The History of the Magnetic Compass

The compass is a simple instrument with an enormous legacy. Its needle swings to align with Earth's magnetic field, offering travellers a reliable reference direction in fog, at night or far out at sea.

Key takeaways

• Early compasses used lodestone, a naturally magnetised mineral.
• Chinese sources describe magnetic direction-finders from the first millennium, and the compass for navigation appears around the eleventh and twelfth centuries.
• The compass reached the Islamic world and Europe by the twelfth and thirteenth centuries.
• A compass points toward magnetic north, which differs from true north.
• Modern navigation uses GPS but compasses remain useful backups.

Lodestone and early devices

Lodestone is a form of the mineral magnetite that is naturally magnetised. People noticed that a sliver of lodestone floating on water or hanging from a thread aligned itself in a consistent direction. Chinese texts from the Han dynasty onward mention devices using lodestone, sometimes spoon-shaped, and these may have been used for geomancy, a practice of choosing favourable sites for buildings and tombs.

Compasses for navigation

By the eleventh century, Chinese writers such as Shen Kuo described needles magnetised by rubbing with lodestone and suspended or floated to point south. Descriptions of compasses used at sea appear in Chinese texts around the early twelfth century. In Europe, Alexander Neckam wrote about a compass needle used by sailors in the late twelfth century, and Arab and Persian sources describe similar use around the same era. Historians debate the exact paths by which the technology spread.

The mariner's compass

Over time, makers improved the design by mounting a magnetised needle on a pivot above a card marked with directions. Later versions added a gimbal, which keeps the compass level on a moving ship, and a liquid-filled bowl to dampen the movement. These advances made it easier to steer reliably, and the compass played a role in the long-distance voyages of the Age of Exploration.

Why a compass points north

Earth behaves as if it has a giant magnet inside, because of movement in its liquid outer core. The magnetic field lines run from the south magnetic region to the north magnetic region. A compass needle is a small magnet that aligns with these lines, so its marked end points toward the Earth's magnetic north pole.

Magnetic north and true north

Magnetic north is not at the geographic North Pole, and it slowly moves over time. The angle between true north and magnetic north at a given place is called magnetic declination, and it varies with location. Maps often show the local declination so navigators can correct for it. Iron objects, electric currents and magnets near a compass can also cause errors.

Gyrocompasses and electronic compasses

Ships and aircraft often use gyrocompasses, which use a fast-spinning wheel to find true north independent of magnetism. Smartphones contain tiny magnetometers that act as digital compasses, combined with other sensors for orientation.

Using a compass today

Hikers and sailors still carry magnetic compasses because they need no batteries or signal. Learning to use one with a map is a valuable skill. Keep the compass away from metal and magnets, and check the declination for your area.

How a compass needle works

A compass needle is a small, lightweight magnet balanced on a low-friction pivot, so it can swing freely. Earth's magnetic field exerts a gentle twisting force on it until it lines up with the field. Because the field lines tilt into the ground at most places, called magnetic dip, makers weight one end of the needle to keep it level, and they weight it differently depending on whether the compass is made for the northern or southern hemisphere. Many hiking compasses are liquid-filled, which damps the swinging so the needle settles quickly, and have a rotating housing marked in degrees.

Using a compass with a map

To take a bearing from the map, place the long edge of the compass along the line between where you are and where you want to go, with the direction-of-travel arrow pointing toward the destination. Turn the rotating housing until its orienting lines are parallel to the map's north-south grid lines, with the north mark pointing to the top of the map. Adjust for magnetic declination as needed for your area. Then lift the compass, hold it level and turn your body until the red end of the needle sits inside the orienting arrow. The direction-of-travel arrow now points the way to go. Check the route against landmarks as you walk.

How people navigated before compasses

Navigators have long used the sky. Sailors followed the Sun by day and the stars by night, and the position of the Pole Star helps find north in the northern hemisphere. Polynesian wayfinders crossed thousands of kilometres of Pacific Ocean without instruments, using star paths, the direction of ocean swells, the flight of birds and the colour of clouds. These techniques were learned over years and passed from teacher to student, and a revival of traditional voyaging has shown that they work.

A moving magnetic field

Earth's magnetic field is generated by flowing liquid metal in the outer core and is not constant. The north magnetic pole has been drifting across the Arctic, and in recent decades it moved toward Siberia at a rate of tens of kilometres a year, forcing updates to navigation models. Over geological time the field has reversed many times, with north and south magnetic poles swapping places at irregular intervals of hundreds of thousands of years or more. Rocks record these reversals, which is how scientists know about them.

Electronics and interference

Phones contain magnetometers that act as compasses, and they sometimes need calibrating by moving the phone in a figure of eight. Metal objects, speakers, magnets, power lines and car bodies can disturb a compass reading, so step away from them when taking a bearing. Always carry a magnetic compass as a backup in remote areas, because it needs no power and works as long as it is kept away from interference.

Frequently asked questions

Did the Chinese invent the compass?

Evidence suggests that magnetic direction-finding devices were developed in China, though the technology also appeared in other regions and its transmission is debated.

Does a compass work at the poles?

It becomes unreliable near the magnetic poles, where the field points steeply down.

Why does magnetic north move?

Changes in Earth's molten outer core alter the magnetic field over time.

Why does a compass needle dip?

Earth's magnetic field lines point downward in the northern hemisphere and upward in the southern hemisphere, except near the equator. A needle that is free to tilt would dip, so manufacturers balance compasses for the region where they will be used.

Can a compass work underwater or in space?

A magnetic compass works underwater because water does not block magnetic fields. In space, away from planets, there is no strong, steady field to align with, so a magnetic compass would be useless there.

How do I calibrate my phone compass?

Open a compass or maps app and move the phone in a figure-of-eight pattern a few times, as prompted. Stay away from metal objects and magnets, and remove magnetic cases or mounts while calibrating.

Sources and further reading

• Compass (Wikipedia)
• NOAA National Centers for Environmental Information: Geomagnetism