... This was the first great age of global navigation, after all. Inspired by Columbus, ships were sailing to the Far East and the newly discovered Americas, with vast fortunes awaiting those who navigated the oceans successfully. (And almost certain death awaiting those who got lost.) But sailors lacked any way to determine longitude at sea. Latitude you could gauge just by looking up at the sky. But before modern navigation technology, the only way to figure out a ship’s longitude involved two clocks. One clock was set to the exact time of your origin point (assuming you knew the longitude of that location). The other clock recorded the current time at your location at sea. The difference between the two times told you your longitudinal position: every four minutes of difference translated to one degree of longitude, or sixty-eight miles at the equator.
In clear weather, you could easily reset the ship clock through accurate readings of the sun’s position. The problem was the home-port clock. With timekeeping technology losing or gaining up to twenty minutes a day, it was practically useless on day two of the journey.