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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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