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From: Marcel Tschudin (no email)
Date: Fri Jun 16 2006 - 08:24:36 EDT
On 6/16/06, George Huxtable <> wrote:
>
> 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
aty at mintaka dot sdsu dot edu
Yes, I only can recommend it. I guess that this will becom THE reference
text on refraction.
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
This happens also at great inversions such as e.g. above around +0.12°K/m
The limit can be calculated from the equation
n*r*sinZ=no*ro*sinZo
where no, ro and Zo are the (constant) values at the observer and n, r and Z
the values at a height above ro. Now with sinZ=no*ro*sinZo/(n*r) is >1 the
ray can't penetrate the layer at height r and therefore bends back and is
trapped.
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.
I guess you ment -0.0065 ?
Alex wrote:
"The water was very cool (and always is) here. I mean most people do not
dare to swim in Kiel till the beginning of August:-) But the air was hot, at
least that was what I felt:-)"
Isn't that just the type of condition to create a high temperature gradient?
Marcel
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