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On Wed, 1 Dec 2004, Frank Reed wrote:

> Picture the Sun sitting on the horizon

I agree with your explanation, which
coincides with the explanation by George.

From what I read, error in the dip due to refraction
is usually much less less than 1', that is not totally
negligible but small.

By the way, Shufeldt in his paper
"Irradiation and refraction at low altitudes and the working
navigator" (Journ of The Inst. of Nav., 4,2, 1954)

claims that it is possible to obtain good position lines
from the Sun on extremelly low altitudes. He means less than 3 d.
He uses some Russian tables of refraction for such altitudes
published in Moscow in 1930.

He says that he took 380 sights at such altitudes, and with very
careful correction of the sights (including water and air temperature
and other meteorological info, irradiation etc.) his position
line was never mostly within 2 miles of his true position.

One place in this paper was expecially interesting for me:
he says that

"to my surprise, attempting to establish index error with
a close degree of accuracy proved to be the most difficult
part of the whole operation".

This is very well confirmed by my own experience.
(I recently posted the data of such observations).
Unfortunately, he does not try to explain why this is so.

Alex.

   

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