To maintain an accurate estimate of a vessel’s position, it is necessary to keep track of both its north-to-south position, or latitude, and its east-to-west position, or longitude. A noon observation with a quadrant indicates only a craft’s latitude. If a navigator in 1820 had a chronometer—an exceptionally accurate timepiece adapted to the rigors of being stored on a ship—he could compare the time of his noon sight with the time in Greenwich, England, and calculate his longitude.
