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    Re: Perpendicularity check
    From: Alexandre Eremenko
    Date: 2004 Sep 23, 00:40 -0500

    Dear Frank,
    Thank you very much for your suggestions,
    especially on the star-to-star distances.
    (My main concern at this moment is checking my new sextant,
    and learning how to use it properly).
    On your on-line almanach, I am just curious:
    Why do you need to know my coordinates to tell me the Sun's GHA?
    I found the same strange feature in other on-line almanach
    as well.
    On Wed, 22 Sep 2004, Frank Reed wrote:
    > I have put together a very accurate
    > Nautical Almanac tool on my web site. It
    > can display almanac data for the standard
    > navigation objects accurate to about
    > 1 arcsecond for the period from 1750 to 2050.
    > The main address for the
    > various tools is
    > www.HistoricalAtlas.com/lunars
    > By the way, I think the "Complete On-board Celestial Navigator"
    > is very
    > clever, and its accuracy is sufficient for
    > the realities of standard late 20th
    > century position line navigation.
    I also think so. But my primary purpose at this moment is checking
    my sextant. After I reworked my sight reductions with more
    precise Almanach data, a strange thing happened:
    it looks like all the results just shifted by a fixed amount of
    0.7 to 0.8'. This seems very strange to me.
    The "Complete on Board" has some systematic bias against
    the real almanach? Incredible.
    Concerning the sight reduction,
    I also used the Complete-on-Board first
    and the errors were up to 3'. After some thinking I decided that
    this is not surprising because the Complete-on-Board tables
    do rounding on each step to the whole minute.
    So I wrote a simple program (on a spreadsheet) to do the
    spherical triangle with high precision.
    This improved my results, and now the average error
    (over 5-10 observations with few minutes intervals between
    the sights) is about 0.3'
    if I exclude the observations that stick out, apparently because
    of crude mistakes in my measurement or in my sextant reading.
    The Davis artificial horizon I ordered has not arrived yet,
    so I am using a plate with water, and the water surface
    is perturbed by the slightest air motion).
    But I am curious whether this .3' error can be also eliminated...
    and what is the sextant's ultimate precision.

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