Earth's magnetic field
The magnetic pole near the geographic North Pole is, in magnet terms, a south pole; that is why a compass's north end points to it.
Earth behaves like a bar magnet
A compass works because Earth has its own magnetic field. NOAA describes it as very similar to the field a bar magnet would make if it sat at the centre of the planet. More than 90 percent of the field we measure is produced inside Earth, in the outer core, where molten iron flows and carries electric currents. The rest comes from magnetised rocks in the crust and from currents high in the atmosphere and in space.
Here is a naming trap. A compass needle's north pole is attracted to the magnetic pole in the Arctic. Unlike poles attract, so that Arctic magnetic pole must be, in magnet terms, a south pole. OpenStax calls the usual name, north magnetic pole, a misnomer. Geographers keep the name for the place; physicists remember that Earth's magnet has its south-type end in the north. Field lines leave Earth near the geographic South Pole and enter near the geographic North Pole.
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Tilted, and not quite at the poles
The magnetic axis does not line up with the axis Earth spins on. Using the World Magnetic Model for 2020, NOAA gives the best-fitting magnet, the geomagnetic dipole, a tilt of 9.41° from the rotation axis. The 2025 model puts it at 9.21°. The real field is lumpier than a perfect dipole, so scientists also mark the dip poles, the places where the field is vertical and a compass cannot settle on any direction. These are the magnetic poles in the everyday sense.
Because magnetic north and true north differ, a compass bearing needs a correction called declination: the angle between magnetic north and true north, counted positive to the east and negative to the west. NOAA's rule is true bearing = magnetic bearing + declination. If the declination where you stand is 10° west (−10°) and your compass reads 45°, the true bearing is 35°. Declination changes from place to place and year to year, which is why maps print the date it was measured. Near the magnetic poles NOAA marks blackout zones, where the horizontal field is below 2,000 nT and compasses cannot be trusted.
Poles on the move
The field changes because the flow in the core changes. The north dip pole has been drifting across the Canadian Arctic toward Siberia. Surveys fixed its position in 2001 and 2007, and the 2007 survey put its speed at about 55 km per year, heading north-northwest. NOAA's December 2025 report on the field measured about 36 km per year for the north dip pole and 9 km per year for the south dip pole during 2025. Because the field keeps changing, NOAA and the British Geological Survey publish a new World Magnetic Model every five years; the 2025 model was released in December 2024. Navies, aviation agencies and many navigation systems use it to turn magnetic readings into true directions.
Over much longer times the field even flips: north and south swap. Rocks record this, because lava that cools below about 700 °C locks in the direction of the field at the time. NOAA notes that the last reversal was about 750,000 to 780,000 years ago, that reversals have been spaced anywhere from 5 thousand to 50 million years apart, and that a reversal would take several thousand years. Nobody can say when the next one will come.
| Source of Earth's field | Mostly the liquid iron outer core (over 90%) |
| Arctic magnetic pole, in magnet terms | A south pole |
| Declination | Angle between magnetic north and true north (east +, west −) |
| True bearing | Magnetic bearing + declination |
| Dip pole | Where the field is vertical |
| Last field reversal | About 750,000 to 780,000 years ago |
- Earth's field is like a bar magnet's, made mostly in the outer core.
- The magnetic pole in the Arctic acts as a south pole, so it attracts a compass's north end.
- The dipole is tilted 9.41° (2020 model); true bearing = magnetic bearing + declination.
- The north dip pole moves tens of kilometres a year; models are renewed every five years.
- The field has reversed many times; the last time about 750,000 to 780,000 years ago.
- In magnet terms, the magnetic pole near the geographic North Pole is:
- A south pole
- A north pole
- Not a pole at all
- Both at once
- Where is most of Earth's magnetic field produced?
- In the liquid outer core
- In the Moon
- In the oceans
- In the ozone layer
- Magnetic bearing 45°, declination 10° west. True bearing in degrees?
- Magnetic bearing 120°, declination 8° east. True bearing in degrees?
- By how many degrees was the geomagnetic dipole tilted from the spin axis in the 2020 model?
- At 36 km per year, how far does the north dip pole move in 10 years, in km?
- How often is a new World Magnetic Model released?
- Every five years
- Every month
- Every hundred years
- Only once
- What is a dip pole?
- A place where the field is vertical
- The spin axis
- Any place on the equator
- A rock that is magnetised
- When did Earth's field last reverse?
- About 750,000 to 780,000 years ago
- About 1600
- About 10,000 years ago
- It has never reversed
- Why are compasses unreliable very close to the magnetic poles?
- The horizontal part of the field is very weak there
- The field is zero everywhere there
- It is too cold for magnets
- The poles repel compasses
Sources for this course (16)
- OpenStax College Physics 2e (openstax.org/books/college-physics-2e): 22.1 Magnets (all magnets have N and S poles, north-seeking and south-seeking, like poles repel and unlike attract, poles cannot be separated, the magnetic pole near the geographic North Pole is a south magnetic pole, sunspots come in N/S pairs)
- 22.2 Ferromagnets and electromagnets (iron, cobalt, nickel, gadolinium; neodymium alloys; domains grow to millimetre size; Curie temperature of iron 1043 K (770 °C); Oersted saw a compass needle deflected by a current; electromagnets in scrapyard cranes, particle accelerators and MRI; electric current is the source of all magnetism; no monopoles observed)
- 22.3 Magnetic fields and field lines (direction = where a compass north end points; four rules: tangent, density = strength, never cross, closed loops from N to S outside)
- 22.7 Magnetic force on a current-carrying conductor (F = I l B sin θ, 1 T = 1 N/(A·m), example 20.0 A, 5.00 cm, 1.50 T gives 1.50 N; motors)
- 22.9 Magnetic fields produced by currents (Oersted 1820, right hand rule 2, B = μ0 I ÷ (2π r), μ0 = 4π × 10⁻⁷ T·m/A, Earth's field about 5.0 × 10⁻⁵ T, 25 A at 5.0 cm gives twice that; loop centre B = N μ0 I ÷ 2R; solenoid B = μ0 n I, 2000 loops over 2.00 m with 1600 A gives 2.01 T, MRI; ferromagnetic cores)
- 23.1 Induced emf and magnetic flux (Faraday's iron-ring apparatus, emf only while the field changes, magnet pushed into a coil, faster motion gives more emf, generators, Φ = B A cos θ)
- NIST CODATA 2022 vacuum magnetic permeability μ0 = 1.25663706127(20) × 10⁻⁶ N/A² (physics.nist.gov/cgi-bin/cuu/Value?mu0)
- NOAA NCEI Geomagnetism Frequently Asked Questions (ncei.noaa.gov/products/geomagnetism-frequently-asked-questions): field like a dipole at Earth's centre, over 90% from the outer core, intensity roughly 25,000 to 65,000 nT (0.25 to 0.65 gauss)
- 1 gauss = 100,000 nT, declination = angle between magnetic north and true north (east positive)
- inclination, dip poles, dipole axis 9.41° from the rotation axis (WMM2020, 2020.0)
- north magnetic pole moving about 55 km per year north-northwest (2001 and 2007 surveys)
- last reversal about 750,000 to 780,000 years ago, reversals spaced 5 thousand to 50 million years, a reversal would take several thousand years, models renewed every five years, blackout zones where H < 2000 nT, compass balance weights differ by hemisphere, true bearing = magnetic bearing + declination
- NOAA NCEI news "World Magnetic Model 2025 Released" (17 December 2024)
- NOAA NCEI "December 2025 State of the Geomagnetic Field" (north dip pole 36 km/yr and south dip pole 9 km/yr since 1 January 2025)
- USGS, "Pre-gilbertian conceptions of terrestrial magnetism" (pubs.usgs.gov/publication/70009843): Gilbert's De Magnete (1600) first suggested that Earth behaves as a great magnet. Royal Institution, Michael Faraday's ring-coil apparatus (rigb.org): made August 1831, success on 29 August 1831, the first electric transformer
- APS News "This Month in Physics History" (Aug 2001): Faraday's electromagnetic rotation experiments of September 1821, the principle behind the electric motor.
Checked: 2026-10-17