Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one step. Facts come from NOAA, BGS and OpenStax; the pictures are not to scale.
Tap the buttons and slide the sliders.
Turn the brass compass case round. Does the needle turn with it?
The dial turns with the case; the magnetised needle, free on its pivot, keeps aligning with the field.
Slide to turn the compass. Does the needle move?
Drag the magnet around the compass. Which end of the needle points at the magnet’s red N end? Watch the iron filings move too.
Near a bar magnet its field swamps Earth’s (~25,000–65,000 nT at the surface). Unlike poles attract; the filings trace the field lines.
Drag the magnet near the compass. The needle turns to face it!
Simplified: the magnet is modelled as two point poles.
Drag the compass around the planet. Does its needle follow the filing lines?
A dipole tilted ~11° from the spin axis. This model needle tilts freely, so it lines up with the full field; a flat pocket compass follows only the horizontal part, which vanishes at the magnetic poles.
Slide the compass around Earth. Watch the needle.
A simplified dipole seen side-on; not to scale.
Drag the compass around the Arctic map. Where is the gap between true north and magnetic north biggest?
Declination = angle between true and magnetic north. It depends on location, changes sharply near the magnetic pole, and changes over time as the field drifts.
Simplified: here the needle aims straight at magnetic north; really it follows the local horizontal field.
Spin the bowl first. Then stroke the needle a few times and spin it again. What changes?
Stroking in one direction lines up the needle’s magnetic regions; floating on water lets it turn freely to lie north–south.
Tap the magnet button three times. Then spin the bowl!
Drag the turtle north and south. How steeply do the field lines dip into the ground?
Idealised dipole: tan I = 2 tan(latitude). Inclination is 0° at the magnetic equator and 90° at the magnetic poles.
Slide the turtle up and down. Watch the needle tip.
Idealised model, not real data.
A compass has a little needle whose red end points roughly north. Sailors use it at sea, even in fog, when they can't see the sun or the land.
The needle is a tiny magnet. It sits on a sharp pin, so it can spin. Magnets have two ends. Different ends pull together, and the same ends push apart.
Deep inside Earth there is hot, runny metal. It swirls and swirls, and that makes Earth a giant magnet. The tiny magnet in the compass lines up with the giant one.
With a grown-up, rub a needle with a magnet, the same way, again and again. Float it on a leaf in a bowl of water. It turns to point north!
Earth's magnet also helps make the northern lights. And find out how GPS works, another way to find your way.
A compass works because its needle is a tiny magnet, and the Earth is a giant one. The needle is a thin, light, permanently magnetised sliver of metal balanced on a sharp pivot, so it can spin almost without friction.8 Earth's magnetic field pushes on it and turns it until it lines up with the field. Over most of the planet, that line runs roughly north to south, so the needle's north-seeking end points (roughly) north.811
That's the whole trick: a compass doesn't "know" where north is. It just lines up with the magnetic field of the planet under your feet.
A compass is a device that shows direction. The best-known kind, the magnetic compass, is a magnetised needle that turns to line up with Earth's magnetic field.10 Once you know which way is north, you can work out every other direction: east is a quarter turn to the right, south is behind you, and west is a quarter turn to the left.11
A typical hiking compass has three parts:
Every magnet has two ends, called its north pole and its south pole. Hang a magnet so it can swing freely and the same end always turns to face north, which is where the names come from.9 Magnets follow one rule: opposite poles attract, and like poles repel.9
That's why a fridge magnet held near a compass makes the needle swing wildly. The nearby magnet is far stronger than Earth's field at that spot, so the needle turns to face it instead. It's also why a compass goes wrong next to iron, steel, speakers and electronics. On ships and planes, where there's metal all around, navigators have to correct for this. The error is called deviation.10
Earth behaves like a huge magnet. Its magnetic field is very similar to the field you'd get from a bar magnet placed at the centre of the planet.111 But there's no solid magnet down there. More than 90% of the field we measure is made inside Earth, in the outer core.1
The outer core is a layer of molten iron and nickel about 2,250 km (1,400 miles) thick. Heat makes it churn, like boiling water in a pan, and Earth's rotation makes it swirl. Moving, electrically conducting metal carries huge electric currents, and electric currents make magnetic fields. Scientists call this self-sustaining engine the geodynamo.76
Now the twist. A compass needle's north end is pulled toward the Arctic. Opposite poles attract, so the magnetic pole in the Arctic must really be a magnetic south pole. It's called the North Magnetic Pole because of where it is, not what kind of pole it is.9
A compass does not point to the North Pole at the top of the globe (true north, or geographic north). Earth's magnetic axis is tilted about 11 degrees from the axis it spins on, so the magnetic poles and the geographic poles are in different places.1
The angle between true north and the direction your compass points is called magnetic declination. Sailors and pilots call it variation.10 It's different from place to place: in some places it's close to zero, in others it's large, and it's measured as east or west of true north.1 To turn a compass reading into a true direction, you add the declination (counting east as positive).2
To be precise, a compass doesn't aim at one point on the map. It lines up with the horizontal part of the magnetic field where you're standing, and that field has bumps and wiggles.2 The map in the "Two different norths" chapter uses a simpler idea, with the needle aiming straight at the magnetic pole, to show why the gap changes as you move. Its numbers are illustrative, not real declinations for any place.
| True (geographic) north | Magnetic north | |
|---|---|---|
| What it is | Where Earth's spin axis meets the surface | Where Earth's magnetic field points straight down |
| Does it move? | No (for navigation) | Yes, it wanders3 |
| What points to it | Maps, GPS, a gyrocompass10 | A magnetic compass (roughly) |
Yes. The explorer James Clark Ross first located the North Magnetic Pole in northern Canada in 1831. Since then it has wandered across the Canadian Arctic toward Russia.3 In the early 2000s surveys measured it moving about 55 km a year; it has slowed a little since (NOAA pole positions).1 Because the field keeps changing, the World Magnetic Model, which planes, ships and phone compass apps rely on, is updated at least every five years.5
Over much longer times, the whole field has flipped many times, so that north and south magnetic poles swapped places. The last reversal was about 780,000 years ago.3
At the magnetic poles, the field is vertical: straight down at the North Magnetic Pole, straight up at the South Magnetic Pole. The horizontal part that turns a compass needle drops to zero, so a compass can't show a direction.1 Even well before you reach the pole, the push on the needle gets so weak that friction in the pivot wins, and the compass starts to behave erratically.8 Close to iron, steel, magnets or electronics, a compass can also be pulled off course.10
No single person. People in ancient China and ancient Greece both noticed magnetism.10 They knew about lodestone, a naturally magnetic rock (a form of the mineral magnetite).11 In China, early magnetic "south-pointing spoons" were used for spiritual purposes, such as arranging buildings by feng shui, before they were used for navigation.10
At first the compass was a backup for days when the Sun, stars and landmarks were hidden. Later it became an essential tool, and it helped ships cross open ocean during the Age of Exploration, which brought trade and new knowledge but also disease, colonisation and enslavement.10
Some very early compasses were just a magnetised needle stuck to a piece of wood or cork, floating in a dish of water.10 You can make one in ten minutes:
Many animals have their own magnetic sense. Magnetic compasses have been found in all major groups of vertebrate animals, and the field is handy because it's there day and night, in any weather.14 Migrating birds use the Sun, the stars and Earth's magnetic field to find their way, and many fly at night.16 (More in how do birds migrate?)
Sea turtles go further. The field lines meet the ground at a different angle, called the inclination or dip, depending on how far north or south you are: flat at the magnetic equator, vertical at the magnetic poles. Sea turtles can sense this angle and the field's strength, which helps them tell whether they are north or south of a place.151 There's strong evidence that turtles and salmon may remember the magnetic "signature" of their home area (a turtle's nesting beach, a salmon's home river) and use it to return years later.15 The turtle toy in the last chapter uses an idealised, perfectly bar-magnet Earth, so its angles are a model, not real measurements.
A GPS receiver works out your position from satellite signals, and for finding your way it has begun to replace the compass. But a magnetic compass needs no battery and no signal, so ships and airplanes still carry one as a backup.810 See how does GPS work? for the other half of modern navigation. Earth's magnetic field also steers the particles that make auroras: what causes the northern lights? And for the magnet side of the story, try how do magnets work?
The needle is a small, light magnet balanced on a pivot so it can spin freely. Earth acts like a giant magnet, and the needle turns until it lines up with Earth's magnetic field, with its north-seeking end pointing roughly north.
Earth's liquid outer core makes a magnetic field that runs roughly north-south. The magnetic pole in the Arctic is actually a magnetic south pole, and because opposite poles attract, it pulls the north end of the needle toward it.
A simple pocket compass is good enough to keep you heading the right way, but it points to magnetic north, not true north. The difference, called declination, can be several degrees or more depending on where you are, and it changes over time. Nearby iron, steel, magnets and electronics can pull the needle off, and close to the magnetic poles compasses become unreliable.
Hold the compass flat in front of you and let the needle settle. The coloured (usually red) end points to magnetic north. Turn your whole body until the red end points straight ahead, then turn the dial so N sits under it. Now you are facing north: east is to your right, west to your left and south behind you.
With a compass, the red or marked end of the needle points north and the other end points south. Without one, an old-fashioned analogue watch and the Sun can give you a rough north-south line. In the Northern Hemisphere, point the hour hand at the Sun; the line halfway between the hour hand and the 12 points roughly south.
No single person. People in ancient China and Greece knew about lodestone, a naturally magnetic rock. The Chinese scholar Shen Kuo (1031 to 1095) wrote the first known description of a magnetic compass needle, and by the 1100s sailors in China and Europe were using compasses to navigate, possibly having discovered them independently.
Not well. A compass follows the local horizontal part of Earth's magnetic field, not a line to the pole, so near the North Pole compass north and true north can be very different. Near the magnetic pole the field points almost straight down, and a normal compass can't show a direction at all.