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    Re: Using a sextant from the bottom of a canyon
    From: Bill B
    Date: 2014 Feb 14, 16:01 -0500

    On 7/4/2013 6:31 PM, Frank Reed wrote:
    > It's a great idea though, and it's got me thinking of a variant for
    > sextant enthusiasts who do not happen to find themselves at the bottoms
    > of canyons and instead are stuck in suburbia on an early summer day at
    > local noon. If you have a good, straight roofline to your south it would
    > be analogous to the top edge of the canyon wall (and also straight and
    > possibly very close to horizontal so verticality might be less of a
    > problem). In this case, it might make more sense to measure the angle
    > from the reflected Sun in the A.H. to the roofline and then from the
    > roofline to the true Sun. Around noon, the "hang" at constant altitude
    > is long enough that you could alternate a couple of sights like this and
    > add up to get the Noon altitude. This might work really well.
    
    In theory, it's straight forward. In practice, a bit more problematic IMHO.
    
    Going back to a conversation perhaps 7 years ago when I attempted to
    calibrate my sextant for use as a 500 meters or less rangefinder (with
    little success), George Huxtable pointed out the distance between the
    horizon glass image and horizon mirror image (horizon glass center and
    index mirror axis) may well not be a constant, especially in the case of
    a front silvered mirror, so trig is less than adequate. Arriving at
    parallax for differing distances and angles would require precise
    optical equipment or an abundance of doggedness.
    
    So right off the bat you have the problem of parallax at short
    distances, plus parallax that changes with distance to the laser
    line/roof line/fence line etc. and may change slightly with the angle
    measured.
    
    I had considered (neighbors permitting) using one the highly accurate
    rotating laser that paints a horizontal circle. Problem being I would
    have to calibrate parallax for each horizontal angle and every distance.
    
    I did try a laser level on a fence approx. 30 feet away (only spot on
    the property where there is not a tree canopy). Eye level becomes
    critical at that distance. I set two marks into the ground so my feet
    were in exactly the same spot each time. Sadly even the movement of the
    rib cage while breathing can result in minutes of an arc differences.
    Holding your breath at exactly the same spot is difficult. Posture
    matters, as does the sole thickness of various shoes. The difference in
    objective lens height between a star scope and prism scope is about 5'
    for my set up.
    
    BTW, this all began years go when I made capturing Venus at meridian
    passage a project after Alex asked me how close to sunset could I see
    Venus. The real challenge is how close to the Sun can I see Venus.
    Figured after Venus passed in front of the Sun there would be a time
    when it would rise in the east just before sunrise. At roughly 55d N I
    could see Venus a *long* time before sunset :-)
    
    Back on track, my final solution for the height of eye problem was to
    make a Kamal of sorts. A board with toe marks to stand on with a line
    coming up and hooking on the sextant handle.
    
    I still use the marks on the ground to place the board. Those marks
    place me due south of an orange dot painted on a fence top about eye
    level. As long as the height of the dot and sextant, and distance to the
    fence remain constant solving the parallax problem is simply.
    
    As Frank noted using the sun at meridian passage. Especially easy if you
    precalculate the time to catch it exactly over the fence's orange dot.
    Also precalculate the Hs and compare that to what your sextant reads. A
    couple of those and you have your a constant you can add or subtract. (I
    call it my fence dip factor.)
    
    I love it. Shine a flashlight on the dot at night and catch the meridian
    passage of stars and planets. After a lot of trial and error I developed
    a system with a 2' SD.
    
    Accuracy past that is a problematic as the fence is slightly out of
    focus with the star scope, and way out with the prism scope. Sometimes I
    get lucky. A 0!3 moon observation last fall.
    
    You could calculate that factor series of vertical angles with targets.
    Meridian passages keep me busy on a summers eve.
    

       
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