Earth's Magnetic Field

I've been learning Android on my own for some days in my free time, and, now that I've created a simple compass application for Android, I want to write about the Earth's magnetic field, that, can be calculated using the data of the sensors of the modern mobiles.

Some days ago I watched on TV a documentary about a scientist who said that the Earth's magnetic field changes around 250.000 years on average, and that last time that changed was around 750.000 years, and that now is moving faster and faster, what could mean that could change soon. I don't know if that would happen soon or not.

The Earth's magnetic field (the geomagnetic field) is the magnetic field that extends from the Earth's inner core (the polarity of the Earth's magnetic field is recorded in sedimentary rocks) to where it meets the solar wind, a stream of energetic particles emanating from the Sun.

It allows the life in the Earth, protecting us from energetic charged particles emanating from the Sun or the space, deflecting most of them.

Humans have used compasses based on the Magnetic North Pole for direction finding since the 11th century A.D. and for navigation since the 12th century.

The Earth's magnetic field can be represented by a three-dimensional vector. Its angle relative to true North is the declination (D) or variation. Facing magnetic North, the angle the field makes with the horizontal is the inclination (I) or dip. The intensity (F) of the field is proportional to the force it exerts on a magnet. Another common representation is in X (North), Y (East) and Z (Down) coordinates.

I'm not going to explain much more about it, but focus on the measure units of the Earth's magnetic field intensity:

Varies according the location on the Earth, being greatest near the poles (Canada, Siberia, Antarctica) and weaker near the Equator. It is often measured in gausses (G) but is generally reported in nanoteslas (nT).

  1 T = 10,000 G

1 nT =  10⁻² G

1 G = 100,000 nT

The field ranges of the Earth's magnetic field intensity are between approximately 25,000 nT and 65,000 nT (0.25G – 0.65 G).

Copyright: http://www.mnh.si.edu/earth/text/4_1_5_0.html

As we can see the geographic pole does not necessarily coincide with the magnetic pole.

References:
 - http://en.wikipedia.org/wiki/Earth%27s_magnetic_field
 - http://web63.justhost.com/~xentrop1/C%C3%B3mo-fuerte-es-el-campo-magn%C3%A9tico-de-la-tierra.php
 - http://www.mnh.si.edu/earth/text/4_1_5_0.html