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Marcel Tschudin wrote:
> Why not measuring the sun's horizontal diameter? This measure is
> (almost?) not affected by refraction.

I discovered the "almost" a few days ago, when programming a function to
compute refracted semidiameter. The values at the 12 and 6 o'clock
position angles (in the horizontal frame) were exactly what I expected
for a Sun-sized object, but at 3 and 9 o'clock they were about .2" too
small.

After some debugging, I realized there was nothing wrong with my code.
Refraction reduces the semidiameter in every direction. When the left
and right limbs refract up, they move on great circles which converge at
the zenith. 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".

In this case, the algorithm was smarter than its designer.

Later I remembered that Jean Meeus ("Astronomical Algorithms", chapter
on refraction) says the Sun's horizontal diameter is reduced slightly by
refraction. His figure (credited to Danjon) is .6", practically
independent of altitude.

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