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Alex mentioned the paper I recommended-

> "I've just received an offprint of a new article by Andrew T Young,
of
| > the Astronomy Deparment, San Diego State University,
"Understanding
| > Astronomical Refraction", which has recently appeared in the
journal
| > "The Observatory"(Vol. 126, no. 1191, pp. 82-115, 2006 April.)"

and asked-
| Have you seen the paper? Is it available on the web?

Yes, I've kindly been sent a reprint. I must be on his refraction
mailing-list, having discussed a lot of details about refraction with
him in the past. I don't know whether it's on the web. I can no longer
find Andy Young's email address at SDSU, but you

could try asking
them. I have always found him to be a most helpful character.

In my opinion, his paper is the sort of thing you might want to keep
in printed form, rather than as web ephemera, but I take a somewhat
old-fashioned attitude toward such things. I get the picture that to
some (here I exclude Alex) if it isn't available online then it
doesn't truly exist.

Anyway, now I consider myself somewhat better informed by Andy's lucid
exposition, and can try to comment further about Alex's problems with
dip; if dip really is the underlying reason for his sextant
discrepancies.

Imagine that in the Kielefjord, on the day Alex was observing, there
was a temperature inversion in the air over the surface of the water.
Here we are considering just the lower few feet, between the level of
the water surface and Alex's height of eye; probably just the lower
couple of metres, depending on Alex's height and how far up the beach
he was standing. If in that region the temperature gradient, with
increasing height, was as great as - 0.115 degrees C

per metre, that is
sufficient to bend light downwards, towards the water surface, so that
it's curvature exactly matches the curvature of the surface. In that
case, light would be "trapped" into following the water surface. In
that case the visible horizon, the boundary between sea and sky, would
appear to be exactly horizontal, no matter what your height of eye. So
the actual dip under thise conditions would not be the text-book value
that Alex took corresponding to his height of eye, but zero instead.
Wouldn't that, on its own, account for most of Alex's observed
discrepancy? If the gradient were higher still, that would give rise
to a reversed dip.

Note that we are talking here about the temperature at the water
surface being only a quarter-degree or so cooler that it is at eye
level, which doesn't seem to be a great deal. However, that gradient
is a lot greater ( and in the opposite direction) than the value taken
for the Standard Atmosphere, which is only +.0065 degrees C per metre.