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The Earth as a sphere

In this topic, the Earth will be considered to be a perfect sphere, with a radius of 6 400 kilometres. (see Introductory Notes)

There are two types of circles that can be drawn on the surface of a sphere.

A great circle is the largest circle that can be drawn on the surface of a sphere, as the centre of the great circle is also the centre of the sphere. The radius of a great circle on Earth will therefore be 6 400 kilometres which is the same as the radius of the Earth sphere. As shown above, the equator and a circle drawn through both the North and South poles are great circles. The meridians of longitude are all great circles.

A great circle track is the shortest distance between two points on the surface of a sphere and forms the basis of many navigational activities for long distance travel by air or sea. A key calculation as part of this process is the finding of way-points; these are points that the traveller should pass through to maintain their journey on the great circle.

A small circle does not have its centre at the centre of the sphere. The radius of a small circle will be LESS than the radius of the sphere. For Earth, it means that the radius of a small circle will be less than 6 400 kilometres. The parallels of latitude (apart from the Equator) are all small circles.

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