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    Re: False Horizons
    From: Fred Hebard
    Date: 2007 Nov 20, 10:02 -0500

    oops, I should have said 1 arcminute of latitude = 1 nautical mile).
    On Nov 20, 2007, at 9:31 AM, Fred Hebard wrote:
    > 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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