Background on Earth and Magnetism
What are magnets and
what is a magnetic field?
Ancient Greeks were the first to discover that certain
stones obtained from the city of Magnesia attracted certain metals,
an amusing phenomenon the origin of which was a mystery to the Greeks.
These stones were magnets very similar to the ones we put on our refrigerators.
Today we know that the magnets generate something called a magnetic
field everywhere. Two magnets that do not even touch other can interact
with each other via magnetic fields. We live in an environment not
only filled with atoms and molecules but also filled with magnetic
and other fields. Even though these fields don't interfere with our
everyday life, they exist and we can see them using a compass.
Why do compasses point
towards the Magnetic North Pole?
Earth has a magnetic field that is produced by its core.
The core is believed to contain liquid iron, which is at high temperatures
and high pressures. We have a limited understanding of earth's magnetism,
as we cannot generate conditions similar to that of earth's core in
a laboratory. However, the magnetic field generated by earth's core
can easily be detected using a compass. A compass is a simple navigational
tool made out of a needle-shaped magnet, which is freely suspended.
The needles are the directed towards the north magnetic pole due to
the earth's magnetic field. Note that the magnetic north pole is different
than the true north pole, which is defined as the point where earth's
rotational axis intersects earth's surface. The angle between magnetic
north and the true north direction is called magnetic declination.
Was the Earth's magnetic
field always the way it is now?
No. For reasons that we do not understand, the position
of the magnetic north pole and therefore the magnetic declination
angle changes over time. The Buddhist astronomer Yi-Xing first measured
the magnetic declination in China about AD 720. The position of the
North magnetic field has been closely monitored in the last hundred
years and has been shown to be moving with an average speed of 10
km per hour. This is a very significant motion; for example, due to
this motion the magnetic declination in Seattle changed by 1° in the
last 10 years. We have also biological and geophysical evidence that
the Earth's magnetic field has reversed many times.