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    Re: Lewis and Clark lunars: more 1803 Almanac data
    From: Frank Reed CT
    Date: 2004 Apr 17, 06:21 EDT
    Ken M wrote:
    " In the few lunars that I've done (I'm a complete beginner at this), the moon's apparent motion with respect to the ecliptic stars/planets has always been around 27"/min. I know that this can change due to refraction and parallax, even for a star or planet that's right on the moon's path, but I don't know what the limits of this change are. I assumed that the apparent motion would remain pretty close to the true motion. So for a first order approximation ... "

    Sure. It's a reasonable first-order approximation. But the rate can easily be different from this 1st order "half a minute every minute". I spent a little time earlier tonight generating predictions of apparent (topocentric) lunar distances for the estimated position of the L&C camp on December 3, 1803.

    For every half hour, starting at 0400 GMT on Dec 3, 1803, I find for Aldebaran lunars (apparent center-to-center distances):
    60.994
    61.268
    61.522
    61.761
    61.987
    62.200
    62.403
    62.597
    62.785
    62.966
    63.143
    63.314
    63.483
    63.650
    63.814
    63.967

    For every half-hour starting at 0600 GMT, I find for Regulus lunars:
    18.183
    17.981
    17.782
    17.587
    17.398
    17.212
    17.030
    16.851
    16.673
    16.494
    16.313
    16.129

    For refraction, I assumed that it was good and cold (middle of the night in December!), but standard refraction would not make much difference. Notice that the rate of change varies substantially during the night.

    The above LDs are center-to-center topocentric distances for the supposed camp site near St. Louis. These LDs include the effects of refraction and parallax. I think they're right, but I did only basic error-checking.

    Note that the rate of change in distance is not emphatically inconsistent with the reported observations. Lunars do sometimes change that slowly... BUT the distances themselves still just don't make any sense at all. I've experimented with various possibilities. I considered local time at variance with the LAN sight the next day. That didn't help... I considered various (systematic) errors in recording the numbers. No luck there... I experimented with errors in near limb versus far limb --even near limbs taken from the gibbous side of the Moon. But still no luck... (there might still be a little life in that idea though).

    I have no real hypothesis, but maybe it's just plain junk data. I've seen in some 19th century whaleship log books occasional cases where it's very likely that data has been transcribed into the "official" log from someone's notes with blatant transcription errors. Maybe this case is nothing more than sloppy copying, after-the-fact.

    Frank E. Reed
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
       
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