Thinking some more, it is probably unreasonable to measure angles to sun, moon, horizon accurate to 1 part in 1000 or 10000 from deck of an ancient ship at sea. (Also probably unreasonable to observe moons of Jupiter under same conditions, had they even known about them at the time.) If so, then the antikythera device would not have been a useful ship-navigation device, but might perhaps have been an experimental prototype for one, perhaps even invented by Archimedes. That would explain why only one such device has ever been found and why they clearly were not heavily used. This theory all is a total fantasy, but not necessarily wrong. It would have been a feasible idea, in principle, to do ship navigation using moon+sun predictions vs observations. Later, it actually was feasible, and actually done, to navigate using chronometers and sun observations with sextants, but this was not easy. First, building accurate chronometers was difficult. Second, making accurate sextant observations from a moving ship, is also pretty damn difficult, harder the smaller the boat. Third, it helps to have good charts.