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I mentioned photgraphing a meter stick to get the angular scale, and Marcel,
you replied:
"Why not measuring the sun's horizontal diameter? This measure is (almost?)
not affected by refraction. The value can be calculated quite accurately and
only changes a few percent during the year. One only has to be careful that
the sun is not overexposed."

Yes. Certainly that will work. The advantages of the meter stick are small:
1) you have multiple points that you can measure so you can potentially get
a "sub-pixel" measurement. 2) It can be extended to longer distances (use a
tape measure, but then you have to be careful to give it a curve so that it
is an arc of a circle or you have to do a little math) and that can supply
the angular scale all the way across the field of view, which is usually
variable.

I mentioned measuring the Sun's flattening as a surrogate for altitude, and
Marcel, you replied:
"This doesn't look to me as a reliable procedure. There are often
situations where differential refraction produces odd shapes"

I agree, and that's what I told Ken. He was annoyed. :-)

But it's all a question of details. If you limit yourself to Sun altitudes
above, let's say, two degrees, most of the complexity in refraction goes
away. Then it's just a question of accuracy. How reliably can one measure
the flattening using a digital camera? A few tenths of a minute of arc?? How
much of a change in altitude corresponds to a tenth of a minute of arc
change in the flattening? Naturally, if you can see the horizon, you would
get better results directly measuring the distance to the horizon. So this
is a procedure specifically designed for those cases where the horizon is
obscured.

For more background on Ken Gebhart's "sun squash", read here:
http://fer3.com/arc/m2.aspx?i=027082&y=200601

And you wrote:
"BTW: using a digital camera has yet an other advantage. Sometimes the
sun can't be seen with the eyes. However, if the clouds in front of
the sun are sufficiently thin the sensor of the digital camera still
can see the IR image of the sun. In those cases it's possible to take
"shots" without actually seeing the sun (except on the screen of the
camera)."

Very cool! (pardon the pun) I'll have to try that.

 -FER



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