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On 17 Dec 2003 at 21:09, Frank Reed wrote:

> If you observe an object in the sky at an
> altitude of 15 degrees, the sea level refraction is 3.6 minutes at sea
> level, but in Denver it would be 3.1 minutes.  That's not a big deal
> for standard line of position sights, but it would be a major source
> of error for land-based lunars practice.

I would guess that many locations at high altitude are also going to
be in mountainous country where the lunar stars might be just above
the raised horizon of a mountain. In such cases the refraction due to
temperature changes will probably be signficant (at least, the stars
twinkle a lot when they're close to the mountain (in an angular
sense)). I wonder what the minimum angle is that one could get away
with before the error becomes intolerable. Is refraction a big
problem for navigation at sea when sighting the horizon?

The celebrated land-based navigator David Thompson used temperature
and altitude to correct for refraction. His altitudes came from
occasional boiling-point measurements and estimated reckoning. They
were sometimes out by several thousand feet (although they were
usually better than that).
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