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Roughly speaking, the process was like this.
When Sun is on a large altitude refraction is negligible.
After few hundreds years of careful observations
(by Ellinistic greeks and Babylonians)
you are able to predict where exactly in the sky the
Sun should be. Then, when the Sun is at a smaller altitude
you see the difference between the computed position
and the observed one. This difference is the
refraction, approximately.

Of course you have to know in advance that refraction
exists at all. This was derived from some
Moon eclipse observations
as it is beautifully explained in the paper.

Alex.

By the way, I found a paper by Edm. Halley
"Some remarks on the Allowances to be made
in Astronomical observations for the refraction of the air",
in the Royal society transactions
(I suppose it is in public domain by now).
A correction table for refraction is included,
as I understand it was made by Newton himself.
Comparison with modern Almanac shows that the
difference is less than 0.1' for altitudes over 20d
and 0.2' for alts over 15d.

Hundred years later in the same journal they were
already discussing
0.01" accuracy:-)

On Thu, 4 Jan 2007, Guy Schwartz wrote:

>
>
> Lets take the sun for example, how would one know if they were viewing it
> with or without refraction to start the analysis?
>
> ----- Original Message -----
> From: "Lu Abel" 
> To: 
> Sent: Thursday, January 04, 2007 7:11 PM
> Subject: [NavList 1977] Re: refraction
>
>
> >
> > Not much different than measuring bending of the stick, Guy, except with a
> > bit of patience required.
> >
> > If I track the rising or setting of a body, I should see a uniform rate of
> > motion in the sky as long as refraction is insignificant.  Once it becomes
> > significant, however, the rate should change.
> >
> > I could, for example, calculate the expected time a body would disappear
> > below the horizon based on its motion through the sky.  But I would
> > observe it to set later.  The time of observed setting would lead me to a
> > value for refraction, ie, the amount the body is below the horizon and
> > therefore the amount of refraction.
> >
> > Measurement of factors such a pressure, humidity, and temperature could
> > then be used to refine the parameters.
> >
> > Lu Abel
> >
> > Guy Schwartz wrote:
> >> Inserting a 4 foot stick half way into a big tank of water at a 45 degree
> >> angle, the refraction can be measured or closely approximated.
> >> How were the refraction adjustments initially calculated for the
> >> atmosphere?
> >> What was the methodology used? I don't think it was as easy as the stick
> >> in the water method?
> >> Thank you.
> >> Guy
> >> >
> >
> > >
> >
> >
> > --
> > No virus found in this incoming message.
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> > Version: 7.5.432 / Virus Database: 268.16.5/616 - Release Date: 1/4/2007
> > 1:34 PM
> >
> >
>
>
> >


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