# NavList:

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

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From: Marcel Tschudin
Date: 2008 Jul 9, 12:47 +0300

```Bill B. wrote:
> The above ramblings aside, the list is devoted to "traditional" navigation.
> Can you please tell me how to measure 0.x" with a marine sextant?

The table values and sextant you use for practical navigation should
have a higher accuracy than your measurements. This is a sort of "tool
making" subject.

Paul wrote:
"With a body of 16' semidiameter, and sea level refraction, this
decreases the semidiameter about .27". I tested this at several
altitudes from 10 degrees to 80, and it didn't vary more than .01[sec
arc]".
I couldn't figure out how you obtained this value. Looking at it
geometrically by linear interpolation: about 0.5 deg refraction at the
horizon and 0 refraction in zenith direction results in a much larger
value. Looking at it using differential refraction shouldn't result in
a more or less constant value (0.27"). I would appreciate if you could
explain the cause of the effect in more detail.

Greg Rudzinski wrote:
> Solar diameter 07/07/2008    31.46' = .291 M.O.A. per pixel
> Pixel solar diameter         108

I guess it's the IR sensitivity of the digital cameras which is the
reason that the pictures of the sun, without using additional filters,
tend to be overexposed, this even with the sun very close to the
horizon and the exposure maximal reduced (by 2 units). However,
depending on the situation at the horizon, with the sun close to it
and the exposure reduced by 2 units, the apparent horizon may only be
recognised with great difficulties. In my case I obtain the most
reliable sun sizes from photos where the sun is viewed through heavy
haze/smog or when the sun is behind thin clouds. Where necessary I
filter then the colours in the photo by removing all red and blue,
measuring only the green image.

Marcel

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