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On 3/28/07, Alexandre E Eremenko  wrote:
> ...
> I have is the chapter of Chauvenet where he explains
> the work of Bessel, and attaches tables computed by Bessel's
> formula with refraction in fractions of a SECOND.

Alex, I would be cautious with such an information. It's certainly
true right overhead in zenith direction where there is no refraction.
Things become more difficult when approaching the horizon. It still
might be true under "normal" observer conditions down to somewhere
around 70 deg ZD. I think that what Frank explained is about
appropriate for navigational purposes under "normal" observation
conditions. Under "normal" conditions I mean an observer somewhere at
mid-latitudes, under average (stable) weather conditions and possibly
also not too high up in the sky.

Bill, yes, all the parameters you mentioned are of concern when
calculating refraction at high zenith distances; even the linear
relationship of refraction from pressure and temperature  fails there,
they become highly exponential.

Andy Young's Observatory paper, which Richard mentioned is highly
recommendable. For those who don't have access to it, they can find a
lot of its content in Andy's labyrinth of over hundred Web-pages on
this subject. There are two entries into the labyrinth
- The Tables of Contents: http://mintaka.sdsu.edu/GF/bibliog/toc.html
- The Alphabetical Index: http://mintaka.sdsu.edu/GF/bibliog/alphindex.html

Marcel

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