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    Re: Longitude by lunar altitudes
    From: Frank Reed
    Date: 2010 Jan 5, 00:15 -0800

    Brad, you wrote:
    "I have a two issues of the same navigational text book (one issued about 1856 and the other issued about 1900) which explicitly discusses longitude by Moon’s altitude. "

    Sorry if I was misleading in my previous message. I did not mean to suggest that the method was unknown. It was widely known to the serious experts in nautical astronomy, but almost unknown to practicing navigators. It has come up again and again over the past 250 years in articles and books where authors somewhere in the middle ground between experts and practitioners would discover it independently. But it never made it into the standard, widely-used navigational manuals. It certainly never made it into common practice among sea-going navigators. There have been a number of people even in the past fifty years who "re-discovered" the method of finding longitude (really, GMT) by lunar altitudes. Chichester did. John Letcher did, and his comments on how it would be so much easier for navigators to learn since navigators are comfortable with taking altitudes echoes similar comments from 150 years earlier.

    I suspect that the book you're referring to is Chauvenet's. It would roughly fit the time span you mentioned, and I do believe he had a section on it, though I haven't double-checked. Bear in mind that Chauvenet has very little connection with practical navigation. Chauvenet was a land-bound nautical astronomer and one of the most prominent mathematicians in the USA in his era. His book included everything that he could squeeze in for every possible user from the common sailor to the professional astronomer in the best observatories of the day. He did write about longitude by lunar altitudes (somewhere, if not in his most famous textbook) and recommended it as a method that might be most useful to astronomers and surveyors on land, but not at sea.

    So WHY was longitude by lunar altitudes impractical for sea-going navigational use? Some of us, me included for a while, have much too cavalierly dismissed it on grounds that really don't matter that much in actual practice so long as you make reasonable rules on when it can be used. But there really is a considerable problem with that method if you're using an almanac from that era. At what intervals are the Moon's RA and Dec published in the almanac? At what intervals are the lunar distances published? Why is there a difference? If there's a lot more calculational work, the method ceases to be a practical replacement for the standard method of lunar distances.

    You concluded:
    "Being overwhelmingly busy right now at work, I can’t scan it in and post the method. I will try to get this to the list when I can."

    Don't worry. If it's Chauvenet, as I suspect, PLEASE don't bother scanning and posting. Several editions of his treatise are available online.


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