Why does the needle return to one direction?
A compass needle is a small magnet that can turn freely. Earth’s magnetic field turns it toward the local field direction. It swings back and forth before settling.
Turn the case and the needle can find its direction again. That direction need not be exactly the north shown on a map, and it can differ between places.
Bring another magnet close and it affects the needle too. Take it away and watch whether the original direction returns.
Disturb it yourself
- Let the needle settle and note its direction. Spin the compass case—does the needle turn with it?
- Bring a magnet slowly from one side and watch the needle lean; try the other side.
- Approach with an iron spoon, a phone, a hair clip. Which ones are “real” disturbances?
Consider: does the needle deflect the same amount with the magnet due east as due north?
Horizontal component, declination and the real Earth
More: a closer explanation
A compass needle is not an ordinary iron strip; it is a small magnet free to rotate. Earth itself carries a magnetic field, and at your location that field has one definite horizontal direction. Displace the needle and the forces on its two ends combine into a torque that twists it back; damping settles the swing.
So the needle does not point at “the North Pole dot” but at the local field direction under your feet. Map north is geographic north (the spin axis); the north a needle follows is magnetic north; the angle between them is declination—different everywhere and drifting slowly over the years.
Bring a magnet or a lump of steel nearby and the needle responds to the sum of all fields: a strong enough disturbance overrides Earth, and removing it lets the original direction return.
Open: why does the needle lie flat?
Earth’s field usually dives into the ground at an angle (magnetic inclination). The needle is constrained to spin about a vertical axis, so it can only follow the horizontal component of the field. A dedicated instrument, the dip circle, lets its needle tip vertically—and near the poles it dips sharply, because there the field is almost vertical.
The torque grows with field strength and the sine of the misalignment; φ is the field direction, θ the needle’s.
Open: Earth has no buried bar magnet
Earth’s main field looks, on large scale, like a tilted dipole, but it is generated by flowing liquid metal in the outer core (the geodynamo)—not by any solid magnet. Declination changes place to place and year to year; nautical charts list local variation and must be updated, and serious navigation cross-checks with the stars or satellites.
Limits of the model
The teaching model uses smoothed dipoles for Earth and the bar magnet, and declination is a fixed preset, not live data. Magnetic storms, local geological anomalies and oddly shaped magnetized objects are out of scope—before trusting a needle outdoors, check for disturbances.