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    Re: Measuring (and Calculating) Dip
    From: Bill B
    Date: 2014 Feb 14, 16:00 -0500

    On 3/3/2013 1:46 PM, Bruce J. Pennino wrote:
    > Then marking trees in my back yard so I could string a taut clothesline
    > from tree to tree.....maybe a few streamers hanging down. Call that my
    > horizon. Dip = zero?  Then I don't have to use an AH. Unfortunately the
    > trees in my yard are close. I
    This approach is full of problems when using a hand-held sextant. I set
    up a simple range due south using the top of a neighbor's fence (approx
    6 ft high) with an orange target dot, and two fender washers nailed into
    the ground approx. 30 feet from the fence for my toes. Due to the tree
    canopy blocking the heavens, observation position was only 30 ft from
    the fence. Purpose was to catch bodies at meridian passage--especially
    Venus during the day. Illuminating the dot with a flashlight allows for
    catching meridian passages through the night.
    Parallax becomes a real bugger, and even with the eye on the same level
    as the fence dot, what I term a "parallax correction" must be
    determined. Height of eye is critical at this distance. Posture,
    breathing (lungs full or empty). distance feet are spread etc. can
    change results by 7' either way. Using a 7x scope with the objective
    lens raised over the ocular lens as opposed to a straight scope can make
    for a 3'+ difference. Focus/depth of field is also a problem, especially
    with the 7x35 prism scope. I had to determine which portion of the
    blurred orange dot to use.
    After determining a number for dip plus parallax (brute force method) I
    am able to come withing 2' on average.
    I did consider a surveyors laser transit projected on the fence for a
    broad horizon, but in the end unless all the objects illuminated by the
    beam are equidistant, calibrating parallax becomes a tedious challenge
    at short distances.
    Bill B

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