# Newton has his say

To add to the foregoing discussion between Becky and Thony over different solutions to the longitude problem, it is interesting that the Jupiter's satellites option does not disappear from the discussion as quickly as you might think, despite what it did to Louis XIV's dominions. John Harrison might also have sympathised with Louis' frustrations, as it was, of course, land observations of Jupiter's satellites which were used to check the accuracy of his watch on the trials in 1762 and 1764.

The physician and mathematician John Arbuthnot considered the problem of longitude in his 1701 pamphlet An Essay on the Usefulness of Mathematical Learning in a Letter from a Gentleman in the City to his Friend in Oxford, in which he put forward finding a means to measure longitude as one of the greatest benefits that mathematics would bring. He specifically talks about the efforts of Edmond Halley on his Paramore expeditions which Becky has mentioned below, but he also notes how 'from the Observation of Jupiter's Satellites, we have a ready way to determine the Longitude of places on the Earth.'

More pertinent to the history of 'the Board' however is Isaac Newton's opinion given to the parliamentary committee set up to consider passing the 1714 Act. In this, Newton explained that: 'for determining the Longitude at Sea, there have been several projects, true in the Theory, but difficult to execute: One is by a Watch to keep time exactly, but, by reason of the motion of a Ship, the variation of heat & cold, wet and dry, and the difference of Gravity in different Latitudes, such a Watch hath not yet been made: Another is by the Eclipses of Jupiter's Satellites: but by reason of the length of Telescopes requisite to observe them, and the motion of a Ship at Sea, those Eclipses cannot yet be there observed. A third is, by the place of the Moon; but her Theory is not yet exact enough for this purpose: it is exact enough to determine her Longitude, within two or three degrees, but not within a degree.' You can see that Jupiter's satellites come second in Newton's list and that he notes the problems associated with this method but doesn't deem it as improbable as an accurate watch.

Even more interestingly, a copy of this opinion crops up in the Barrington Papers which I discussed in my last post. Barrington referenced the opinion in his justification to Parliament that further methods for solving the longitude might be found, notwithstanding Harrison's success with his chronometer. Just as Becky has discussed the long life of a magnetic solution to finding longitude, Galileo's dream of using Jupiter's satellites did not vanish into thin air.

Image credits: Cropped image of BGN/1 © Jonathan Betts.