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Alex wrote-

>I recently found an atricle which seems
>very disturbing to me:
>"Refraction near the horizon" by Schaefer and Liller,
>Publ. Astr. Soc. Pacific.
>You can read the paper on
>http://www.math.purdue.edu/~eremenko/dip.pdf
>The main point of the article is that the refraction
>near the horizon (and thus the dip of the horizon)
>varies unpredictably and the variation is enormous.
>
>The authors do not discuss Cel Nav in this paper,
>their main concern is refuring "Archaeoastronomy".
>
>I don't care about Archaeoastronomy, but their main
>conclusions seem to imply that the refraction
>near the horizon (and thus the dip) is uncertain
>to more than 1/2 of a degree.
>
>Then how is traditional CelNav based on altitude measurements
>possible at all?
>
>Any comments?

=============

From George-

No, it's not nearly so bad as Alex makes out.

The alarming figures thar Alex quotes relate to the uncertainty in
observing an object such as the setting Sun. It includes all the bending of
the light along the path as it comes in from space and skims the horizon to
arrive at the observer's eye.

Fortunately, the light-path involved in sighting the horizon itself by the
observer is MUCH shorter, and therefore the refraction along that shorter
path (a component of the dip) is much less.

George.

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