|Act of Parliament|
|Long title||An Act for Providing a Publick Reward for such Person or Persons as shall Discover the Longitude at Sea|
|Citation||13 Ann. C 14|
|Text of statute as originally enacted|
|Text of the Longitude Act as in force today (including any amendments) within the United Kingdom, from the UK Statute Law Database|
The Longitude Act was an Act of Parliament of the United Kingdom passed in July 1714 during the reign of Queen Anne. It established the Board of Longitude and offered a monetary reward (Longitude Prize) for anyone who could find a simple and practical method for the precise determination of a ship's longitude. The prize has never been officially paid out.
As transoceanic travel grew in significance, so did the importance of accurate and reliable navigation at sea. Scientists and navigators had been working on the problem of measuring longitude for a long time. While determining latitude was relatively easy, early ocean navigators had to rely on dead reckoning to find longitude. This was particularly inaccurate on long voyages without sight of land and could sometimes lead to tragedy, as during the Scilly naval disaster of 1707 which claimed the lives of nearly 2,000 sailors. This brought the problem of measuring longitude at sea into sharp focus once more. Following the Merchants and Seamen Petition, which called for finding an adequate solution and was presented to Westminster Palace in May 1714, the Longitude Act was passed in July 1714.
For details on many of the efforts towards determining the longitude, see History of longitude.
The Longitude Act monetary prize was £10,000 (worth over 1.26 million in 2013)) for anyone who could find a practical way of determining longitude at sea to an accuracy of not greater than one degree of longitude (equates to 60 nautical miles (110 km; 69 mi) at the equator). The prize was to be increased to £15,000 if the accuracy was not greater than 40 minutes, and further enhanced to £20,000 if the accuracy was not greater than half a degree. The prize has never officially been paid to anyone, though John Harrison received grants and awards that exceeded this.
- During daylight hours, latitude can be found from the altitude of the sun at noon with the aid of a table giving the sun's declination for the day. At night, latitude can also be determined altitudes of stars with known declination crossing the meridian (including Polaris, the northern pole star).
- Sobel, Dava, Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time, Fourth Estate Ltd., London 1998, p. 6, ISBN 1-85702-571-7
- UK CPI inflation numbers based on data available from Gregory Clark (2014), "What Were the British Earnings and Prices Then? (New Series)" MeasuringWorth.
- Longitude Act 1714
- Alexi Baker (July 2013), Longitude Acts, Board of Longitude project, University of Cambridge Digital Library