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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunar Distance in Wikipedia
From: George Huxtable
Date: 2007 Aug 22, 10:15 +0100
From: George Huxtable
Date: 2007 Aug 22, 10:15 +0100
Jim van Zandt and Renee Mattie have both attended to that Wikipedia article, and between them they are transforming it into something really worthwhile. I have a few comments, though. Can anyone see a way to credit Clive Sutherland with that nice drawing? Not that he has any wish to copyright it; he doesn't. =================== The article states, under "History", "Lunar distance tables last appeared in the British Nautical Almanac for 1904, and in the USNO Nautical Almanac for 1912". I don't know about the US version, but have suspicions that the 1904 date is a bit out. Lecky, in "Wrinkles" (actually, a later editor, in my 1917 edition) states on page 758 "The Nautical Almanac for 1907 ... is without tables of lunar distances ..." May, in "A history of marine navigation", 1973, page 40, states "In 1909 the Nautical Almanac ceased to publish the necessary tables..." Hewson, in "A history of the practice of navigation", 1983 ed., page 241, states "... in 1908, lunar distances were omitted from the Nautical almanac" All three authors were writing from a British background, and would have been referring to the British almanac. So here we have three authors, all differing from that 1904 date, and from each other! It isn't a vitally important matter, but let's get it right. Has the British "Nautical Almanac" for those years been digitised, and does any listmember have easy access? It would then be the work of only a few minutes to discover the year of disappearance. Otherwise, I will ask Catherine Hohenkerk, of HM Almanac Office, who has a complete stack of back-numbers. ================== The last paragraph, under "Method", now states- "Knowing Greenwich time and the altitudes of the moon and the other body, the navigator can apply the intercept method to find his latitude and longitude. Alternatively, the navigator can first determine local time, and then longitude.[1]" That is unarguably true, now, but gives quite the wrong slant, because the first alternative was not generally available in the period we are concentrating on, ending in the mid 19th century. There were two reasons- First, Sumner's line of position method was not published until 1843, and the intercept method improvements, by St Hilaire, not until the mid-1870s. By this date lunars were in serious decline. Second, Moon predictions, in terms of GHA and dec, were not provided in such a way as to allow Moon altitudes to be accurately calculated, in early almanacs. In the 1767 almanac, and for some time after, these quantities were provided only at Greenwich noon and midnight, between which the Moon's declination could change by more than 2 degrees; far too infrequent for proper interpolation. By 1864, the next almanac I have, Moon dec was predicted for each hour, just as in modern almanacs. I don't know when the omprovement took place. So I think the intercept method should NOT be quoted as the primary method of position finding, to go with a discussion of lunars. Instead, the emphasis should be on direct determination of longitude from difference between local time and Greenwich time. What do others think? ================ Under "Errors", there's an explanation of why an error of 1' in lunar distance gives rise to an error of 30' in longitude, and a conclusion "So lunar distance can never be a precise way to determine longitude. After that, someone has inserted {cite}. I wonder why? One follows from the other. Although I am all in favour of rigour, in presenting evidence for statements that are made, I feel that this is being unduly pernickety. And I am not convinced that evidence presented in a citation from a book or a journal article is always of value, anyway, as you can see from the contradictory statements, above, about the date of omission of lunar distances from the almanac. 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 -~----------~----~----~----~------~----~------~--~---