The problem of finding the longitude, particularly at sea, was the driving force behind a great deal of scientific endeavour in the eighteenth century.
Finding the latitude of a ship at sea was not a problem by this time. It depended on finding the altitude of the Sun at noon with a backstaff, cross-staff or, from the 1730s, an octant. Tables were also used which depended on the time of year. But the longitude, which depended on time, remained unattainable. In 1726 Jonathan Swift wrote in ‘Gulliver’s Travels’: 'When I become immortal … I shall then see the discovery of the longitude, the perpetual motion, the universal medicine'
While many methods had been suggested for finding the longitude, only two were really serious contenders. One was to use the motion of the Moon with respect to the Sun and stars as a timepiece. However the data on both the star’s positions and the Moon’s motion was not adequate. The necessity of finding the longitude was so great that Greenwich Observatory had been founded in 1676 to find the data necessary. After a series of devastating shipwrecks the Board of Longitude was set up in 1714 and offered a prize of up to £20,000 ‘for a due and sufficient Encouragement to any such Person or Persons as shall Discover a proper Method of Finding the said Longitude’.
The components needed for finding the longitude by lunar distance were in place by the 1760s. John Flamsteed, the first Astronomer Royal at Greenwich, produced star maps which were published in 1725. Sextants (improvements on octants) were available and tables of the Moon’s position had been compiled by Tobias Mayer. However, the calculations took Neville Maskelene, the then Astronomer Royal, four hours. The next project was to publish tables with the computations worked out which was done by 1767 in the Nautical Almanac. The Almanacenabled longitude to be found with good instruments in half an hour. Because it used Greenwich as the reference point, Greenwich time became standard.
The story of John Harrison, a Lincolnshire clockmaker, and the development of his timepieces is well known. Harrison eventually won £10,000 in 1773 at the age of 80 for ‘H4’, a watch which led to the development of chronometers and the solution of longitude by the comparison of time. The widow of Tobias Mayer received £3,000 for his calculations for the lunar distance method in 1765.
While Harrison, Mayer and Maskelene had all contributed to solutions for the longitude problem, it was the next generation of makers who could provide instruments at reasonable prices in large quantities for navigators to buy and learn to use.
When leading instrument makers such as Jesse Ramsden and the Troughtons began to use dividing engines for their scales, smaller and lighter instruments could be produced in large numbers without loss of accuracy. John Arnold and Thomas Earnshaw made chronometers for £60 and these had become commonplace by 1820.