US Navy returns to navigating by stars amid hacking fears
It was how Odysseus sailed the seas, how Columbus reached the Americas, and how Lawrence of Arabia found his way across the vast, featureless deserts of the Middle East.
For millennia, travellers used the stars to guide them on their journeys - a technique which, in recent decades, has been replaced by modern technology.
But now the US Navy is reinstating classes on celestial navigation for all new recruits, teaching the use of sextants - instruments made of mirrors used to calculate angles and plot directions - because of rising concerns that computers used to chart courses could be hacked or malfunction.
"We went away from celestial navigation because computers are great," said Lt Cmdr Ryan Rogers, the deputy chairman of the Naval Academy's Department of Seamanship and Navigation. "The problem is there's no backup."
The era of celestial navigation ended with the launch of satellites in the 1990s, which evolved into the Global Positioning System (GPS). While celestial navigation can calculate your position within 1.5 miles, by 1995 GPS could pinpoint your location within feet, and the system has never been shut down.
Today, 31 satellites circle the Earth, each twice a day, costing American taxpayers about $1bn (€874m) a year.
"The perceived need for sextants was taken away," said Peter Trogdon, president of nautical instrument company Weems & Plath in Eastport, Maryland.
Mr Trogdon, said sales of sextants plunged after the arrival of GPS. "There's only a few thousand sold a year," he said. "Most of those are sold to yachtsmen who want to have a backup."
"If you can use GPS, it's just so much more accurate," said Lt Cmdr Ryan Rogers. But, he added, "we know there are cyber vulnerabilities."
Recruits to the academy in Annapolis, Maryland, have this autumn seen study of the stars return to their curriculum for the first time since it was dropped in 2006. It was reinstated for navigators in 2011, but not for the whole navy.