St Helena or bust

20 Jan 2011

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This year we'll be talking a lot about Nevil Maskelyne, and for good reason. Not only is it the bicentenary of his death - which a number of organizations including the NMM will be commemorating - but Maskelyne is also an important part of the Board of Longitude story for many reasons. One of these is his role in the development and promotion of the lunar distance method (or lunars).

In that context, today marks another anniversary, because it's 250 years to the day since a 28-year-old Nevil Maskelyne set out on a voyage to St Helena. Maskelyne was travelling there to view the transit of Venus that was due to take place at the end of May, although he did a lot else while he was there. To get to the South-Atlantic island he had an 11-week voyage on the East India Company ship, the Prince Henry. He didn't waste his time on board, however, using the voyage to test the lunar distance method. Maskelyne's surviving journals and other records show that he and his assistant Robert Waddington made observations of the sun, moon and stars using a 'Hadley's quadrant' or octant. They then used the observations and a new set of astronomical tables by Tobias Mayer, an astronomy professor from the University of Göttingen, to calculate the ship's longitude, which Maskelyne compared to the dead reckoning estimates of the ship's officers. By the end of the voyage, Maskelyne reported that his longitude reckoning was only 1½° in error compared to errors of up to 10° from some of the ship's officers, and after his return to England in 1762 he wrote that 'longitude can always be found within a degree or very little more'. The following year he published The British Mariners Guide, which described the lunar distance method (as did Waddington in A practical method for finding the longitude and latitude of a ship at sea, by observations of the moon). This formed the basis of his successful promotion of the method, which he championed on the Board of Longitude as Astronomer Royal, and for which he led the publication of the Nautical Almanac from 1766.