Dead Reckoning

Captains of the 15th, 16th and 17th centuries used to determine their position at sea (distance east or west of home port) by this method. The captain would throw a log overboard and observe how quickly the ship receded from the temporary guidepost. He noted the crude speeedometer reading in his ship's logbook, along with the direction of travel, which he took from the stars or a compass, and the length of time on a particular course, counted with a sandglass or a pocket watch. Factoring in the effects of ocean currents, fickel winds and errors in judgment he then determined his longitdue. Too often, the technique of dead reckoning marked him for a dead man.

Before the invention of the ship's clock by John Harrison in 18th century, ship's captains really didn't know where they were longitudally . . . . this is interesting when you think about Magellan circumnavigating the globe.