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    Re: Lewis and Clark lunars.
    From: George Huxtable
    Date: 2004 Apr 24, 11:49 +0100

    Bill Noyce commented-
    >> The big problem I still have is the Regulus measurement in the middle.
    >> I can't see them reading from one side of the nonius at one series,
    >> switching to the other side for the next series, and then switching back.
    >Is there any chance they used a different instrument for Regulus?
    Response from George-
    No, I doubt it. They had another instrument with them, a wooden
    quadrant/octant. But it had a wildly-different index error. And L&C were,
    as usual, careful to state their index correction as being -8' 45"; though
    it was stated just once for that night, not for each set of observations.
    In this early part of their journey, so far there has been no need to use
    their quadrant/octant, which was required later, in the Summer period, when
    high Sun meridian altitudes took the reflected noon Sun angle above 120deg
    and outside the range of the sextant. Then the quadrant/octant was
    available, used in back-observation mode for angles 90deg to 120deg.
    However, the octant was, for the first time, got out later on that day 3
    Dec 1803, to measure noon Sun altitude while the sextant was tied up over
    noon, locked in angle to measure equal-altitudes before and after noon.
    Unfortunately, there seems considerable doubt and confusion as to whether
    in that part of the journey the (immense) index correction for the octant
    should have +2deg or +1deg 20', which so far I have been unable to resolve.
    Fred Hebard asked-
    >A one-day error would come close to 15 degrees.  It also would place
    >the objects at a different spot in the sky, changing refraction and
    >parallax, possibly accounting for the remaining difference from the
    >moon's rate of movement of about 13 degrees/day.  Do moon altitudes
    >accompany the lunar distances?  These would settle this question
    My comments-
    Even in an extreme case, parallax and refraction combined can make a
    difference of no more than about 1 degree.
    No, L&C measured no altitudes with their lunars, which is a great pity,
    seeing they were all set up to measure by reflection in water. They left
    the Moon and star altitudes to be calculated, which may be one reason why
    nobody was able to disentangle their lunar data after their return (we seem
    to be discovering other reasons). Star reflections may have been invisible
    in a water trough, and for star altitudes they carried a tiltable mirror
    and spirit level. Not very accurate, perhaps, but star altitudes were
    required only to make a refraction correction, which at high star altitudes
    is a very small one. So they could have done a proper job, but they didn't.
    If they had given us some altitudes, we could have resolved the question of
    which stars they were observing by drawing in some position lines, seeing
    that we now know their real position so precisely.
    Ken Mudrew refers, in passing, to David Thompson's navigation in North
    America. Thompson was a real navigator. I wonder if Ken is in touch with
    the work of his fellow-Calgarian, Jeff Gottfred, and the series of articles
    he published in Northwest Journal?
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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