# NavList:

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

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Re: Refraction - Corrigendum: Humidity
From: Marcel Tschudin
Date: 2005 Aug 27, 16:03 +0300

```The content of my previous mail needs a correction. The error is the result
of a misunderstanding.

> Here the influence of humidity on the refractive index of air in figures:
>
> This page from ESO describes the procedure on how the index can be
> calculated:
>
> The tabulated values on page 17 of the following manuscript from Stanford
> correspond to the "dry part" of the index, as veryfied with the above
> "procedure" for standard condition.
> http://www.stanford.edu/group/efmh/FAMbook/Chap10.pdf
>
> Finally Andrew Young shows on his Web page the refractivity of air at
> standard condition including the "wet part" as calculated by Peck and
> Reeder
> http://mintaka.sdsu.edu/GF/explain/optics/disp.html
>
> Now, what effect does this have on refraction?
> Using 0? altitude at sea level (height=0m), standard temperrature (=15?C)
> and pressure (=1013.25hPa)  and a wavelength of 595nm (orange, e.g.
> sunrise or sunset).
>
> By integration over the light path:
> Index with "dry part" only =1.000277  -> Refraction = 30.27 minutes
> Index with "dry part" and "wet part" =1.000293  -> Refraction = 32.17
> minutes
> using a generally mentioned
> approximative index of 1.0003 -> Refraction = 33.03 minutes
> and, last but not least, Bennett's formula -> Refraction = 34.48 minutes

The differences between the values shown in the Stanford document and those
out of the graph on Andy Young's Web page are (mainly) due to different
temperature scaling and NOT due to the influence of humidity. It seems that
the values in the Stanford document are scaled to 15degC whereas the graph
on Andy Young's Web page to 0degC.

Marcel

```
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