Terrestrial longitude is given as the angle between two planes which both pass through the Earth’s centre and both poles. The reference plane passes through Greenwich in the United Kingdom and the second plane passed through the actual location point. Longitude angles are always measured parallel to the equator, and are typically given in degrees. The line of this reference plane on the Earth’s surface is known as the prime meridian and defines the line of zero longitude.

Figure 1 - Lines of terrestrial longitude on the Earth's surface.

Longitude angles also define the relative directions of East and West on the Earth’s surface, where the direction of increasing longitude is always East and decreasing longitude is always West.

Important Longitudes

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