Essential Information

Royal Observatory

18 Sep 2014

Longitude Solutions When Isaac Newton testified to a Committee of the House of Commons in 1714 about four potential longitude solutions, he pointed out that there were key problems with all of them, but what were those proposed solutions? And what were the problems with them?   Time Keepers The simplest solution to the problem of longitude was one of time difference. You can determine how far east or west you are of a given location, for example Greenwich, if you know the difference between local time (determined by the sun) and time in Greenwich. All you need is a clock that is set to Greenwich time. So far, so good. But therein lay the problem: eighteenth century clocks did not keep accurate time at sea. The most accurate clocks at the time were pendulum clocks, but this was impossible with the rolling of a ship. On top of this, the effect of changes in temperature and pressure could cause the components of the clock to expand and contract and cause the lubricating oil to change its consistency, which could lead to great inaccuracies in timekeeping. Even a small inaccuracy could lead to an error of hundreds of miles, stranding your ship on the sandbank or on the rocks. Jupiter's Moons Making a clock that could keep accurate time at sea seemed an impossible task. However, there were celestial clocks, events in the heavens that occurred so regularly that you could set your clock by them. Through the telescope on his balcony in Padua, the Italian astronomer Galileo Galilei,  observed, “the clock of the heavens”. He noticed that the moons of Jupiter move in a very predictable way, so regular that you could set your clock by them. Observing Jupiter’s moons proved useful for determining longitude on land. However,  it proved difficult to observe the motion of Jupiter's  moons from the deck of a moving ship. One longitude projector, Christopher Irwin, attempted to create a marine chair as a way of combatting the movement of the ship to aid in the observation of Jupiter’s moons. However, when Nevil Maskelyne tested in on his voyage to St Helena in 1761, he wrote back to the Board, 'Mr Irwin's marine chair affords no convenience or advantage to an observer in using a telescope for observing the celestial phenomena but sea, but rather the contrary' Lunar Distance John Flamsteed, the first Astronomer Royal, observed another celestial clock, the movement of the moon. Longitude could be calculated from the position of the moon relative to certain stars. However, the motion of the moon is very complicated, following a pattern that repeats only once in every 18 years. To employ this method, very precise observations of the movement of the moon relative to that of the stars were needed, over a prolonged period of time.   Signalling: This was one of the methods that did not use time difference, and was proposed by William Whiston. A gifted mathematician, Wiston succeeded Isaac Newton as Lucasian professor of Mathematics at Cambridge. However, his unorthodox religious views, soon got him the sack. Wiston's suggestion was simple. He suggested that rockets could be fired from signalling ships at fixed locations in the oceans. Mariners could then determine their distance from the ship from the delay between seeing the flare and hearing the sound. Direction could be determined using a compass. Elegant as this seemed to the theorist, there was the problem of keeping the signalling ships manned and supplied. The Admirals on the board of Longitude, quickly pointed out that the ships would need to be manned by thousands of sailors. Men who would be at the mercy of the elements, in danger of starvation and hard pressed to stay sober!   Magnetism This suggestion came from Astronomer Royal, Edmond Halley, who suggested determining east/west position using variations in the earth's magnetic field.  The difference between geographical north and magnetic north vary, so that a pattern can be plotted over the globe. This also sounds like an elegant solution to finding longitude.  However, the earth's magnetic field is not static, it changes over time, making this method unreliable.   This blog post has been created in conjunction with our Visitor Experience tour guides. VE guides deliver free tours of Ships, Clocks & Stars every Thursday evening at 5pm, 6pm & 7pm.