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Finding Longitude: how clocks and stars helped solve the longitude problem. Richard Dunn and Rebekah Higgitt. 2014


Finding Longitude: how clocks and stars helped solve the longitude problem

By Richard Dunn and Rebekah Higgitt. 

Published by Collins. 


ISBN 978-0-00-752586-7. 

Also available as an eBook with ISBN 978-0-00-752587-4. 


This book was written to accompany the exhibition “Ships, Clocks and Stars: the Quest for Longitude” held at the National Maritime Museum, Greenwich from July 2014 to January 2015.1  It is not a catalogue of the exhibition, though all of the items in the exhibition are reproduced in the book, but the story of how the quest for finding a ship’s position at sea led to the Longitude Act of 1714, the ideas put forward, the trials that took place, and the commercial developments that meant finding longitude became affordable and common-place. 


The book begins with the division of the world in 1494 so that newly discovered lands would belong to either Spain or Portugal, which required a means of telling where the division fell at the other side of the world.  It then goes on to describe how instruments for latitude were developed over the years leading to the cross-staff and backstaff of the seventeenth century.  The problem is with longitude: the difference in time between a fixed point and the local time of a ship’s current position.  Developing accurate methods would enable ships to travel more predictably, more quickly, and with less risk—leading to greater profit. 


The first incentive scheme to reward the solver of the longitude problem was “established in Spain, the leading power of maritime power of the sixteenth century, by Philip II in 1567.”  Other governments followed, including the Dutch in 1600 and 1738.  The British passed the Longitude Act in 1714, “An Act for Providing a Publick Reward for such Person or Persons as shall Discover the Longitude at Sea.”  The reward was up to £20,000, but the successful method had to be “Tried and found Practicable and Useful at Sea” by means of a long distance sea trial.  Other sums could be advanced to people to help them experiment with schemes that seemed promising.  The Act also appointed Commissioner of Longitude, who later formed a standing body, or a “Board”. 


The book describes several of the schemes put forward, with wonderful illustrations of the instruments, maps, and books produced to promote them.  Many proposed schemes were too fanciful to be taken seriously, so it was not until the 1730s that “a scheme emerged with sufficient credibility for the Commissioners to consider a reward.”


The story of John Harrison and his developments of sea clocks may be well known, but the close-up photos of them that appear in this large-format book make it stand out from others that I have read.  The purpose of his clocks was to tell the time of “a reference point of known longitude” to be compared to local time at the current position of the ship.  Instruments for determining the latter were also being developed, and this book bring them out of the shadows.  Such instruments were used to measure the altitude of the sun from the horizon before and after midday, or of a star.


John Hadley proposed in 1731, at a meeting of the Royal Society, the use of an instrument with two mirrors “to use double reflection for indirectly viewing one of the objects; the other being viewed directly.”  This method meant “observations were not greatly affected by the ship’s motion - distinct advantage over other instruments.”  A trial at sea provided its effectiveness, and it became very popular.  The instrument was called a quadrant as “it could measure angles up to 90° or a quarter of a circle.”  Quadrants were taken in Endeavour,2 and indeed, on all of Cook’s voyages, one being given to Gerassim Gregoriev Izmailov at Unalaska Island in 1778.3


Tobias Mayer4 used quadrants in his quest to determine accurately the locations of places on land, leading to his map of Germany that showed “how uncertain the locations of even the major cities were.”  He also designed an instrument for shipboard observations.  He was urged to submit his ideas on lunar tables to the Commissioners of Longitude, but he “doubted that they would reward a foreigner…  Discussions of his ideas were taking place in England by the end of 1754” and a copy of his instrument was tested at sea in 1757.  Later tests showed that his tables with the right instrument “could determine a ship’s longitude to within 1°, a result that brought Mayer’s proposal within the limits of the 1714 Act.”


On 9 February, 1765 the Commissioners considered the results of the latest trials of Harrison’s clock and Mayer’s lunar tables.  Both were deemed successful and both rewarded.5  Another Longitude Act went through Parliament later that year.  “The trials were over.  It was up to the Commissioners to bring the new methods into practice.”  It was felt that the two methods were complimentary. 


Nevil Maskelyne, Astronomer Royal, was an enthusiastic supporter of the lunar tables method, and “quickly produced a system for producing the Nautical Almanac and Astronomical Ephemeris.”  The calculations needed to compile the tables were undertaken by a group of people, known as computers, one of whom was William Wales.6  The first Nautical Almanac was published in 1766 with the data for 1767, and published yearly thereafter.  A copy was taken in Endeavour and used for determining longitude, and on each of Cook’s voyages.7 


Although Harrison’s watch (now known as H4) had been successful,8 the 1765 Act demanded that it would be possible for other makers to produce similar timekeepers. Harrison had to disclose the workings of his watch, and Larcum Kendall was appointed to make a replica, which he said would take two years, but it was not ready until 1770.  Maskelyne also published drawings and descriptions of Harrison’s watch in 1767, allowing other makers to have a try.  One such person was John Arnold.


The book moves on to consider the trials of the two methods during Cook’s voyages.  Charles Green,9 a former assistant to Maskelyne, taught Cook how to use the lunar method during the Endeavour voyage to Brazil.  When they had left England, only the 1768 and 1769 editions of the Nautical Almanac were available for them to take.  After that, they had to undertake the onerous calculations themselves.  During Cook’s Second Voyage the lunar method was compared with Kendall’s watch (now known as K1) and three of Arnold’s timepieces.  K1 did so well it was described by Cook as his “trusty friend”,10 and taken again on his Third Voyage.  Arnold’s ones gained or lost time too much and were abandoned. During the Third Voyage another of Kendall’s developments, K3, was taken in Discovery.


Kendall’s K3 was taken in another ship called Discovery, during the voyage to the Pacific led by George Vancouver.  K1 went with the First Fleet to Australia.  K2, completed by Kendall in 1771, went first with Constantine Phipps, in Racehorse, to the Arctic, and then with William Bligh, in Bounty, to the Pacific.  The mutineers took her to Pitcairn Island. 


The time-pieces described so far were expensive to make, and so were the sextants and other instruments needed for measuring longitude.  A very interesting chapter in the book explains how manufacturing methods developed producing cheaper instruments that became more widely available to sea farers.  Thomas Mudge and Thomas Earnshaw developed the timepiece.  The scale marked on a sextant by hand using an engraving instrument was replaced by an engine developed by Jesse Ramsden, increasing accuracy and reducing the cost. 


I thought I knew the story of the quest for longitude reasonably well until I read this book, and realise how many more people were involved in many more ways than I’d ever thought about.


Spread over 250 pages with 150 illustrations, this book is a joy to read, and a reference book to keep and to return to.



Ian Boreham



  1. Cook’s Log, page 43, vol. 37, no. 3 (2014).
  2. Cook’s Log, page 1102. vol. 17 no. 4 (1994).
  3. Cook’s Log, page 25, vol. 26, no. 4 (2003). 
  4. Cook’s Log, page 30, vol. 35, no. 3 (2012).
  5. Mayer had died in 1762, so his reward went to his widow. 
  6. Cook’s Log, page 17, vol. 29, no. 3 (2006).
  7. Cook’s Log, page 1564, vol. 21, no. 4 (1998).
  8. Cook’s Log, page 10, vol. 34, no. 4 (2011).
  9. Cook’s Log, page 1775, vol. 23, no. 4 (2000).
  10. Cook’s Log, page 1968, vol. 25, no. 3 (2002).

Originally published in Cook's Log, page 25, volume 37, number 4 (2014).


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