# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Refraction
From: Kent Nordstr�m
Date: 2008 Jul 4, 16:37 +0200

```Frank asked: Aha. Very nice! So that does appear to explain it. Kent, did
you believe
> that your refraction number was for 0 deg C or 10 deg C?

Firstly I can confirm that the refraction formula given with the factor
60,35 was for the frequency approx. 5500 Angstroem (same as 520 lamda
nanometer).
The formula is absed on Po=1013,25x10exp2 Pa and 273,15 degr. K, which
should be 0 degr. C.
Kent N

----- Original Message -----
From:
To:
Sent: Friday, July 04, 2008 10:57 AM
Subject: [NavList 5668] Re: Refraction

>
> Dave Walden, you wrote:
> "They are functions of pressure, temperature and wavelength.  Two sets of
> conditions can be calculated for easy comparisons.  Those included are for
> the Nautical Almanac conditions and a more conventional set to conditions:
> 760mb, 0 deg C, lamda .54 microns.   The K =60.4 sec, should look
> familiar.
> The Naut Alm STP yields 58.2, also familiar."
>
> Aha. Very nice! So that does appear to explain it. Kent, did you believe
> that your refraction number was for 0 deg C or 10 deg C?
>
> And you wrote:
> "The formulas can be found in Spherical Astronomy by Green.  This is the
> "successor" to Smart's work.  Green was coauthor with Smart on later
> editions and finally wrote his own.  (To my way of thinking, it's
> better.)"
>
> Damn. Another book to buy. :-) What's the full title of Green's book?
>
> And:
> "Also attached is a plot of errors of Bennett and simpler formulas
> (minutes
> refrac vs degrees alt).  There is indeed, nothing magic about Bennett.  It
> get 34 min at 0 deg in case you've forgotten, but the added complication
> to
> do so only makes things worse at higher altitudes."
>
> A simple way to keep the best of Bennett (it is indeed more convenient
> than
> typing in the whole table) is to retain it for altitudes below 15 degrees
> (or some other cut-off) and use some simple function of tan(z.d.) for
> higher
> altitudes.
>
> Anyone have any thoughts about the best color of light to use for the
> refraction formulae/tables? At very low altitudes, the star images really
> are smeared out substantially. What's the standard for astronomy? Is it
> appropriate for sextant users?
>
> -FER
>
>
>
> >

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