# NavList:

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

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Re: True Distance vs. Observed Distance
From: Marcel Tschudin
Date: 2012 Sep 17, 22:09 +0300
Greg,

I like your drawing skills, especially the ship ;-)

Your star positions are likely given in astrometric coordinates (without atmosphere). These positions are then converted  to air-free local coordinates of the observer. Refraction is applied to these local coordinates by deducting its value from the ZD (or added to its altitude) for each of the two bodies depending their air-free ZD (or altitude). The azimuths are unaffected by refraction and remain therefore unchanged. The distance is then calculated between the two refracted positions (ZD1ref, Azi1 and ZD2ref, Azi2).

Note that refraction has in this case to be calculated from the true ZD (or altitude) using e.g. the formula from Saemundsson (1986, see http://en.wikipedia.org/wiki/Atmospheric_refraction there the second formula with R=1.02 cot .... ). Looking at Bill's Excel spread sheet "A Nav Ang Sep Refract Mas" shows that he did indeed use this formula. I was however not able to calculate with it the star distance of Greg's observation.

Marcel

On Mon, Sep 17, 2012 at 7:47 PM, Greg Rudzinski wrote:

Linked is a diagram showing how refraction changes observed distance between celestial bodies. For the diagram the bodies have a distance about 45* with the zenith directly above the observer. The refraction correction is added to the observed distance to get a true distance. If starting with a true distance from a GC calculator then the refraction correction is subtracted from the true distance to get the observed distance.

Greg Rudzinski
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