“…nothing is so much wanted and desired at sea, as the discovery of the longitude, for the safety and quickness of voyages, the preservation of ships, and the lives of men…”

The Longitude Act, 1714

For any maritime nation, investment in long distance trade, outposts and settlements overseas made navigation, including the ability to determine a ship’s longitude, increasingly important. As nations including Spain, the Netherlands and France, sought to dominate the world’s oceans, each offered rewards for solving the longitude problem. But it was in Britain that the approach paid off as a result of the 1714 Longitude Act.

What is longitude?

Global position is described by two coordinates, latitude and longitude, measured in degrees. Lines of latitude measure positions north and south and run parallel to the equator. Lines of longitude run pole to pole and measure positions east and west. Latitude is easy to measure from the Sun. Longitude presents a bigger challenge.

Longitude solutions

Most proposals for finding longitude were based on the principle of time difference and aimed to allow sailors to determine the time at the reference point for comparison with their local time from the Sun. By 1714, the most promising ideas seemed to be to carry the reference time with a mechanical clock or to use astronomical observations to find it. Much effort had already gone into both methods. Accurate pendulum clocks existed by the early 18th century, but attempts to make them work at sea failed due to the motions of the ship and changes in humidity and temperature.

On the astronomy side, Charles II founded the Royal Observatory in 1675 to carry out observations ‘to find out the so much desired longitude of places for the perfecting of the art of navigation’. If an accurate catalogue of the positions of the stars was made, the Moon’s motion relative to the stars could be used as a celestial clock to calculate Greenwich Time. This was known as the ‘lunar distance method’. In principle, sailors would measure the Moon’s position relative to a star and use tables of its predicted position to calculate the time at Greenwich (or another chosen reference). The problem was to predict the Moon’s complex motions and to perfect instruments to make the necessary observations.

The Longitude Act

In 1714, the British Government offered, by Act of Parliament, £20,000 for a solution which could find longitude to within half a degree (equivalent to 2 minutes of time), and a group later known as the Board of Longitude was set up to assess submissions and offer rewards. These experts included the Astronomer Royal at Greenwich and other scientific, maritime and political leaders.

The Solution(s)

One of the remarkable things about the longitude story is that two practical solutions were developed at the same time.

In the field of mechanical timekeeping, John Harrison, a working-class joiner and clockmaker with little formal education came closest to receiving the reward money through his extraordinary mechanical talent and determination, culminating in his marine timekeeper, H4. This would become the instrument known as the marine chronometer. At the same time, the work of John Hadley, German astronomer Tobias Mayer and others perfected the instruments and astronomical tables necessary for the lunar distance method.

Greenwich was central to the story. Above all, Astronomer Royal Nevil Maskelyne’s observations at the Royal Observatory, his work on the Nautical Almanac and the Board of Longitude demonstrated the complementary nature of astronomical and timekeeper methods, ultimately leading to the successful determination of longitude at sea. As solutions were developed, the Royal Observatory became a testing site for marine timekeepers and the place at which the astronomical observations needed for navigational tables were made. It was this work that would eventually lead to Greenwich becoming the home of the Prime Meridian, zero degrees longitude for the world.

Authors: Richard Dunn, Senior Curator for the History of Science and Curator of Ships Clocks & Stars: The Quest for Longitude exhibition, Royal Museums Greenwich and Sheryl Twigg, Press & PR Manager, Royal Museums Greenwich

To mark the tercentenary of the Longitude Act of 1714, Ships Clocks & Stars: The Quest for Longitude opened at the National Maritime Museum on 11 July 2014.

You can read more about the history of challenge prizes on the Nesta website.