Key Ideas

• The Earth itself behaves like a giant magnet, which is why a compass needle always points North.
• This happens because of a swirling liquid iron and nickel core deep inside the Earth, a process called the geodynamo.
• Magnetic North isn't exactly true North; the gap between them, called declination, varies by location, and both drift over time.
• Cooling rocks lock in the direction of the magnetic field at the moment they form, which is how scientists know the poles have completely flipped many times.
• The Earth's magnetic field shields us from the sun's charged particles, the solar wind, and creates the aurora near the poles.
• Losing a magnetic field can cost a planet its atmosphere, as it likely did on Mars, and some animals navigate using Earth's field directly.

• Geodynamo: The process by which the Earth's swirling liquid iron and nickel outer core generates the planet's magnetic field.
• Declination: The angle between the direction a compass points (magnetic North) and true geographic North, which varies by location.
• Paleomagnetism: The magnetic direction locked into rock as it forms, used by scientists to study the Earth's ancient magnetic field.
• Solar wind: A stream of charged particles constantly blasted out by the sun, mostly deflected around Earth by its magnetic field.
• Aurora: The glowing lights near the poles caused by charged solar particles colliding with gases in the upper atmosphere.
• Pole reversal: A complete swap of the Earth's magnetic North and South poles, which has happened many times over the planet's history.

1. If the Earth's magnetic poles drift over time, would an old map's declination reading still be accurate today? What would you check before trusting it?
2. Mars lost its magnetic field and, over time, lost most of its atmosphere. What do you predict would happen to Earth if its core stopped swirling tomorrow?
3. A rock formed a million years ago locks in the field's direction at that moment. What would finding two rock layers with opposite locked-in directions tell you happened between them?

• A compass, or the homemade floating-needle compass from an earlier lesson
• A bar magnet, thread, and something to hang it from
• A straight stick, chalk or small markers, and a sunny midday