# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Measuring (and Calculating) Dip - sighting technique
Date: 2013 Mar 22, 16:39 -0700

To: Bill Morris

Hi Bill,

I have a specific question about your observations. I hope you can recall what you saw when the dip angle changed by ~30 arc seconds.

Did the horizon remain in focus? Or did it blur?

Further, do you know the depth of focus at the magnification of your observation? This is a theodolite specification. You'll see why in the following paragraphs.

If the wave height correction is really (HeightOfEye - 1/2 Wave Height), then the distance to the horizon must also change.

An example: When your height of eye was 1.89 meters, the distance to the horizon is nominally 3860 meters * sqrt(1.89), which is 5306 meters (2.87 nautical miles). If the correction is (HoE- 1/2 wave height), and the wave height was 1 meter, then the distance to the horizon must be 3860 * sqrt(1.89-.5), which is 4550 meters (2.46 nautical miles).

So if the horizon remained in focus, AND the correction is (HoE - 1/2wave height), then the depth of focus must be at least 755 meters (0.41 nautical miles). Under the assumption that the depth of focus of the theodolite is not that large, then the horizon must have blurred!!!!

Conversely, if the horizon remained in focus AND the depth of focus is not that large, then the correction, as defined, cannot be true.

On your next round of dip measurements, could you try to see what happens?
1) the time period over which the ~30 arc second delta occurs.
2) Does the horizon remain in focus as the delta occurs
3) The depth of focus of your theodolite.

Best Regards

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