How Does a Compass Work? Magnetic North Is a South Pole

Hold a compass flat and turn slowly on the spot. The case turns with you, but the needle swings back to the same line every time and settles with its north end pointing roughly north.

That end is the north pole of a small magnet, named for the direction it seeks, and unlike magnetic poles attract. So the end of Earth’s field that the needle’s north end lines up toward has to be magnetically south, and the magnetic pole in the Arctic is exactly that.

How does a compass work?

A compass works because its needle is a small magnet that can turn freely. Earth’s magnetic field, generated mostly by flowing liquid iron in the planet’s outer core, exerts a torque that swings the needle into line with the horizontal direction of the field where it sits. Its north end then shows magnetic north, which usually differs from true north.

The basic requirement, in NOAA’s geomagnetism FAQ, is that “the compass needle must be free to rotate and align with the magnetic field.” Most modern compasses also seal the needle in a liquid-filled capsule, which damps its movement so it settles sooner.

The pole in the Arctic is magnetically a south pole

Magnet poles were named for what they do. Hang a magnet freely and one end turns north, so the two ends became the north and south poles, “or more properly, north-seeking and south-seeking poles,” as the OpenStax College Physics textbook puts it.

Compasses were in use before magnetism was fully understood, the University of Colorado’s geomagnetism group notes. The needle’s ends already had their names, and the magnetic pole near the geographic North Pole was called north too, for where it lies. In physics terms it is a south magnetic pole, where magnetic field lines enter the Earth, and convention keeps the name “irregardless of the physics.”

The field also slants into the ground in the north: at the north magnetic pole, the north end of a dip needle points straight down. Compasses built for the northern hemisphere therefore carry a small weight on the south end of the needle to hold it level, and southern-hemisphere models carry it on the north end.

The needle follows the field where it sits

Field lines do not run straight to the poles, and the needle follows the lines. Its heading “can also differ substantially from the direction to the Magnetic North Pole,” the Colorado group writes, “since magnetic field lines are not just great circles connecting the magnetic poles.”

“It is a common misconception that the needle of a magnetic compass points to a particular spot on the earth’s surface,” Larry Gedney wrote in a 1985 column for the University of Alaska Fairbanks Geophysical Institute. The poles, he explained, are “not centers of attraction at all, but simply blurry localities” where the field lines meet the surface at right angles. They also shift tens to hundreds of kilometers with daily variations and magnetic storms.

“The compass points in the directions of the horizontal component of the magnetic field where the compass is located, and not to any single point,” NOAA’s geomagnetism FAQ states.

Magnetic north and true north rarely agree

The angle between them is the magnetic declination, and it can shift over short distances: “A mile or two away the magnetic declination may be considerably different,” NOAA warns. It also drifts over the years.

In 1985 a compass over most of Alaska pointed between 20 and 30 degrees east of true north, Gedney wrote. At Greenwich, compasses pointed west of true north from around 1660, by as much as 24 degrees around 1820 in NOAA’s historical field model. In September 2019 they pointed true north again, for the first time in about 360 years, Royal Museums Greenwich reported.

Magnetic storms can swing the needle within hours, worst at northern latitudes, where the field lines dip steeply. During the same disturbances that drive intense auroral activity, the magnetic observatory at Sitka had recorded erratic deflections of over 10 degrees in the horizontal field.

The earliest clear description already saw the needle miss due south

By Joseph Needham’s reckoning in Science and Civilisation in China, the earliest clear description of a magnetic needle compass in any language is in the Dream Pool Essays of the Song-dynasty statesman Shen Kuo, written around 1088. In Needham’s translation, a needle rubbed with lodestone “is able to point to the south. But it always inclines slightly to the east, and does not point directly at the south.” Shen advised hanging it from “a single cocoon fibre of new silk.”

Needham called that “a clear statement of the magnetic declination,” four centuries before 1492, the traditional date of its discovery by Columbus. Europe caught on before Columbus: by about 1450, Needham found, German makers of compass sundials were already marking the declination on their dials.

Magnetic north sped up, then braked

The north magnetic pole wanders. James Clark Ross located it in northern Canada in 1831, NOAA records. By 2019 it had been in Canada for 400 years and was heading toward Russia, according to the British Geological Survey, which tracks its speed:

Period	How fast magnetic north moved
1600 to 1990	About 10 km a year
The 2000s	Up to 55 km a year, a record
2019	Around 50 km a year
By 2025	Slowed to about 35 km a year

BGS called the slowdown “a deceleration that has not been seen before.” Liquid iron moving in the outer core drags the field along “in an unpredictable way,” and it is the same iron core that took most of Earth’s gold down with it as the planet formed.

Phones and runways use a five-year forecast of the field

A phone has no needle. It reads the field with a built-in digital compass and corrects for declination with the joint US-UK World Magnetic Model, which NOAA says “comes pre-installed in Android and iOS devices, thereby bringing its use to more than a billion devices around the world.”

Because changes in the field “are difficult to predict,” the model is reissued every five years. The current version, WMM2025, was released on December 17, 2024, and is valid until late 2029. The 2015 edition did not last its five years: in February 2019 NOAA put out an out-of-cycle update because the pole was “moving quickly away from the Canadian Arctic toward Siberia.”

Under the FAA’s Aeronautical Information Manual, a runway’s number is “the whole number nearest one-tenth the magnetic azimuth of the centerline.” Runway 17 points about 170 degrees from magnetic north, and NOAA calls runways “perhaps the most visible example of a navigation aid updated to match shifts in Earth’s magnetic field.”

The needle in your hand is a small north pole, lined up by a field whose Arctic end is magnetically south, and it answers to the field where you stand, not to a spot on a map. That spot is still drifting, about 35 km a year. Strictly FYI.