Compass, astrolabe and caravel: finding the way at sea

A sailor in 1480 could work out how far north he was from the height of the Pole Star, but nobody had a reliable way to tell how far east or west a ship had gone.

Three questions out of sight of land

Once the coast drops below the horizon, a navigator needs to answer three questions. Which way is the ship heading? How far north or south is it? How far east or west has it come? In the 15th century the first question had a good answer, the second had a workable one, and the third had none. Sailors guessed their east to west position by dead reckoning: they estimated the ship’s speed, kept track of the time and the compass heading, and marked a best guess on the chart. Currents and leeway, the sideways drift of a ship pushed by the wind, made that guess worse every day. A reliable way to find longitude at sea only came in the 18th century, with accurate marine clocks.

The compass

A magnetised iron needle, left free to turn, settles pointing roughly north and south. The earliest clear description of such a needle comes from China. The scholar and official Shen Kuo described how to make and use one in his Dream Pool Essays, published in 1088, and a few decades later the writer Zhu Yu, whose family lived in the port of Guangzhou, described sea pilots using a needle to steer when they could not see the sun or stars. In Europe the first known mention of a magnetised needle used by sailors is in a book by the English scholar Alexander Neckam, written around 1190. Historians still argue whether Europeans learned of the compass from Asia or worked it out for themselves.

Latitude from the sky

The Pole Star sits almost exactly above the North Pole, so its height above the horizon is roughly equal to the observer’s latitude. At the equator it lies on the horizon; at Lisbon it stands about 38 degrees up. To measure that angle, sailors used a quadrant or a mariner’s astrolabe, a heavy brass ring marked in degrees with a sighting rule that turned on a pin at the centre. The navigator hung it from his thumb so that gravity kept it upright, turned the rule until the star showed through both sights, and read the angle. South of the equator the Pole Star disappears, so Portuguese pilots learned to measure the height of the noon sun instead and correct it with printed tables that gave the sun’s position for each day of the year.

The caravel

The ship that carried the Portuguese down the African coast was the caravel, a small, light vessel of the 15th and 16th centuries. Its hull drew little water, so it could explore shallow coasts and river mouths. Its lateen sails, large triangles hung from long sloping yards, let it sail closer into the wind than the square-rigged ships of northern Europe. That mattered on the way home, when the wind along northwest Africa blew steadily against a ship heading north. Later caravels often mixed square sails on the foremast with lateen sails behind. Two of Columbus’s three ships in 1492, the Niña and the Pinta, were caravels.

Henry the Navigator and Cape Bojador

Prince Henry of Portugal (1394 to 1460), remembered as Henry the Navigator, paid for voyage after voyage down the coast of Africa. For years his captains turned back at Cape Bojador, on the coast of today’s Western Sahara, about 220 kilometres south of the Canary Islands. Shallow water stretched far out from the shore, the waves and currents were violent, and the steady north wind made it hard to come home once a ship had passed. Sailors called the sea beyond it the Green Sea of Darkness.