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The longitude prize was a prize offered by the British government through an Act of Parliament in 1714 for the precise determination of a ship's longitude.

The measurement of longitude was a problem that came into sharp focus as people began making transoceanic voyages. Determining latitude was relatively easy in that the altitude of Polaris, the northern pole star, was equal to the observer's latitude. But two lines of position are needed to determine location. The most desirable second position line was a line of longitude.

In one incident in 1707, Admiral Cloudesley Shovell and his fleet were afloat in fog and thought they were in the middle of the ocean; they ran aground and over 2000 men died. That incident in the general context of British maritime endeavours led to the establishment of a prize for finding a method of measuring longitude.

"The Discovery of the Longitude is of such Consequence to Great Britain for the safety of the Navy and Merchant Ships as well as for the improvement of Trade that for want thereof many Ships have been retarded in their voyages, and many lost..." Parliament, in 1714, voted to offer a reward (£10,000 for any method capable of determining a ship's longitude within one degree; £15,000, within 40 minutes, and £20,000 within one half a degree) "for such person or persons as shall discover the Longitude."

John Harrison was the man who solved the problem of measuring longitude. While most efforts had focused on a precise catalogue of stars, to be used together with the moon's position to determine longitude, Harrison attempted to build a precision clock which kept the time of the home port. This, together with determination of the local time using the height of the sun, would allow mariners to calculate longitude. With support from the Board of Longitude set up to administer the prize, he started in 1730 to build several spring-driven clocks, finally succeeding in 1761 with a determination of better than half a degree.

However, the board refused to believe that longitude could be determined without astronomical measures, first awarding only half the prize and then dragging the process out with more demands for evidence and several copies of the clocks.

Finally in 1773, King George III persuaded Parliament to award the prize to Harrison, bypassing the board.

Dava Sobel's 1996 bestseller Longitude (ISBN 0-14-025879-5) recounts Harrison's story. A film adaptation of Longitude was released by A&E in 1999, starring Michael Gambon as Harrison and Jeremy Irons as Rupert Gould.

Today a sailor has a number of choices for determining accurate positional information, including radar and GPS, the satellite navigation system. Even with all these modern methods of determining longitude, a marine chronometer is also normally carried as a backup.

• Nevil Maskelyne
• Lunar distance
• Inducement prize contest

• Royal Observatory Greenwich: John Harrison and the Longitude Problem
• Nova Online: Lost at Sea, the Search for Longitude