Earth's Magnetic Field and the Compass Explained

Almost all of Earth's magnetic field comes from deep inside the planet. Around 1,900 miles beneath your feet lies the outer core, a layer of liquid, extremely hot metal that is mostly iron. This material conducts electricity and is constantly moving, because heat flows outward from the depths. Earth's rotation twists these flows even further.

Moving, conductive metal produces electric currents, and those currents in turn produce a magnetic field. The field strengthens the currents and the currents strengthen the field: scientists call this the geodynamo. Common estimates attribute about 95 percent of the field to this mechanism. The rest comes from sources such as magnetized rocks in the crust and currents in the upper atmosphere.

Compared with a neodymium magnet, Earth's field is very weak. At the equator it measures about 30 microtesla, and near the poles roughly twice that. In central Europe the total strength is around 48 microtesla. For comparison, a small neodymium magnet easily reaches ten thousand times that at its surface.

Weak as it is, the field does a huge job. Far out in space it forms the magnetosphere, a kind of shield that deflects charged particles from the solar wind. Without it, the solar wind could slowly strip away the atmosphere, as probably happened on Mars long ago. The northern and southern lights show you where some of those particles still get through.

The angle between compass north and true north is called magnetic declination, or variation. It depends on where you are and changes over the years. Keep in mind that the needle does not aim straight at the magnetic pole. It follows the direction of the field right where you are standing, which is why declination has to be measured and modeled instead of worked out with simple geometry.

If magnetic north lies east of true north, declination is east (positive); otherwise it is west (negative). In practice:

• Look up the current value: Good topographic maps print the declination in the margin. For an up to date value at your location, try the NOAA declination calculator.
• Do the math: Add east declination to your compass bearing and subtract west declination to get the true bearing.
• Avoid interference: Iron, cars, power lines and magnets nearby will throw off the reading.

Smartphones rely on global field models such as the World Magnetic Model for their compass apps as well. It is updated regularly, most recently in its 2025 edition.

Many animals use Earth's magnetic field to find their way. In 1972, Roswitha and Wolfgang Wiltschko showed with European robins that migrating birds sense the tilt of the field lines. Rather than telling north from south, they distinguish "toward the pole" from "toward the equator."

• Sea turtles read the strength and tilt of the field like a rough map and use it to navigate across entire oceans.
• Salmon and spiny lobsters also orient themselves magnetically.
• Homing pigeons, bees and even mole rats have been shown to respond to magnetic fields.

Exactly how this sense works is not fully understood. In birds, a light dependent process in eye proteins called cryptochromes is considered the leading candidate. Whether humans have a magnetic sense remains an open question.