# NavList:

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

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Re: Refraction near the Horizon ? Ob servation vs. Calculation
From: Marcel Tschudin
Date: 2013 Apr 5, 17:22 +0300
Bruce,

With the example I intended to show how the "traditional" formulae which are "generally" used for calculating refraction and dip agree with the observations. The formula used for dip is mentioned in Bowditch and probably also in the N.A. (I do not have a copy of it). I know that G. G. Bennett's (are you still on NavList?) refraction formula is a widely accepted and also a widely used one. Could it be that his formula is even mentioned in a supplement of the N.A.? The formula for *estimating* the dip does indeed result from an assumed terrestrial refraction k which describes the curvature of the light path and a mean Earth radius.

Regarding the statistics: The mode-value is the most likely value (in the Fig. the pink squares and in the Tab. the value under "Mode<�H>"). The error bars (in the Fig. the pink bars and in the Tab. the values under "Mode-sigma" and "Mode+sigma") describe a value range; it is expected that 2 out of 3 observations are within this range.

Marcel

On Fri, Apr 5, 2013 at 4:15 PM, Bruce J. Pennino wrote:

Thank you for posting this information. The text is just excellent and well written. My math/statistics  background limits me , but I'll study the results.

I was interested to see that you use the equation for dip 1.76 sq rt H meters, which is the same as 0.971 sq rt H feet.  I gather that this is the universally accepted equation, which I've proven to myself is from basic trig knowing the average radius of the earth? Right?

Bruce

----- Original Message -----
Sent: Thursday, April 04, 2013 6:28 PM
Subject: [NavList] Refraction near the Horizon – Observation vs. Calculation

The following link leads to a first, gross description of the collected measurements, their related data and to two examples showing how the dataset can be used. ( I hope my version of English is sufficently comprehensible.)

The intention is now trying to find estimates for refraction and dip which agree even better with the measurements than those shown.

Comments, suggestions and critics are welcome.

Marcel

P.S: May be one understands now why I encourage members of NavList to compile a similar dataset with measured dips.

View and reply to this message: http://fer3.com/arc/m2.aspx?i=123335

View and reply to this message: http://fer3.com/arc/m2.aspx?i=123350

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