in 5º increments. The astrolabe could be stabilised vertically, using gravity or the visual horizon, and pinholes in a rotating arm aligned with the Sun, Moon or stars to determine an astro position-line. The simplest check would be a noon latitude check using the Sun. Accuracy is quoted as 5º (300 NM) at sea; on land it was thought to be within 0.5º (30 NM). A marine sextant can be read to 1′ (1 NM) and should yield a position-line to within 5 NM.
Photograph by David Mearns; further reading at the links below . .
