18.2 Form of the Earth

The general shape of the earth.

• Strictly speaking, the Earth is not a sphere, but a spheroid.
• Which means it is slightly flattened at the poles.
• And because the Earth rotates around its North and South pole axis, it is called an Oblate Spheroid.
• But in practice the earth is regarded as a perfect sphere. 

Axis and Direction of Rotation of the earth; 

• The poles of the Earth are the extremities of the Earth's axis of rotation, and are fixed locations which the earth rotates about.
• They are referred to as the North and South Geographic Poles, also known respectively as True North and True South
• The direction of the Earth's rotation is from West to East, or, to an observer looking down on the North Pole, it is turning anti-clockwise.

Geographic and Magnetic Poles; 

• The magnetic pole is a point where the Earth's magnetic field goes vertically down into the planet.
• The geographic and magnetic poles are not situated in the same place.
• The north magnetic pole is currently about 400km south of the north geographic pole, but can move to about 1,000km away.
• The earth is essentially a large magnet with a field of magnetic force, or flux, around it.
• A freely suspended compass needle will align itself with the flux lines and under most conditions a stable direction reference is provided with reference to the magnetic poles.