What the compass offered
For a ship far from land, the sun and stars show direction only in clear weather. A magnetised needle, free to turn, settles roughly along the Earth’s magnetic field and points close to north whatever the weather, and so gave sailors a reference in cloud, fog and darkness. The wind could be traced on a compass card, a course could be held, and the bearing of a landmark could be recorded. It did not by itself tell a navigator where he was; it fed dead reckoning and made charts more useful.
Origins and the road to the sea
The origins of the compass are still debated. Chinese sources describe the directional property of lodestone, a naturally magnetic iron ore, from an early date, and early devices were used for orientation and divination on land. A Chinese text of about the early 12th century is often cited as one of the first to mention a magnetic needle used by sailors, and from that period the instrument seems to have been used on ships in Chinese waters. Whether the technique reached Europe and the Islamic world by transmission or arose independently is unclear.
In Europe, a monk and scholar, Alexander Neckam, mentioned a needle used by sailors in writings from around 1200. Other references appear in the following decades. A popular tradition credits Flavio Gioia of Amalfi with inventing the compass around 1300, but no solid evidence supports a person of that name as the inventor, and modern historians treat it as a legend, though Amalfi was a major maritime city.
From floating needle to pivoted card
The earliest sea compasses are usually described as a magnetised needle thrust through a piece of straw or cork, floating in a bowl of water. The next step was a needle balanced on a pin in a dry box, which was steadier. The needle was soon attached to a circular card marked with the directions of the winds, so that the whole card turned. By the 14th century, this was the typical form of a European mariner’s compass, with the north point commonly marked by a fleur-de-lis. The card, divided into thirty-two points, matched the pattern of lines on the portolan charts.
Ships’ compasses were housed in a box, called a binnacle, which also held a lamp so that the card could be read at night. To keep the card level when the ship rolled, makers mounted it in gimbals, a pair of pivoted rings; the technique appears in descriptions from the 16th century.
Variation
Navigators found that the needle does not point exactly to true north. The angle between magnetic north and true north, called variation or declination, differs from place to place and changes slowly over time. Early compass makers sometimes offset the needle or the card to take account of it for the home region. As Atlantic voyages lengthened, it became clear that the variation changed greatly across the ocean, and traditional accounts of Columbus’s first voyage say that he noticed the needle behaving differently as he crossed. Charts and sailing directions started to carry information on variation, and by the 17th century, observations of it were collected and published.
Better makers, better compasses
Over the 18th and 19th centuries, instrument makers improved the card, the needle and the pivot. A liquid-filled bowl damped the swinging of the card and helped it stay steady in rough seas; a patent for a liquid compass was taken out in 1813 by Francis Crow, and later variations became common, particularly on naval ships. Lighter cards and needles with several parallel bars of steel were tried to reduce friction.
The shift to iron and steel hulls in the 19th century caused a new difficulty. The iron in a ship distorts the magnetic field around the compass, causing an error called deviation that changes with the direction in which the ship is heading. Matthew Flinders, the navigator and surveyor, had examined the effect of iron on compasses in the early 19th century and proposed a vertical bar of soft iron to counter some of it. The Astronomer Royal, George Airy, carried out experiments in the 1830s and 1840s on an iron ship, using small magnets and soft iron masses to correct a compass. Navigators also began to swing the ship, steering it through a series of headings and comparing the compass readings with known bearings, so that a table of deviation could be made.
The physicist William Thomson, later Lord Kelvin, designed a new compass in the 1870s with a light card and a set of small needles, and a binnacle with adjustable iron spheres and magnets to compensate for the ship’s field. It became widely used in the Royal Navy and merchant ships.
The gyrocompass
In the early 20th century, a different principle was introduced. A gyrocompass uses a fast-spinning wheel that, through the Earth’s rotation, settles on true north instead of magnetic north. Hermann Anschütz-Kaempfe in Germany and Elmer Sperry in the United States developed practical versions in the 1900s and 1910s. It was not affected by the iron of a ship in the same way and gave true bearings, and it is now standard on large vessels. The magnetic compass is still carried as a backup, as it needs no power.
A modest instrument with a long history
The compass has none of the glamour of a chronometer or a radar screen, and it is simple in principle. Yet it gave sailors a dependable sense of direction in nearly all conditions for some eight hundred years, and it underlies the lines of portolan charts, the courses of explorers and the dead reckoning of ordinary coastal traders.