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    Re: Beginner with inaccurate results
    From: George Huxtable
    Date: 2005 Aug 31, 09:54 +0100

    Thanks to Ken Muldrew for putting right my misunderstanding about the Davis
    artificial horizon; that it is in fact a liquid horizon rather than one
    using a spirit-levelled glass.. This means that all I said about the use of
    the art. horizon was irrelevant and can and should be ignored.
    A discrepancy of 5 arc-minutes, or 5 miles, is a serious matter that should
    have a discernable cause. Getting on-land Sun altitudes by reflection in a
    liquid is a rather precise technique; better than measuring altitudes at sea.
    Could it be a refraction error in the glass cloche above the liquid
    reflector, due to imperfections in the glass plates, not being
    plane-parallel? Test for this is by measuring with the cloche (or the whole
    device) reversed through 180 degrees, which should reverse any discrepancy.
    But I would be surprised to find such large errors in the cloche.
    Otherwise, we need to ask some silly questions. One that comes to mind is
    the GPS position that was quoted. Was this taken from (roughly) the same
    position from which the observation was made, or could it have been from a
    few miles away? Was it displayed to a WGS84 datum, or had it perhaps been
    converted to a local Norwegian mapping datum? Not that I would ever expect
    such datums to differ by a matter of several miles!
    Asbjorn reises the question of refraction in his shades. That is indeed a
    weakness of some plastic sextants, but 5' error is far more than I would
    expect. Can he avoid changing shades between ascertaining index error and
    observing with the art.horizon, perhaps by choosing a clear-glass cloche
    rather than a coloured one?
    Asbjorn poses an intriguing problem, to me and to others, and it would be
    good to get to the bottom of it.
    At 23:33 30/08/2005, you wrote:
    >On 30 Aug 2005 at 22:59, George Huxtable wrote:
    > > My suspicion falls on the glass artificial horizon, and the spirit level
    > > that was used to set it horizontal.
    >Asbjorn is apparently using a Davis artificial horizon which is a tray for
    >liquid with a cloche that can be covered with dark or clear glass, so I
    >don't think that is the source of his mysterious (but apparently
    >systematic) error. Hopefully Asbjorn will correct me if I'm mistaken about
    > > At 11:24 30/08/2005, Asbjorn wrote
    > > >I use a Davis artificial horizon with colored glass.
    >Ken Muldrew.
    Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222,
    or from within UK 01865 820222.
    Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
    5HX, UK.

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