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    Re: False Horizons
    From: Nicol�s de Hilster
    Date: 2007 Nov 20, 15:40 +0100

    Isonomia wrote:
    > As a separate question to my other post, I want to use a sextant on
    > land to demonstrate to children that you can locate your position with
    > the sun.
    > The obvious first problem is getting an accurate fix on the "horizon",
    > and  the obvious answer (even without a book) was to use a reflection.
    > All good in theory, but in practice it is full of problems.
    > 1.  there is the problem of keeping a bowl of water still with kids
    > jumping up  and down,
    > 2. trying to get children to stay one position long  enough to keep
    > the reflection in view
    > 3. using the filters makes it even worse because they can't see the
    > bowl of water and  the sun's reflection is quite a bit darker.
    > Eventually for my younger child  I tried setting up a plank in the
    > garden and tying the sextant up with  string to give a level, but the
    > darn sun moved so quick that it ran off the  end before I had a chance
    > to set it up with any accuracy. I'd appreciate any ideas!
    Perhaps making a simple quadrant would do for the kids and instead of 
    using two visors to look through you could make one with one small open 
    visor and a larger closed one to cast the smaller visor's shadow onto 
    the larger (so they do not have to look at the sun). After that exercise 
    you can explain that a sextant does a similar job using two mirrors.
    > 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!
    Last post on refraction in this list was NavList 3850 (check the 
    archives at http://fer3.com/arc/sort2.aspx?y=200711md), giving three 
    different formulae.
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