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    Re: Children's land-locked "Sextant"
    From: Gary LaPook
    Date: 2007 Nov 27, 23:33 -0800
    Gary LaPook wrote:

    Here is a link to a site where you can download for free varius navigational publications including the American Practical Navigator, HO 9, Bowditch. Table 14 is the dip short of the horizon table.
    http://164.214.12.145/index/index.html
    gl

    Rebecca Lowry wrote:
    Hi,
     
    I'm also in the process of learning.
     
    Any chance you could send, or email, a couple to me as well?
     
    thanks,
    Wayne Lowry
    93915 prairie rd
    Junction city, Oregon 97448
    USA

    Greg Rudzinski <gregrudzinski---.com> wrote:

    I suggest demonstrating a polaris observation to your young students
    using the far shore of a lake as a horizon. Bowditch table 22 will
    give you the dip correction. A pub. 249 vol. 1. table 6. correction
    needs to be made (up to 44 minutes of arc). Table 6. requires a local
    hour angle of Aries entry. I have a homemade pocket table that
    simplifies some of this. E-mail me a mailing address and I will send
    you a few.

    Greg Rudzinski
    gregrudzinski---.com

    On Nov 27, 7:20 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
    >
    > HISTORY & WHAT I'VE STOPPED CONSIDERING
    >
    > 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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