Rehabilitating Nevil Maskelyne - Part Two: Why lunar distance?

10 Feb 2011

Another element of the accusation that Maskelyne and the Commissioners of Longitude were wrong-headed in their treatment of John Harrison, is the idea that they were short-sighted to continue to consider the lunar-distance method as well, and that they were over-invested in it due to self-interest. Today many people have the impression that the lunar-distance method -- and earlier proposed methods of finding the longitude at sea that were based upon magnetic variation or observations of the moons of Jupiter -- was as laughable as the so-called 'powder of sympathy'. If that were true, then surely Maskelyne and the other Commissioners were stodgy / unintelligent / prejudiced / etc. for not having seen from the beginning that timekeepers were the way to go!

However, many intelligent people including Isaac Newton were not sure that clockmakers could make timekeepers hardy, affordable and regular-running enough to fully shoulder the burden of finding longitude at sea - at the time of the Act of 1714 but also well into the second half of the eighteenth century. Before Harrison, the technology just wasn't there, and seagoing timekeepers were not perceived in terms of precision and reliability. The longitude proposals which involved astronomical observations, or indeed compasses, had a somewhat greater affinity with centuries-old navigational practice than did precision timekeepers as well (although mariners sometimes used sand-filled hourglasses). As Katy mentioned in her last post, when Newton testified to a Committee of the House of Commons in 1714 about four potential longitude solutions, he pointed out that there were key problems with all of them (timekeepers, lunar-distance, observations of the eclipses of Jupiter's moons, and a much-publicized and sometimes parodied plan by Humphry Ditton and William Whiston involving moored ships launching mortars into the air at regular intervals along established trade routes).

As the decades passed, innovations in astronomy and technology and relatively successful sea trials made it seem more likely that the lunar-distance method and timekeepers could both provide a solution. Many people, including Maskelyne, thought that it might be most effective to employ the two together, if only reliable enough timekeepers could be produced. (The astronomer pointed out that in theory, errors in lunar-distance observations and calculations made aboard ship would not carry over from one day to another, whereas any unpredictability in the going of marine timekeepers would only become magnified as the days passed.) The astronomical data and computations upon which the abbreviated version of the lunar-distance method was based, still needed to be precise enough and widely available for years in advance to be of widespread use - which increasingly came to pass from about the 1760s onwards as Maskelyne published his Nautical Almanacs and associated Tables. However, weather could still pose a problem to observations, calculations still took some time to complete, and older mariners were sometimes reluctant to learn the skills necessary to use the lunar-distance method.

In order to be useful for finding the longitude at sea, timekeepers need to have a very predictable going rate (i.e. the amount of time gained or lost each day) and to be able to stand up to changing conditions aboard ship but also, in the end, to be produced widely and far less expensively than could Harrison's inventions. At first, the Harrisons and the Commissioners differed over whether John's timekeepers had been put to acceptably rigorous and defined testing with respect to the first set of requirements once they were trialled at sea and at observatories on land. Then in the 1760s and 1770s, the Commissioners were particularly concerned about whether or not the timekeepers could be reproduced for widespread use, since otherwise they would be of little use to navigation and trade as a whole.

The Commissioners periodically encouraged both Harrison's timekeepers and the lunar-distance method over the years - but the former actually received the first known award of money from the Commissioners and accounted for most of its expenditures until 1763, which challenges the clockmakers' beliefs that the officials were prejudiced against them or timekeepers in general. (Professor Eric G. Forbes, who was of the opinion that the German astronomer Tobias Mayer (left) could equally be called the 'discoverer' of longitude at sea, came to the same conclusion because the Commissioners approached Mayer's improved tables for the lunar-distance method just as critically as they did Harrison's claims.) By the time Parliament awarded £8750 to John in 1773, he had already received grants of money from them totalling more than £13,000. When he asked for more funding in 1746, he told the Commissioners that their periodic financial support of him was much appreciated and vital, since 'the Difficulty in the Contrivance & the Nicety in the execution of many parts of these new designs have so entirely ingrossed his time & thoughts for many years past as to render him quite incapable of following any gainfull employment for the support of himself & family'.

To find out how a number of 'cultural' differences may have contributed to the Harrisons' conviction that Maskelyne and the Commissioners were plotting against them, see Part Three of 'Rehabilitating Nevil Maskelyne' tomorrow.