From The Telegraph:

…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.”