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    Re: Children's land-locked "Sextant"
    From: Jim Hickey
    Date: 2007 Nov 27, 11:27 -0800

    The Davis Mark 3 sextant runs about $35 and would be good for a group
    of kids to get their hands on. Since there are no optics, it is pretty
    easy to line up the suns image using an artifical horizon.
    I blieeve Omar Reis on his web site shows how to make a sextant out of
    a CD.
    If I was very careful with the one I had and plotted the angles, I
    could get a LOP within about 4 minutes.
    You may also want to consider making an strolabe there are some kits
    available or you could fabricate one fairly easily. The theory behind
    a LOP might be a little deep for the age group but they might get more
    out of the astrolabe. You would also eliminate the horizon issue.
    On Nov 27, 10:20 am, Isonomia  wrote:
    > I'm camping next summer with a load of 11year old kids and wanted to
    > do some celestrial navigation and plot a position to within
    > 10-20miles.
    > Has anyone ever built a simple theodolite type sextant out of basic
    > DIY material and managed to obtain an accuracy that would allow a
    > basic position plot and if so how?
    > In particular I would like to build a form of "sextant" based on
    > measuring the angle above a horizontal plane of the sun using the
    > sun's shadow.
    > I'd welcome comments, suggestions or practical experience on:-
    > 1. How to create a horizontal plane to within a few minutes accuracy?
    > 2. How to obtain a good shadow/image, e.g. has anyone tried glass
    > lenses?
    > 3. How to measure an angle from the horizontal to the sun's image to a
    > few minutes?
    > All contributions greatfully received.
    > Mike
    > I started by considering using a sextant with an artificial horizon.
    > However, whereas it is a skill to find the sun and line it up with the
    > horizon (sea=go up, sky=go down), trying to line up two images of the
    > sun is an art. And aligning two images of stars is so difficult I've
    > only managed a sensible result on 50% of my tries.
    > My next idea was to use a real artificial horizon, in the shape of a
    > string set at a distance to give a low arc error. However, after a bit
    > of calculation regarding the distance (30-100m) and the length of the
    > string (10-30m), I've realised that any string big enough to see is
    > going to dip considerably unless it is under such extreme tension that
    > it is a positive hazard in an open area.
    > I then considered laser levels - but I'm not looking through a sextant
    > with potential laser reflections all over place - even if one could
    > see the laser line in daylight at the distance required.
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