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    Re: False Horizons
    From: Fred Hebard
    Date: 2007 Nov 20, 09:31 -0500

    You do need to know the semidiameter of the sun when using a
    reflected artificial horizon for precision work, as you match up the
    edges of the reflected and direct images.  I suppose centering the
    disks would work to a few minutes of arc (1 arcminute = 1 nautical
    mile).  Matching the edges you can easily get the error under 0.2' of
    arc, with practice.  Timing is the most important parameter as the
    sun moves about 1 arcminute in 4 seconds.
    The refraction table on the inside cover of the Nautical Almanac
    would work (it's inside the cover of the the orange almanac, but
    further in on the blue one).  The table also incorporates additional
    corrections, if I recall correctly.  The dip table is separate.
    Running a level string at eye height is a good way of making an
    artificial horizon and would be the best for kids, in my opinion, due
    to the difficulty of finding the image in the artificial horizon.
    You might want to calculate the error involved in having the string
    deviate from eye height.  Obviously, positioning of the string and
    observing point with respect to the sun's amplitude at various times
    of day are important.
    Good luck with getting kids interested for very long!  Soccer or
    basketball might be a better activity for long-term participation,
    even if you're a lousy athlete!
    Fred Hebard
    On Nov 20, 2007, at 6:43 AM, Isonomia wrote:
    > Refraction errors on false horizons
    > The great advantage with a false horizon for children (as opposed to
    > the sea), is that there is no need to pay for icecreams! AKA, taking
    > the children to the sea side - but  more importantly there is no need
    > to know the size of the sun, the height of  the waves, the height of
    > the person or anything else like that as they all  cancel out.
    > The great disadvantage is that you still have refraction through the
    > atmosphere (though not of the horizon!) and the only table I have
    > doesn't  list refraction of the horizon separately from that of the
    > direct view of  the sun so I would appreciate some information on this
    > error!
    > >
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