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    Re: Moon - Antares
    From: Frank Reed
    Date: 2009 Jan 23, 21:26 -0800

    Brad, you wrote:
    "I have experimented with Mr. Reed's online lunars tool and have found that I 
    can obtain what appears to be the correct result for the immersion of Antares 
    for my lat, lon vs that which is illustrated online at the USNO for the June 
    It should be accurate for something like that within a few seconds of time, 
    but bear in mind that there is no "limb correction" in the lunar distance 
    calculator (I may add one at some point). The lunar limb has great basins and 
    great mountain ranges along it which can affect the exact timing of 
    And also:
    "I cannot get the tool to function for the emmersion of the same event.  I 
    suspect that Mr. Reed's program does not function for the non-illuminated 
    limb.  That would be an odd lunar, an un-illuminated limb is not used."
    Yes, that's right. There's no way to treat an emmersion as a lunar distance, 
    but perhaps there could be in some sort of "expert mode". Oh, and please call 
    me "Frank" or "Frank Reed" --no need to be formal with someone like me. 
    And you concluded:
    "Bowditch's navigator, mid 1800's, offers a method of solving this event and 
    in particular offers it up as a method of correcting your chronometer."
    This section in Bowditch was really irrelevant to most users. I suspect it was 
    something Bowditch wanted to see included in later editions for his own 
    particular, personal reasons. I've never seen a single case in 19th century 
    logbooks of anything remotely resembling an occultation calculation although 
    it's certainly possible that some land-based users of Bowditch's book tried 
    that procedure. It was probably just too much work for navigators at sea, and 
    anyone with the high competence necessary to work an occultation probably 
    would never be without a good sextant (which would let you do lunars any old 
    time as opposed to occultations which are rather rare events).
    And you concluded:
    "If your chronometer was accurate, however, it could be flipped over to determine longitude."
    Actually, what you get from an occultation with a known GMT is equivalent to a 
    "lunar distance line of position" with the distance equal to zero. It's not 
    necessarily a longitude. It could even be a latitude. Of course, if you 
    assume an exactly known latitude, then any line of position, except one 
    parallel to latitude, crossed with that known latitude will yield a 
    longitude. But this is a modern point of view... From a historical point of 
    view, an occultation, after some reduction work, could give GMT just like a 
    chronometer. That combined with a time sight (an ordinary altitude sight of 
    some body near the prime vertical converted to a value for local time 
    --effectively using your sextant as a sundial) would then give you longitude.
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