by Dava Sobel, author of Longitude
Skipper Rich will soon cross the Equator, 0º latitude, the great dividing line between Earth’s northern and southern hemispheres. Having traveled from 46º30´N at the start, he will go as far as 56º south, below the tip of Cape Horn. As he circumnavigates the globe, he will also travel through the full 360º of longitude.
Fortunately for Rich, his navigational equipment keeps him constantly informed of his precise position. Earlier sailors had to rely on a mixture of guesswork and hope to do that, and only rarely figured out exactly where they were. They could tell their latitude easily enough by the height of the sun or known guide stars above the horizon – in clear weather, at least. But longitude always posed serious problems. The most popular means, known as dead reckoning, called for a log on a knotted line to be thrown overboard. The navigator, using a sand glass to time how quickly the line paid out, gauged the ship’s speed along its course. Then he factored in the effects of ocean currents and winds on their progress, to estimate a position east or west of homeport.
Not until the end of the 18th century were the necessary instruments – the sextant and the chronometer – invented to determine longitude at sea.