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    Re: False Horizons
    From: Mike L
    Date: 2007 Nov 20, 08:59 -0800

    > 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.
    Fred Hebard
    Even a very tight string dips by 1cm over 5m, assuming you want a
    horizon of at least 10deg width, then this is equivalent to a 5m
    string at a distance of around 30m. 1cm I calculate to be around 1' of
    error at 30m.
    If in addition you assume +/- 1cm eye height error, this is another 1'
    That's not forgetting the problem of getting it level in the first
    place. Even my water level is only 10m long so, that requires setting
    the level four times (0-10m, 10-20m, 20-30m and end-end of string), so
    I'd be lucky to get the whole setup accurate to +/-1cm.
    And .... by that time the sun has moved, gone behind a cloud, or
    someone has tripped over the string!
    Contrast that to the simplicity of filling a bowl with water!
    Now even for an adult, keeping the height within 3mm is going to be
    difficult so the temptation is to increase the distance to 30m or even
    100m allowing 1cm or3cm error in height. But the further away the
    horizon, the longer it needs to be (at least 10deg wide I thought
    seemed about right), which means
    at 10m it must be 1.7m long level to +/-3mm,
    at 30m it must be 6m long level to +/-1cm
    at 100m it must be 17m long and level to +/-3cm.
    This means likely errors at 30m are:-
    +/-1cm dip in string = 1'
    +/-1cm error in setting sextant heihgt = 1'
    If yo allow +/-1cm error in place sticks etc, the total error is
    beginnning to mount even without having added on errors in using the
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