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Close up of Wh.1490 scale

Astronomy was crucial for British imperial power at sea.

Observatory data-gathering and the practical skill of observing sun and stars aboard ship both played a vital role in tackling the naval challenge of the age: finding location at sea accurately. In Britain, improving the tools and techniques necessary for safe navigation became a major concern for political and scientific elites.

Latitude from the sun

Observing celestial objects was an essential skill for navigators in the age of imperial expansion. Instruments like the backstaff (Image 1) and octant (Image 2) were used to measure the altitude of the sun or pole star above the horizon, from which both local time and latitude (location north–south) could be found.

Backstaff, by Edmund Blow, London, 1736 and Octant, by John Goater, London, c. 1750
Image 1: Backstaff, by Edmund Blow, London, 1736 (Wh.2314). Image 2: Octant, by John Goater, London, c. 1750 (Wh.0355).

Longitude from the moon

Finding latitude at sea was relatively straightforward. Finding longitude (location east–west) was not. The instrument shown in Image 3, the sextant, would provide the most reliable and widespread solution.

Sextant, by Jesse Ramsden, London, c. 1785
Image 3: Sextant, by Jesse Ramsden, London, c. 1785 (Wh.2122).

The challenge for instrument-makers was dividing the sextant’s scale precisely enough that it could take measurements the backstaff and octant couldn’t: of the angle between the moon and certain prominent stars, down to a fraction of a degree.

These measurements provided a roundabout solution to a tantalisingly simple problem. To determine longitude, navigators needed both local time and time at a reference point (chosen to be Greenwich). Clocks proved inaccurate on rough seas, so astronomers offered a celestial solution—navigators could calculate Greenwich time by comparing measured ‘lunar distances’ with astronomical data tables.

The stakes were high: In 1714, following several losses to the British fleet, Parliament had offered a massive £20,000 prize for a solution to finding longitude at sea. For the ‘lunar distance’ method to prove practicable, it required affordable, accurate sextants, and lots of them.

The dividing engine

It was the famed London instrument-maker Jesse Ramsden who developed the first practical solution to making sextants quickly, accurately, and in large numbers. His answer was a machine: the dividing engine (Image 4).

Dividing engine, copy of Ramsden’s second engine, London, c. 1800.
Image 4: Dividing engine, copy of Ramsden’s second engine, London, c. 1800. This example was in work dividing sextants for nearly 150 years! (Wh.3270)

To find longitude, a sextant’s divided scale needed to measure angles between moon and star accurate to better than one-sixtieth of one degree. Ramsden could divide the scale on his sextants by hand, but this was time-consuming work.

The mass-production of sextants required an industrial solution, so the Board of Longitude funded Ramsden to mechanise the process.

His first dividing engine, with 2160 teeth cut at precise regular intervals and driven by an endless screw, could do in minutes what it had taken weeks to do manually. Ramsden’s design was made public and rival makers soon built copies, including the Whipple’s example.

As a mass-produced commodity, sextants soon became an essential tool of open sea navigation. They were often used in conjunction with another famous technology developed with the support of the Board of Longitude: the marine chronometer.

Joshua Nall

Joshua Nall, ‘Astronomy at Sea’, Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, 2020.

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