Papers of the Board of Longitude : Correspondence regarding methods of establishing longitude by lunar distances

Papers of the Board of Longitude

<p style='text-align: justify;'>This volume gives a sense of how lunar methods for finding longitude at sea developed in the later period of the Board of Longitude's existence, from 1802 to 1828. By this time, the method of lunar distances was well established. Using a sextant to measure the angle between the Moon and the Sun, or a fixed star, a navigator could determine their longitude by reference to the tables printed in the Nautical Almanac. However, navigators needed to make a number of corrections to these measurements in order to account for the effects of parallax and atmospheric refraction. This process of 'clearing' the lunar distance typically required a lengthy set of complicated calculations and tended to impede the regular use of the lunar distance method. The correspondents writing to the Board in this volume proposed a variety of means to simplify these calculations in the hope of reward.</p><p style='text-align: justify;'>All the correspondents contained within this volume are based in Britain, ranging from Penzance to Aberdeen. The majority are from naval backgrounds, either in active service, such as Captain Spencer [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections/objects/127676.html'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>] of HMS Owen Glendower [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections.html#!csearch;authority=vessel-336929;vesselReference=vessel-336929'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>], or retired, such as Mr Billingsley of the East India Company. One correspondent, Mr Violani, a French prisoner of war held at Chatham Dockyards for over 12 years, offers his method as part of a <a href='' onclick='store.loadPage(1);return false;'> bargain for better treatment (RGO 14/33:469)</a>.</p><p style='text-align: justify;'>The proposed solutions can be divided broadly into mathematical, geometrical and mechanical methods. Indeed, the diversity of approaches taken to clearing the lunar distance is indicative of the range of methods practised for finding longitude at sea, even in this later period.</p><p style='text-align: justify;'>Amongst the mathematical solutions, there is variation. Some methods rely more heavily on trigonometry, such as Mr Adams, a surveyor of artificers' works at Plymouth Dockyards. Others make greater use of logarithms, such as Mr Flanagan, a master mariner based in Liverpool. Those proposing mathematical methods also tend to request that the Board print further supplementary calculating tables in order to aid the uptake of their particular approach. The contents of the Nautical Almanac were therefore actively debated. Naval men, such as Mr Beckerleg, a schoolmaster onboard HMS Caledonia [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections.html#!csearch;authority=vessel-299455;vesselReference=vessel-299455'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>], used their position of influence to gain support for their particular tables in advance of petitioning the Board. In one letter of 1828 Beckerleg points out that his pupils are already familiar with his tables, having worked with them onboard the HMS Caledonia. In response to such requests, the Board sent copies of prospective tables to navigators in the Royal Navy, seeking an assessment. In September 1822 the Board received word from Captain Spencer that Schumacher's tables were of <a href='' onclick='store.loadPage(1);return false;'> 'great service' (RGO 14/33:383)</a>.</p><p style='text-align: justify;'>Geometrical solutions are typically advanced as an aid to purely mathematical methods. Lieutenant Nepean, for instance, points out how navigators can clear the lunar distance more quickly by <a href='' onclick='store.loadPage(1);return false;'> drawing intersecting (RGO 14/33:368)</a> lines at certain points in the calculation.</p><p style='text-align: justify;'>A number of correspondents also propose the use of an instrument or machine to aid calculation. Writing in 1828, Beckerley gives a particularly detailed account of his 'Parallaxometre' complete with a <a href='' onclick='store.loadPage(401);return false;'> coloured drawing (RGO 14/33:465)</a>. He claims that, with the aid of the instrument, clearing the lunar distance could be completed in 'at most two minutes' and also 'by those who are not sufficiently acquainted with the Lunars'. The instrument consists of a large angle, similar to a sextant, marked in degrees. Affixed to this are four connected straight rulers. By manipulating the intersections of the rulers, the instrument computes the correction required for parallax in degrees and minutes. Mr Kirkpatrick of Liverpool also writes to the Board in 1824, describing his 'Lunar Calculator' and suggests a price of £20 per instrument.</p><p style='text-align: justify;'>James Poskett<br />History and Philosophy of Science<br />University of Cambridge<br /></p>