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For millennia, travellers used the stars to guide them - a technique which, in recent decades, has been replaced by modern technology.

Now the United States 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 Lieutenant Commander 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 3km, by 1995 GPS could pinpoint your location within metres, and the system has never been shut down.

Today, 31 satellites circle the Earth, each twice a day, costing American taxpayers about US$1 billion a year.

"The perceived need for sextants was taken away," said Peter Trogdon, president of nautical instrument company Weems & Plath in Eastport, Maryland.

He 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 Rogers. But, he added, "we know there are cyber vulnerabilities."

Study of the stars was dropped in 2006. It was reinstated for navigators in 2011, but it is only now being taught to all other recruits.