# NavList:

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

**Re: Biruni and the radius of the Earth by dip**

**From:**John Huth

**Date:**2011 Jan 5, 10:55 -0500

Ok I see what i was doing wrong. The equation quoted on that website is valid as an approximation that takes into account *both* refraction *and* dip simultaneously. Since it appeared under the heading on refraction, I assumed it was a refractive correction.

Reading further, I see that the typical refraction effect is about 1/6th or 1/7th of the curvature, so your number of 6 arc minutes sounds right.

It gets subtracted because in standard conditions, the refraction "raises" the horizon, right?

Duh....well, it takes some time, but I can learn!

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Keeping up with the grind

On Wed, Jan 5, 2011 at 10:40 AM, Fred Hebard <mbiew---.net> wrote:

be careful too of the angle unit, radians versus degrees.

On Jan 5, 2011, at 10:27 AM, Apache Runner wrote:

Maybe I am misunderstanding. Let me look at the formula again - on that website you had, there was a crude figure of merit for what I thought was refraction that was 1.75 moa * sqrt(h) where h is in meters. It seemed to be for refraction and it gives 40 moa if I plug into it.

Where did you get 6 moa from?

On Wed, Jan 5, 2011 at 10:17 AM, Marcel Tschudin <marcel.e.tschudin---.com> wrote:

John, you wrote:

> Both the dip and the refraction are in the 40 arc minute region for a 500 m

> peak, ....

Could there be a misunderstanding? From a 500m high peak the horizon

has a geometrical dip (without refraction) of about 40 moa. The

refraction increases this geometrical dip by e.g. 6 moa leading to an

observed dip of e.g. 46 moa. In this case we deal with the

"terrestrial" refraction which is considerably smaller than the

"astronomical" refraction since in the "terrestrial" case the length

of the ray path through the atmosphere is much shorter.

Marcel

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Keeping up with the grind

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Keeping up with the grind