If having a different Prime Meridian on every map weren’t enough of a headache, ocean-going navigators had to deal with another problem: clocks that didn’t keep accurate time on the bounding waves. Galileo’s clock was a bust on the ocean because the ship’s movement kept spoiling the swing of the pendulum—so the clock kept stopping. Ferdinand Magellan used sandglasses to keep time and made sure there were 18 of them on each of his ships. Sandglasses were still the most reliable timekeeper aboard a ship. Somebody had to be in charge of turning the sandglasses to keep Prime Meridian time. They probably made the littlest sailor do it—who was also in charge of taking out the trash and pulling weeds.
Why is time so important? Because longitude is time converted into degrees. To know your location you must know 2 times: what time it is where you are and what time it is at the Prime Meridian. If you don’t know both those times, you’re lost.
Back to the beginning of The Western Civ User’s Guide to Time & Space