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Hands holding a magnetic compass.

Compasses

by Chris Woodford. Last updated: September 7, 2013.

When Bob Dylan sang "like a rolling stone... with no direction home," he obviously wasn't carrying his compass. Armed with a simple bit of magnetized metal, you can almost always find your "direction home" in an instant. People have been navigating with magnetic compasses for the best part of 900 years, so there must be something in it! What are compasses and how do they work? And what about the compasses people use in ships and airplanes where Earth's magnetism isn't always a reliable method of navigation? Let's take a closer look!

Photo: A magnetic compass points north because it aligns itself with the magnetic field produced inside Earth. Photo by Staff Sgt. Jacob N. Bailey courtesy of US Air Force.

What is a compass?

Magnetic compass used by astronauts in the 1960s.

The simplest compass is a magnetized metal needle mounted in such a way that it can spin freely. (You can make one yourself by magnetizing an ordinary needle, placing it carefully on a slice of cork, and letting the cork float in a tray of water.) Left to its own devices, the needle turns until one end points north and the other south. You can usually figure out which end is which from the position of the Sun in the sky, remembering that the Sun rises in the east and sets in the west. So if you're looking down on the floating needle at about noon, with the eye on the left and the point on the right, and the Sun in front of you, you know the point is indicating north.

Photo: In the 1960s, astronauts were equipped with little compasses like this as part of their survival gear so they'd know where they were when they came back to Earth. Photo courtesy of NASA Johnson Space Center (NASA-JSC).

How do you use a compass?

Using a magnetic compass.

Compasses you buy are a bit more sophisticated than floating needles but work essentially the same way. They have a lightweight, magnetized pointer mounted on a very low-friction pivot that is sealed inside a small plastic cylinder filled with liquid. The pointer is built into a rectangle of plastic called a compass card, printed with the cardinal points of the compass (north, south, east, and west), and the intercardinal points (north-east, north-west, south-east, south-west).

To use a compass like this, you first figure out which direction is north. You let the needle settle then rotate the compass card so the needle lines up with the north-south axis and the end of the needle colored red, marked with an arrow, or printed 'N' points north. You can then instantly see which direction is south, east, or west and (with the help of a map) set off in the direction you need to go.

Photo: Magnetic compasses are really easy to use, but it helps if you have a map as well so you know which direction is best to head for. Photo by Dominique M. Lasco courtesy of US Navy.

How do compasses work?

Why does a compass point north? Because Earth's north magnetic pole is a south-seeking pole that attracts the compass's north pole

Magnetism is one of the first bits of science we learn in school and just about the first thing we discover is that "like poles repel, unlike poles attract." In other words, if you hold two bar magnets so their north poles are almost touching, they'll push away from one another; if you turn one of the magnets around so one magnet's north pole is near the other magnet's south pole, the magnets will pull toward one another.

That's all there is to a compass: the red pointer in a compass (or the magnetized needle on your home-made compass) is a magnet and it's being attracted by Earth's own magnetism (sometimes called the geomagnetic field—"geo" simply means Earth). As English scientist William Gilbert explained about 400 years ago, Earth behaves like a giant bar magnet with one pole up in the Arctic (near the north pole) and another pole down in Antarctica (near the south pole). Now if the needle in your compass is pointing north, that means it's being attracted (pulled toward) something near Earth's north pole. Since unlike poles attract, the thing your compass is being attracted to must be a magnetic south pole. In other words, the thing we call Earth's magnetic north pole is actually the south pole of the magnet inside Earth. That's quite a confusing idea, but it'll make sense if you always remember that unlike poles attract.

Earth's magnetic field is actually quite weak compared to the "macho" forces like gravity and friction that really dominate our lives. For a compass to be able to show up the relatively tiny effects of Earth's magnetism, we have to minimize the effects of these other forces. That's why compass needles are lightweight (so gravity has less effect on them) and mounted on frictionless bearings (so there's less frictional resistance for the magnetic force to overcome).

Artwork: Earth behaves as though it has a giant bar magnet built inside it. But the magnet is the opposite way around to how you might think, with its south pole up near Earth's actual (geographic) north pole and vice-versa. A compass needle points north because the north pole of the magnet inside it is attracted to the south pole of Earth's built-in magnet. Confusing, eh? Also note that the magnetic north pole and the real north pole don't exactly coincide.

Why compasses can be inaccurate

Compasses are brilliantly useful but they can sometimes lead us astray, because of two quite different problems called declination (or variation) and deviation. Here's why.

Declination (variation)

Earth spins about an axis (a kind of invisible rod) running through the north pole (sometimes called the geographic north pole, at the "top" of the planet) and the south pole (or geographic south pole, at the "bottom" of the planet). But Earth's magnetic field is a bit wonky and doesn't quite line up with its axis of rotation. So the magnetic north pole (the place your compass points toward) doesn't precisely coincide with the real north pole (it's several hundred km/miles) away and the same goes for the magnetic south pole.

In practice, the difference between "true north" and "magnetic north" is small and generally (when you're out and about with a compass and map) you can treat the north a compass shows you as though it were pointing to the real, geographic north pole. If we're being more accurate, the difference between "magnetic north" and "true north" is an angle that varies slightly from place to place (and from year to year, because the position of Earth's magnetic north is constantly changing) and it's called the declination or variation. When really accurate navigation is important (for example, on ships), you have to take the declination into account and correct for it.

Deviation

A compass is designed to react to the magnetic field generated by the swirling hot mass of rock thousands of kilometers/miles deep inside Earth, but there are lots of other things going on, much nearer to your compass, that can throw it well and truly out of whack. If you're inside an iron ship or a car, for example, all that metal can make a big difference. The accuracy of a compass measurement in a certain situation is called the deviation, and it's the angle between where the compass would point if it were perfectly accurate (magnetic north) and where it actually points. If there's a magnet nearby, or you're near a particularly magnetic bit of Earth's crust, or there are fluctuating electric currents generating magnetic fields, your compass needle is going to be affected and its accuracy is going to be reduced. The most sophisticated compasses have compensating magnets or pieces of iron built into them that you can adjust to cancel out any local magnetic effects.

Ship's compasses

Declination and deviation don't matter so much if you're on foot with a map or in a car; generally, there are other things you can use to help you find your way and it's hardly catastrophic if you take a wrong turn or two. On a ship, far from land and in bad weather (so you can't navigate by the sky), it's a whole different matter. Before technological advances like GPS and radar came along, people's lives depended on navigating accurately by compass alone. That's why ship's compasses (sometimes called mariner's compasses) were much more sophisticated than the ones people typically used on land. In a modern ship's compass, the compass card is attached to a float with a number of magnetic needles underneath it and spins freely inside a large glass bowl filled with a mixture of alcohol and water (to minimize friction and absorb vibrations from the moving ship). The whole thing is mounted on gimbals (pivots) in a stand called a binnacle so it stays horizontal even when the ship is pitching (moving up and down) and rolling (rocking from side to side) in the waves.

Other kinds of compasses

Gyrocompass and other navigational systems on a truck.

Gyrocompasses

If magnetic compasses can be tricky to use in ships, imagine how much worse they are in fast-moving aircraft. That's why airplanes (plus large ships and some land vehicles) rely on gyrocompasses. Unlike a magnetic compass, which points the same way because of magnetic attraction, a gyrocompass uses a gyroscope—a fast-spinning wheel, mounted on gimbals, that keeps spinning in the same direction whichever way you turn it. Gyrocompasses are better able to cope with the more "dynamic environment" onboard ships and planes and another advantage is that they can be set to indicate true north (the north pole) rather than magnetic north.

Photo: Gyrocompass and navigational equipment on a truck. Photo courtesy of US Geological Survey.

Astrocompasses

While magnetic compasses and gyrocompasses are set according to the Earth, astrocompasses are aligned with the position of celestial bodies (fixed points in the sky, such as the Sun or stars) and then indicate the position of true north. They're more complex and harder to use than magnetic compasses, but offer a good alternative in places like the polar regions where magnetic compasses and gyrocompasses are unreliable.

Radio compasses

Also called radio direction finders (RDF), these pick up directional signals beamed out from radio transmitters. Automatic direction finders (ADF) on modern aircraft are radio compasses that automatically figure out and display directions using a pointer and dial similar to a traditional, magnetic compass.

Who invented the compass?

No-one knows when or where compasses were invented, but this is what we do know:

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Text copyright © Chris Woodford 2010, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2010) Compasses. Retrieved from http://www.explainthatstuff.com/how-compasses-work.html. [Accessed (Insert date here)]

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