Barbados or bust: longitude on trial

Cross-posted from The H Word blog, where this post first appeared on 9 September 2013.


Barbados beach scene (perhaps not quite what Nevil Maskelyne experienced in 1763)

On 9 September 1763 a young curate and Fellow of Trinity College, Cambridge, set off for Portsmouth. He was to travel to Barbados on a voyage that would test the accuracy and practicality of three different methods of finding longitude at sea. At stake were potential rewards from the Board of Longitude.

The curate, Nevil Maskelyne, was also an astronomer and mathematician who became Astronomer Royal in 1765. I am currently editing a book of essays centred around Maskelyne, which, like the book I am co-authoring on the history of longitude, is due out next year for the tercentenary of the first Longitude Act. Working toward that anniversary, I spotted this one.

Back in 1763, Maskelyne was instructed to do two things. Firstly, he was to make longitude-determining astronomical observations during the voyage and, secondly, to make observations on land when the ship arrived in order to determine the island’s position, a prerequisite for an effective trial.

The three “methods” under trial in 1763 would be deemed successful if they succeeded in predicting Barbados’s longitude to within a degree or half a degree. They were:

1) A marine chair made by Christopher Irwin that was intended to steady an observer to allow him to measure the positions of Jupiter’s satellites at sea. (Eclipses of Jupiter’s moons were already used as a celestial timekeeper* to determine longitude on land: these were the observations Maskelyne made at Barbados.)

2) The latest version of the lunar tables of Tobias Mayer, which helped predict the position of the Moon and allow it to be used as a timekeeper using the lunar-distance method.

3) The latest mechanical marine timekeeper, and first sea watch, made by John Harrison.

Maskelyne and his assistant, Charles Green, were to make the ship-board observations and calculations necessary for the use of the first two methods. Harrison’s watch, now known as H4, would travel out separately with Harrison’s son William.

All of the methods worked in theory; the sea trial was to establish whether they worked reliably in practice. Only Irwin’s chair was a failure. Remarkably, two plausible methods of finding longitude had, finally, come to fruition at almost exactly the same time:

1757: Mayer sent his theory of the Moon’s motion to the Board of Longitude. It proved capable of making pretty good predictions – an object that had defeated Isaac Newton’s best efforts. Harrison, who had received rewards amounting to £2750 during 1737-1757, abandoned the development of his large marine clocks (H1H2H3) and thew his efforts into his watch.

1761: The potential of Mayer’s tables and the lunar-distance method was demonstrated by Maskelyne and his assistant, Robert Waddington, during a voyage to St Helena, where they had been sent by the Royal Society to observe the transit of Venus. Harrison sent his watch on trial to Jamaica and claimed an excellent result. Unfortunately, the trial was declared void because of uncertainties about the longitude of Jamaica and the watch’s rate, Harrison had to make do with another £1500.

1763: The Barbados trial was the really significant one – Mayer’s tables (via the lunar-distance method) and Harrison’s watch were both officially found to have met the necessary criteria. The Board of Longitude had two methods on their hands… potentially.

The lunar-distance method was complex and time-consuming and could only become useful if enough navigators were trained to undertake the required observations and calculations. Ideally, part of the work needed to be done for them, via the publication of regularly updated predictive and pre-calculated tables.

Harrison’s watch had worked well, but the question was whether another such machine could ever be made. Could one be made by another workshop? Could a marine timekeeper be made that was less costly than the exquisite H4?

In 1765, an Act was passed that divvied up the spoils and aimed to help make these potential methods “practicable and useful”. Harrison would receive £10,000 only if he revealed his method (i.e. the mechanism and the methods and materials involved in the construction of his watch) to other artisans. A further £10,000 would be paid out if more timekeepers could be made and successfully tried.

Tobias Mayer had died in 1762, but £3000 was paid to his widow in return for his papers. £300 went to the mathematician Leonhard Euler as a reward for his equations, which had greatly enhanced the accuracy of Mayer’s tables. A further £5000 was held out as a reward for the future improvement of the tables and, perhaps most significantly, the Board committed to the regular publishing of a Nautical Almanac, to be overseen by the brand new Astronomer Royal, Nevil Maskelyne.

The Barbados trial was not a competition or a race for a prize, although Christopher Irwin certainly found his marine chair out of the running. Rather, it confirmed two promising methods that required further investment. The Board of Longitude committed to this, seeing that they were not mutually exclusive. The lunar-distance method could be made available more quickly and was the only means of checking the performance of a ship-board timekeeper.

While Harrison’s paranoid belief that Maskelyne was prejudiced against him and his watch has become the dominant version of this story, it is not backed by the evidence. As Astronomer Royal and Commissioner of Longitude from 1765-1811, Maskelyne was to aid the development of both of the methods that his 1763 voyage had helped to prove.

* The difference in longitude of two places is equal to the difference in their local times.


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