Navigation Before Satellites: Celestial and Dead Reckoning

For most of maritime history, navigators lacking any external positioning reference relied on dead reckoning: starting from a known position, a ship's navigator would estimate its new position by tracking heading (via magnetic compass), speed (historically measured with a "chip log," a knotted rope paid out over the stern and timed with a sandglass, giving rise to the nautical unit of speed, the knot), and elapsed time, plotting the estimated course on a chart. Because dead reckoning accumulates error from currents, wind drift, and imprecise speed and heading measurement, navigators supplemented it whenever possible with celestial navigation, determining latitude and, eventually, longitude by measuring the angle between a celestial body — the sun, a star, or the moon — and the visible horizon.