When the Apollo astronauts went farther from Earth than any human had ever been (or has been since), they relied on the most advanced computers then available to guide their spacecraft around the moon and back. Of course, the computers have since been surpassed by devices we can carry around in our pockets, let alone mount on a spaceship.
But their backup navigational device has not been improved on since the Apollo missions and would probably be recognizable to its inventors: A sextant. The concept dates back to Isaac Newton, but the format used by ships (until modern navigational systems were developed) dates to the first half of the 18th century.
The theory was that if onboard computers failed because of a power loss or some other malfunction, the astronauts could still navigate because of the sextant's ability to measure angles and provide data to calculate their position. The possibility of Soviet interference with signals from the ground was also considered a danger, and the sextant made sure the astronauts could navigate without constant information from ground control.
The article linked at Astronomy includes a video showing how astronauts used the sextant for space navigation as well.
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