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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Maskelyne's "British Mariner's Guide"
From: George Huxtable
Date: 2007 May 30, 22:29 +0100
From: George Huxtable
Date: 2007 May 30, 22:29 +0100
Frank mentioned, in a passage about another topic (Subject: [NavList 2918] Re: Self Sufficiency) - "I've recently been re-reading Maskelyne's "British Mariner's Guide to the Discovery of the Longitude at Sea and Land" (1763) " ================= That's a remarkable work, containing all sorts of interesting stuff. Very advanced for its early date of 1763. Maskelyne impresses me by the clarity and precision of his careful explanations about a complex matter. The only error that I've caught is on page 38, where he says- "... from the observed distance of the sun and moon's nearest limbs ... subtract the sum of the semidiameters of the sun and moon; this gives the apparent distance of the centres of the sun and moon." For "subtract", one should read "add", of course. The tables at the back contain, with much other stuff, data for predicting the position of Sun and Moon, in condensed form, from 1760 to 1780. But the procedures for making those calculations, given in chap IV, (particularly in part 2 for the Moon) are mind-bogglingly complex. They were based on Mayer's observations and analysis. It would be an exceptional navigator who could meet the challenge of making those calculations at sea, though some managed to do so. Notably John Harrison (not the John Harrison of the clock), purser on Dolphin during her circumnavigation, who deduced an accurate longitute for Tahiti in 1766, this leading the way for Cook three years later. Presumably, Harrison was working from the British Mariner's Guide. I have worked my own way through an example, but not without some false-paths and errors on the way; knowing what the right answer had to be, made all the difference. So I can appreciate Harrison's achievement, in getting the right answer for Tahiti, with no way of cross-checking it. Frank may find it of some interest to try such a prediction for himself. Indeed, an interesting exercise, for any skilled computer whiz with time on his hands, might be to download those tables into a spreadsheet, and then automate that prediction procedure, which involves lots of lookups and interpolations. The results, for various dates between 1760 and 1780, could be compared with modern back-predictions for those dates. That task was done in the later Nautical Almanac, starting in 1767, in which two (human) computers, unaware of each other's results, were given the job of calculating Moon positions at 12-hour intervals throughout the year, which were then interpolated to three-hour intervals. Lunar distances were then pre-computed from those results. There's some unexpected stuff in the Mariner's Guide, including advice to mariners about the correct spacing of the knots in a log-line, and the timing of a sand-glass to go with it; a matter about which much confusion existed in 1763. George. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---