Variation, deviation and the Earth's magnetism

Navigation · Chapter 4 · DGCA CPL and ATPL

Why a compass points anywhere at all, why it does not point at the North Pole, how to turn a compass heading into a true one and back, and where the compass stops working.

Remember

A magnet has a red (north-seeking) and a blue end; unlike poles attract. Liquid iron churning in the Earth's core makes it a tilted bar magnet with its blue end in the Arctic.
Three norths: true (the pole), magnetic (the needle), compass (the needle in this aircraft).
Variation: true ↔ magnetic, east least, west best. Deviation: magnetic ↔ compass, east least, west best.
The table: C D M V T, one column at a time; past 360 start again at 000.
Isogonal: equal variation; agonic: zero. Variation changes over the years as the magnetic pole moves.
Dip: 0° at the magnetic equator (aclinic), 90° at the magnetic poles, where the compass fails.

Why does a compass needle point anywhere at all?

What a magnet is

Two ends: red and blue • Every magnet has two poles, one at each end, where its pull is strongest • The red end is called north-seeking, the blue end south-seeking • Cut a magnet in half and each half still has two poles
Lines of force • The space round a magnet where it pulls is its field • Iron filings on a card show it as lines of force • They come out of the red end and go back into the blue end (the dots show the way)
Unlike poles attract • Red and blue pull towards each other • Red and red (or blue and blue) push apart • Keep this rule: it explains the whole compass
A compass needle is a tiny magnet • A needle on a pivot is free to turn • It swings, overshoots and settles along the line of force where it sits • Its red end lies the way the lines run
What a compass really shows • Not "north": it shows which way the field runs where it is • On Earth that field is the Earth's own: next lesson, where it comes from

Where does the Earth's magnetism come from?

Why the Earth is a magnet

Inside the Earth • Crust and mantle: solid rock • Outer core: liquid iron, from about 2 900 km down • Inner core: solid iron at the very centre • The centre is still hot and still cooling down
The liquid iron churns • Heat from the centre keeps the liquid iron moving, like soup on a stove • The Earth's spin twists the flow into swirls along the spin axis
Moving metal makes a magnet • Iron carries electricity: as it flows, electric currents run in it • Electric currents make a magnetic field • The field keeps the currents going: the Earth runs its own dynamo, the geodynamo
Outside: a giant, tilted bar magnet • From outside, the field looks like a huge bar magnet inside the Earth • Its lines leave the southern half and come back in over the Arctic • It is tilted away from the spin axis, and "bent": the two magnetic poles are not opposite each other
Why the red end points north • Unlike poles attract (last lesson) • The Earth's "magnet" has its blue end under the Arctic, at the magnetic north pole, in the Arctic north of Canada and Alaska • So every needle's red end is pulled that way: that is why we call it north-seeking