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Chronometer, in gimbal and fitted box

Navigators used many tools in the 18th and 19th Centuries to track their course at sea. The reality of navigation was a lot messier than stories about precision instruments might suggest.

Mixed navigational arts

No single technology or method ‘solved’ the challenge of accurately determining the position of a ship on the open ocean. Sailors continued to track location by ‘dead reckoning’: charting course (by compass), speed (by log (Image 1)), depth (by sounding lead (Image 2)), and judging currents by experience and lookout.

Harpoon log for measuring a ship’s speed and Walker’s Harpoon II Depth Finder
Image 1: Harpoon log for measuring a ship’s speed, T. Walker, c. 1865 (Wh.2625). Image 2: Sounding lead, “Walker’s Harpoon II Depth Finder”, by T. Walker & Son, London, c. 1875 (Wh.3401).

Typically, this work was then checked against chronometers (if carried) and compared with results calculated from lunar distance observations. All these techniques had their pros and cons, and the safest method was to use them in conjunction.

Untrustworthy clockwork

2-day marine chronometer, by Robert Molyneux, London, 1828
Image 3: 2-day marine chronometer, by Robert Molyneux, London, 1828 (Wh.2581).

Chronometers like the beautiful example shown in Image 3 are often seen as the outstanding, state-of-the-art solution to the problem of maritime navigation in the age of empire.

This is in part due to Dava Sobel’s engaging best-seller Longitude (1995), which has promoted the misleading notion that John Harrison’s marine timekeepers solved the problem of determining longitude at sea.

In reality, although Harrison’s H4 clock (completed 1759) and the chronometers that succeeded it did keep Greenwich time reasonably well aboard ship, many navigators remained sceptical of clockwork. Chronometers were extremely expensive, and their fragility and unreliability meant that several had to be carried together.

As the navigator Andrew Mackay noted in his 1804 instruction manual, the most prevalent rival method—computing longitude by sextant and lunar tables—was already established as a reliable and relatively simple technique:

“Of all the various modes which have been proposed to determine the longitude of a ship at sea, it is probable the method by lunar observations will always claim the pre-eminence, upon this account, that the lunar tables are now brought to a very great degree of exactness, as also the method of constructing instruments proper for taking the necessary observations; and the modes of calculation are far from being difficult.

"The same degree of confidence cannot be placed in time-keepers, as their rate of going is so liable to be altered from the least accidental injury.”(1)

Shipboard astronomers

For major voyages of discovery, professionally-trained astronomers were often hired to join ships, both to assist with navigation and to execute one of principal goals of these imperial enterprises: the claiming of new territory by naming and charting it. In 1791, for example, the Board of Longitude commissioned the astronomer William Gooch to join George Vancouver’s expedition to the Pacific, instructing him to:

“make nautical, astronomical and trigonometrical observations … for fixing the latitude and longitude of various points of … coast and country, and to ascertain their relative positions with respect to each another, and also to make nautical and astronomical observations during the voyage out and home, all tending to the improvement of geography and navigation.”(2)

Once in the Pacific, Gooch recorded in his journal that “No sooner had we left the Marquesas, than we fell in with four new Islands, never before seen by Europeans, ever since which time I have been employed in Surveying them, settling their Latitude, and Longitude, and laying them down on a Chart.”(3)

Unfortunately for Gooch, he would not live to see his observations returned to the Admiralty in London. In May 1792, Gooch and two other sailors were killed during a botched attempt to trade for supplies at Waimea on the island of Oahu.


  1. Andrew Mackay, The Complete Navigator: Or, an Easy and Familiar Guide to the Theory and Practice of Navigation (London: Longman and Rees, 1804), p.169
  2. Board of Longitude’s instructions to William Gooch, [1791], Cambridge University Library RGO 14/9, 61v-63v.
  3. Quoted in: Richard Dunn, ‘Heaving a Little Ballast: Seaborne Astronomy in the Late-Eighteenth Century’, in: Marcus Granato & Marta C. Lourenço (eds.), Scientific Instruments in the History of Science: Studies in Transfer, Use and Preservation (Museu de Astronomia e Ciências Afins, 2014), 79–100, on p.88.

Joshua Nall

Joshua Nall, ‘Navigational Arts’, Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, 2020.

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