On Sun, 22 May 2005 09:52:22 +0100, John Hall wrote:

That's fairly small for visible light, isn't it? I know that when
working with radar, refraction is commonly allowed for by assuming
an Earth radius of 4/3 times the true value.

Light is affected by temperature/density variations in the atmosphere,
think desert mirages or, closer to home, those patches of "water" on
summer roads that disappear as you approach. The latter is a
reflection of the sky in the the hot layer of air close to the road
surface.

--
Cheers
Dave. pam is missing e-mail