Rare Mariner’s Astrolabe Found in Shipwreck Near Oman

Contrary to some reports, it may not be the earliest-known marine navigational tool—but it’s still a spectacular find

The astrolabe is a rare and highly sophisticated navigational tool. David L. Mearns – Blue Water Recoveries Ltd.

In 2014, a team of archaeologists discovered the sunken remains of a 500-year-old Portuguese ship off the coast of Oman. They pulled thousands of artifacts from the wreckage, including a mysterious round object that appeared to have been stamped with the Portuguese royal coat of arms. Now, with the help of 3-D scanning technology, reports Rebecca Morelle of the BBC, researchers at the University of Warwick in England identified the object as an astrolabe—a rare and highly sophisticated navigational tool.

When marine scientists, led by David L. Mearns and his company Bluewater Discoveries Ltd., discovered the artifact, they suspected that it had been used for navigation. But they couldn’t be sure until scanning analysis and 3-D imaging revealed a series of lines, no longer visible to the naked eye, etched around the object. These lines, each separated by five degrees, revealed that the disc was indeed an astrolabe—or more specifically, a mariner’s astrolabe.

It was found amidst the wreckage of the Esmeralda—an ill-fated ship that was part of a fleet of 20 vessels helmed by Vasco da Gama. In 1498, this much-lauded Portuguese explorer discovered a direct route from Europe to India. At the time, the only other known passage to India was controlled by Arab rulers, explains National Geographic’s Sarah Gibbens.

Between 1502 and 1503 da Gama made a second journey to India, and according to Colin Dwyer of NPR, he left several ships behind to patrol the waters off the coast of Oman. The Esmeralda, which plunged into the Indian Ocean during a violent storm, never made it home. Based on the date of the expedition’s departure and an emblem found stamped on the device, researchers have estimated that the astrolabe dates between 1495 and 1500.

A University of Warwick press release claims the Oman astrolabe is the “earliest known marine navigation tool” yet to be discovered. But as Catherine Eagleton, Associate Director of Curatorial Affairs at Smithsonian's National Museum of American History, cautions: “precision of terminology is critical here.”

First, contrary to some media reports, it is not the oldest astrolabe to be found. Over time, the term “astrolabe” has been used to refer to a number of different instruments, explains Eagleton. The most common was the planispheric astrolabe, which was “effectively a map of the sky showing positions of the sun and stars, used for astronomical calculations as well as observations,” Eagleton tells Smithsonian.com. It is not clear when this technology emerged, but it appears to have been around at least by the Roman era; in the second century A.D., Roman mathematician Claudius Ptolemy wrote about a device that resembles a planispheric astrolabe.

Mariner’s astrolabes came into use much later, during the late 15th century. They were much simpler, Eagleton explains. The device measures the angle above the horizon, or altitude, of the sun or a star. “[T]his is essential in calculating latitude, which is needed when navigating at sea,” she says.

The Oman find is exceptionally old for a mariner's​ astrolabe. “It's certainly one of the earliest examples of this particular marine navigational tool,” Eagleton says. Even so, it's unlikely it's the earliest marine navigation tool. As Eagleton says: “the Greeks and Romans were navigating in the Mediterranean, and people were navigating down the coast of East Africa … 2000 years ago. They must have been doing it with something.”

As an example, Eagleton cites the sounding weight, a bell-shaped piece of lead that was dropped into the sea to determine the depth of the water and pull up sediment from the sea floor. Sounding weights helped sailors ensure that they would not run aground and, based on the samples that were brought up from the bottom of the sea, allowed navigators to determine where they were situated. “[Sounding weights] were used since at least the 6th century BC,” Eagleton said.

Is the Oman artifact the earliest-known mariner’s astrolabe? It might be. “There’s another one [from] probably around the same date,” Eagleton says. “But whether it’s five years one way or the other is difficult to work out. The dating of metal objects is really difficult in general. Especially if they have been under water, they get corroded, and like this one, you can't read the detail very easily.”

But specificities about its date aside, the Oman astrolabe is a fascinating archaeological find. Its discovery confirms historical accounts about the emergence of the mariner’s astrolabe. According to an inventory of known astrolabes published in the Journal of Maritime Archaeology, the Portuguese historian João de Barros references da Gama’s use of a wooden astrolabe during his 1497 expedition to Saint Helena, describing the device as a new technology. The Oman artifact suggests that the explorer and his crew were indeed using astrolabes in the late 15th or early 16th century.

Also significant is the fact that the Oman astrolabe was discovered amidst the wreckage of an identifiable ship, surrounded by other artifacts that were submerged when the vessel went down. “[T]he more of these instruments we have from known contexts like shipwrecks, the better we can understand the practices of navigation in this period when Europeans were exploring the Indian Ocean,” Eagleton says. “What’s interesting about this instrument isn’t only the date it was made, but the underwater archaeological context in which it was found.”

Editor's Note October 26, 2017: Due to an editing error, an earlier version of this article incorrectly stated that Vasco da Gama traveled between Europe and India in 1948; It was in 1498.

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