Magnetic field

Earth's magnetic field has flipped hundreds of times; the last reversal was about 780,000 years ago. During reversals the field can weaken to under 10% of today’s strength, letting auroras appear near the equator. Since 1840, Earth’s field has already weakened by about 9%.

Migratory birds may ‘see’ Earth’s magnetic field. In European robins, the eye protein cryptochrome 4 changes its electron-spin chemistry under magnetic fields when lit. A 2021 Nature study showed it is magnetically sensitive, linking bird navigation to quantum mechanics.