Editing Nevil Maskelyne (section)

===Measurement of longitude===
In 1760 the [[Royal Society]] appointed Maskelyne as an astronomer on one of their expeditions to observe the 1761 [[transit of Venus]]. He and [[Robert Waddington (mathematician)|Robert Waddington]] were sent to the island of [[Saint Helena]] in the [[South Atlantic]]. This was an important observation since accurate measurements would allow the accurate calculation of Earth's distance from the Sun, which would in turn allow the actual rather than the relative scale of the [[Solar System]] to be calculated. This would allow, it was argued, the production of more accurate astronomical tables, in particular those predicting the motion of the Moon.<ref>{{cite book|last1=Woolf|first1=Harry|title=The Transits of Venus. A study of eighteenth-century science|date=1959|publisher=[[Princeton University Press]]|location=Princeton, NJ}}</ref>

Bad weather prevented observation of the transit, but Maskelyne used his journey to trial a method of determining [[longitude]] using the position of the moon, which became known as the [[Lunar distance (navigation)|lunar distance method]].<ref>Nevil Maskelyne [http://cudl.lib.cam.ac.uk/view/MS-RGO-00004-00150/1 Journal of a Voyage to St Helena], RGO 4/150, Cambridge Digital Library</ref> He returned to England, resuming his position as [[curate]] at [[Chipping Barnet]] in 1761, and began work on a book, publishing the lunar-distance method of longitude calculation and providing tables to facilitate its use in 1763 in ''The British Mariner's Guide'', which included the suggestion that to facilitate the finding of [[longitude]] at sea, lunar distances should be calculated beforehand for each year and published in a form accessible to navigators.<ref name=EB1911/>

In 1763 the [[Board of Longitude]] sent Maskelyne to [[Barbados]] in order to carry out an official trial of three contenders for a [[Longitude prize|Longitude reward]]. He was to carry out observations on board ship and to calculate the longitude of the capital, [[Bridgetown]] by observation of [[Jupiter]]'s satellites. The three methods on trial were [[John Harrison]]'s sea watch (now known as [[John Harrison#H4|H4]]), [[Tobias Mayer]]'s lunar tables and a marine chair made by Christopher Irwin, intended to help observations of Jupiter's satellites on board ship. Both Harrison's watch and [[Lunar distance (navigation)|lunar-distance]] observations based on Mayer's lunar tables produced results within the terms of the [[Longitude Act]], although the former appeared to be more accurate. Harrison's watch had produced Bridgetown's longitude with an error of less than ten miles, while the lunar-distance observations were accurate to within 30 nautical miles.

Maskelyne reported the results of the trial to the Board of Longitude on 9 February 1765.<ref>{{cite web|title=Confirmed Minutes of the Board of Longitude|url=http://cudl.lib.cam.ac.uk/view/MS-RGO-00014-00005/79|website=Cambridge Digital Library}}</ref> On 26 February 1765 he had been appointed Astronomer Royal<ref name="Howse" /> following the unexpected death of [[Nathaniel Bliss]] in 1764; making him ''ex officio'' a Commissioner of Longitude. The Commissioners understood that the timekeeping and astronomical methods of finding longitude were complementary. The lunar-distance method could more quickly be rolled out, with Maskelyne's proposal that tables like those in his "The British Mariner's Guide" be published for each year. This proposal led to the establishment of [[The Nautical Almanac]], the production of which, as Astronomer Royal, Maskelyne oversaw. Taking even occasional astronomical observations was also the only way to check that a timekeeper was keeping good time over the course of a long voyage. The Commissioners also needed to know that more than one sea watch could be made, and that Harrison's methods could be communicated to other watchmakers.<ref>{{cite book|last1=Dunn|first1=Richard|last2=Higgitt|first2=Rebekah|title=Finding Longitude: How Ships, Clocks and Stars Helped Solve the Longitude Problem|date=2014|publisher=Collins|location=Glasgow|isbn=978-0007525867}}</ref>

The Board of Longitude therefore decided that rewards should be given to Harrison (£10,000), Mayer (£3000, posthumously) and others involved in helping to develop the lunar-distance method.<ref>{{cite web|last1=Higgitt|first1=Rebekah|title=Barbados or bust: longitude on trial|website=[[TheGuardian.com]]|date=9 September 2013|url=https://www.theguardian.com/science/the-h-word/2013/sep/09/history-science-longitude-maskelyne-barbados|access-date=15 April 2015}}</ref><ref>{{cite journal|last1=Howse|first1=Derek|title=Britain's Board of Longitude: The Finances, 1714-1828|journal=The Mariner's Mirror|date=1998|volume=84|issue=4|pages=400–417|url=http://blogs.rmg.co.uk/wp-content/uploads/sites/5/2013/03/howse.pdf|access-date=16 April 2015|doi=10.1080/00253359.1998.10656713|archive-date=23 May 2014|archive-url=https://web.archive.org/web/20140523173104/http://blogs.rmg.co.uk/wp-content/uploads/sites/5/2013/03/howse.pdf|url-status=dead}}</ref> Harrison was told that a further reward of £10,000 would be forthcoming if he could demonstrate the replicability of his watch. Although Harrison and his son later accused Maskelyne of bias against the timekeeping method, charges repeated by authors such as [[Dava Sobel]] and [[Rupert Gould]], Maskelyne never submitted a method or an idea of his own for consideration by the Board of Longitude. He was to play a significant role in having [[Marine chronometer|marine timekeepers]], as well as the lunar-distance method, developed, tested and used on board voyages of exploration.<ref name="Howse" />

Since the observations that fed into the ''[[Nautical Almanac]]'' were made at the [[Royal Greenwich Observatory|Royal Observatory, Greenwich]], the [[Greenwich meridian]] became the reference for measurements of longitude in the Royal Navy, and on British Admiralty charts. It was subsequently chosen for adoption as the international [[Prime Meridian]] in 1884.<ref>(JR Wills The Royal Society)</ref><ref>{{cite book|last1=Dunn|first1=Richard|last2=Higgitt|first2=Rebekah|title=Finding Longitude: How Ships, Clocks and Stars Helped Solve the Longitude Problem|date=2014|publisher=Collins|location=Glasgow|isbn=978-0007525867|page=221}}</ref>