Determining Longitude and Trading Humans

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Timeline of longitude and British slave trade

A long time ago, before sleek objects appeared in our pockets that could tell us our geolocation by a 7 m accuracy by calculating the time it takes for a signal to reach them from at least four different satellites; before those satellites that make the global positioning system (GPS) were sent to orbit the Earth, people didn’t where they were.

Of course, there were maps and landmarks and you could determine your location on the north-south axis of the Earth by observing the position of the sun at noon. But things got tricky on the high seas, where there are no static guides to use for navigation, no watermarks. You could determine your latitude with the position of the sun, as mentioned, but figuring out longitude, the location on the east-west axel, was impossible.

Image result for Hipparchus, the Greek astronomer


Latitude and longitude were first specified by Hipparchus, the Greek astronomer (190-120 BC). Instead of the prime meridian line passing through Greenwich Observatory like now he had it pass through Rhodes. Most importantly, he argued that absolute time could be determined by measuring time between lunar eclipses. This absolute Rhodes time could then be compared with the local time wherever you were on earth. However, as sufficiently accurate clocks did not yet exist, this method did not work.


Then there was the Dutch astronomer Gemma Frisius who in 1530 proposed to calculate a ship’s position by setting a clock on departure and keeping it at absolute time. The absolute time could then be compared with the local time (measured at noon). The issue here was again time, timekeeping and lack of sufficiently accurate clocks. In addition, in 1612 Galileo Galilei suggested to use the moons of Jupiter and use their location as an universal clock. However, this was impossible on the unstable surface on board a ship.

Sir Cloudesley Shovell, by Michael Dahl, about 1702-5 (National Maritime Museum)

Because of the longitude issue, ships were not able to determine their exact position whilst at sea. This meant that you might sail towards an entirely wrong destination for many sea miles without knowing. And when you did indeed reach a coast, you didn’t really know which coast it was. Take the case of Admiral Sir Cloudesley Shovell sailing with his fleet and approaching the coast of England on 22 October 1707. The Admiral mistook his location and didn’t realize the land he was approaching was that of the Isles of Scilly, leading him to crash four of his ships into the rocks and drowning not only himself but 1400 plus of his sailors.

Thus, due to the difficulty of determining longitude, sailing was risky and slow. If you were trying to avoid unknown waters and took the safe route, you risked getting looted by pirates and privateers. But if you didn’t take the safest route or if you got lost on the Atlantic, there was a great chance of facing the fate of Admiral Sir Cloudesley Shovell. Knowing the exact position of your ship also meant that you could sail directly to your destination without sailing around aimlessly.

There had been attempts long before the 1700s and all great European maritime powers had vested interests in making sailing safer and quicker, thus gaining a strategic and economic advantage over others. The British got into the geolocation-arms-race in 1714 when the Parliament passed the Longitude Act, offering a large sum of money to whoever who could offer a practical solution to the longitude problem.

One of the permanent exhibition galleries called Time and Longitude at the Royal Observatory in Greenwich is dedicated to the British quest for finding longitude and features the solution, namely clocks made by the clockmaker John Harrison. The clocks were steady enough to keep absolute time, which could then be compared with the local time. Calculating the difference between the two times, you were able to convert the time difference into degrees of longitude.

What I’d like to suggest is an addition to the Time and Longitude exhibit. Down the hill from the Royal Observatory, on the second floor of the Royal Maritime Museum, in the collection of the Caird Library there is a copy of the wood block print of the slave ship Brookes. The copy archived in Greenwich was printed in 1791, but the Brookes diagram was first printed the year before by the Committee for Effecting the Abolition of the Slave Trade. It shows what a lot of ships sailing under the British flag were carrying: humans, packed tightly in the hull of a ship.

The addition of the Brookes diagram to the Time and Longitude display would serve as a reminder of why Britain was so keen on finding a solution to defining longitude at sea. At the time when John Harrison was working on determining time Britain was becoming the foremost European country engaged in the slave trade. I’d like to suggest that the political support and the financial rewards granted by the Parliament for being able to determine a ship’s geolocation should be seen in the context of transatlantic slave trade.




Atkinson, R d’E (1963) Time and longitude

Browne, Simone (2015) Dark Matters. Duke University Press

Gerrard, Ted (2007) Astronomical minds : the true longitude story

Hochschild, Adam (2012) Bury the Chains. Macmillan

Howse, Derek (1980) Greenwich time and the discovery of the longitude, Oxford University Press

Jennings, J. (1997) The Business of Abolishing the Slave Trade, 1783-1807. London, Portland. Frank Cass.

Wood, M. (2000) Blind Memory: visual representations of slavery in England and America, 1780-1865. Manchester. Manchester University Press.


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