IDIOT’S GUIDES: SCIENCE MYSTERIES EXPLAINED
Why does a compass work?
When you take a long magnet and suspend it on water or on a shaft, it rotates to point
magnetic north. This phenomenon has in many ways built our modern world, enabling
navigation across long distances. But why does it work?
The Earth’s powerful magnetic ﬁeld is unique among the rocky planets of the inner Solar System. But it
doesn’t just help us get from A to B, it protects us from many unseen cosmic dangers.
Magnets on Earth, if they’re light enough, will
spontaneously rotate to point toward the Earth’s
magnetic north pole because the Earth itself is a
Our liquid metallic core spins at a slightly dif-
ferent rate than the rest of the planet, making the
interior of our world a giant electric generator, or
dynamo. Convection in the mantle—the huge layer
of rock between the crust and core—also adds to
this eect. And when a dynamo generates electrici-
ty, it also generates a magnetic ﬁeld.
In addition to being all around us, Earth’s
magnetic ﬁeld extends into space, many
times the diameter of the planet. If we could
see magnetism, Earth would look more like a
comet, with a huge tail of electromagnetic force
streaming from it.
This magnetic ﬁeld has proven to be ex-
tremely useful to humans and many animals.
When we suspend a magnet so it can rotate
freely—techniques include ﬂoating it in water
or attaching it to a pivot as in a compass—the
magnet will spin and align facing the north
magnetic pole of Earth.
But here’s a confusing fact: because the
north poles of magnets point toward north, and
because in magnetism opposite poles attract, in
terms of physics and magnetism the “top” of the
Earth is actually a south magnetic pole! How-
ever, to prevent confusion, we refer to it as the
north magnetic pole on maps. This confusion
came about because humans deﬁned “north” on our maps
before we developed a full understanding of magnetism.
And of course, because there’s no up or down in space,
we’re free to deﬁne whichever end of the planet we like as
Because Earth isn’t a perfect sphere (it bulges around
the equator), and because we’re tilted at a 23-degree angle
with respect to our orbit around the Sun, and because the
interior of the Earth isn’t uniform but has lumpy parts, all
this means the magnetic north pole isn’t at the same place
as the physical north pole. The physical North Pole is the
point around which the Earth rotates.
What’s more, the magnetic north pole moves around—
quite a lot. In 2001, scientists pinpointed the magnetic
pole near Ellesmore Island in northern Canada. It has
since moved beyond Canada toward Russia, at a speed of
about 35 miles (56km) a year.
You can tell when you’re standing on the magnetic
north pole because if you hold your compass out, the nee-
dle will try to point straight down into the ground.
The magnetic ﬁeld of Earth—scientists call it the
magnetosphere—does more for us than let us ﬁgure out
which way is north.
The lines of magnetic force that ﬂow out into space
around us actually prevent certain kinds of particles from
reaching the surface of the planet. The Sun, apart from
providing light and heat, also blasts Earth with danger-
ous radiation. It’s the magnetosphere that protects us
from the more dangerous particles and stops them from
stripping our atmosphere. The magnetic shield is so eec-
tive, spacecraft designers are thinking of ways to have a
spacecraft generate its own mini magnetic ﬁeld to act as a
radiation shield for long journeys, such as to Mars.
Speaking of Mars, the Red Planet has no signiﬁcant
magnetic ﬁeld, and scientists believe that’s why it no lon-
ger has an atmosphere or oceans—these have been blasted
away by radiation over millions of years.
Magnetic South PoleGeographic South Pole
Geographic North PoleMagnetic North Pole
The north magnetic pole is actually the SOUTH pole of the Earth’s magnetic ﬁeld