Spherical Co-ordinate Systems: Latitude and Longitude

Co-ordinate systems, used to measure positions on a sphere will be very important in this course. As as introduction, here is a reminder about how we measure position on the surface of the Earth.

The rotation of the Earth provides a natural way of defining position. The rotation axis meets the Earth's surface at the North and South Poles. The great circle which is perpendicular to the rotation axis is the equator. Any point on the Earth's surface is defined by two co-ordinates; its longitude and latitude.


A point's longitude is measured east or west along the equator. Longitude ranges from 0° to 180° East and 0° to ° West. Lines of constant longitude are known as meridians; they are all great circles. The figure to the right shows several meridians.
There is no obvious zero point for longitude. In practise, a single meridian is chosen to represent zero longitude. Since UK astronomers and clockmakers were instrumental in the development of techniques to measure longitude, the Prime Meridian (as it is known) passes through Greenwich.

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The latitude of an object is the angular distance north or south of the equator. Lines of constant latitude run parallel to the equator. They are called parallels of latitude. The figure to the right shows several parallels of latitude. Latitude goes from 0° at the equator to +90° N at the North Pole or -90° S at the South Pole.

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