Compasses
Last updated: January 28, 2010.
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?
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 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, the
needle is aligned parallel to your body, going from your left to your
right, and the Sun is somewhere on your right, you know that the end
of the needle to your left is 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?
Compasses you buy are a bit more sophisticated than this 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?
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."
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). The needle in your compass is pointing north because
like poles attract: the needle is attracted to ("seeking out")
Earth's magnetic north pole. Since like poles attract, Earth's
magnetic north pole is actually the south pole of the magnet
inside Earth.
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).
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
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:
- ~300-200BCE: Primitive magnetic direction finders are believed to have been invented in China.
- 12th century CE: More sophisticated compasses are invented independently in China, the Arabic world, and Europe and feature compass needles mounted on pins for the first time.
- 13th century: Compasses incorporate compass cards marked with the now-familiar cardinal points and subdivisions.
- 15th century: Navigators realize that compasses point to Earth's magnetic north pole rather than its true (geographical) north pole.
- 16th century: Marine compasses are mounted in gimbals to reduce problems caused by the motion of ships.
- 17th century: Englishman William Gilbert publishes a comprehensive scientific account of Earth's magnetism and uses it to explain why compasses point north.
- 1880s: Scottish physicist William Thompson (Lord Kelvin) develops compasses that can be adjusted to work inside iron-hulled ships.
- 1880s: Dutchman Marinus Gerardus van den Bos patents a gyrocompass. Others develop and refine the invention over the next few decades.
- 1900s: Radio direction finding (RDF) is developed by Italian engineers Ettore Bellini and Alessandro Tosi.
Further reading
