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In the night sky: Orion
In the night sky: Orion

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1.2.7 Finding your longitude

A photograph of the Prime Meridian monument in Greenwich, London.
Figure 17 The Prime Meridian monument found in Greenwich, London. The line on the ground marks where the Prime Meridian runs.

Finding your latitude is only half of the information required to determine your location on the globe. The other piece of information required is your longitude.

Your longitude is your distance east or west in time from the prime Meridian, which runs through Greenwich, south London. The prime Meridian is defined as zero degrees longitude.

A method for finding longitude was eventually devised when clocks that could keep accurate time at sea were made.

A photograph of a John Harrison marine chronometer.
Figure 18 A John Harrison marine chronometer used for establishing the longitude of a ship at sea

Remember, this was in the days long before digital watches, and the clocks of the period were based on the action of springs and pendulums, which did not behave regularly when tossed about on the ocean wave.

You can find out more about how the longitude problem was solved by reading the book Longitude by Dava Sobel, or visiting the Royal Observatory in Greenwich or the National Maritime Museum, also in Greenwich.