# NavList:

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

**Re: dip, dip short, distance off with buildings, etc.**

**From:**Frank Reed CT

**Date:**2006 Jan 11, 01:01 EST

George H, you wrote: "That's a perfectly valid way to assess refraction, but not a particularly new one." Oh, of course. Let's be clear -- all of this was worked out a very long time ago. Terrestrial refraction was a serious practical problem for terrestrial survey work and many people probably wrote about it (I've never looked into it), and to that extent, there is nothing new under the Sun! But it's certainly new to this list as far as I am aware. Just about one year ago, there was a rather long discussion where people were trying to puzzle out the origin of the equation underlying Bowditch's Table XV. It didn't interest me at the time, but it does now. No one seemed to know how to derive it last year or, more importantly, assess its limitations. What I am saying is that this simple technique of changing the curvature of the Earth can be used to analyze not just Table XV [see my reply to Bill] but everything else where terrestrial refraction impacts navigation. For example, consider "anomalous dip". The primary source of dip anomalies (though not the only source) is the variation in the scale height in the layer of the atmophere close to the ocean which in turn changes the effective curvature of the Earth. If I calculate dip based on a lapse rate of -6.5 deg per kilometer (implying a scale height of about 10km), I find that the dip at 5 meters height of eye is 3.9 minutes of arc. If instead the lapse rate is -34.1 deg/km, the atmosphere has constant density and the dip is exactly equal to the geometric value which 4.3 minutes of arc. And if the lapse rate is +25deg/km, then the dip at 5 meters if 3.4 minutes of arc. Naturally a calculation like this assumes that the lapse rate is constant and the atmosphere is more or less the same on the whole path from horizon to observer (so it can't handle really exotic refraction, like mirages). There's also a conceptual aspect to this. Next time you're looking at a distant ocean scene with a few boats off in the distance, perhaps a lighthouse, and beyond the horizon some low hills of an island, ask yourself how the scene would change if the Earth's curvature were a little greater or a little less. That's exactly what you would see under conditions of variable refraction. A temperature inversion would lift the distant hills just as if the Earth was nearly flat. Note that this sort of analysis does not apply to distant objects that are higher than a few hundred meters (so no mountains) and it has the same limitations that I mentioned above, but it covers a very large portion of the possible variation in refraction. Finally, the practical value of this is not that we can start calculating dip and all the rest based on the specific temperature profile since, of course, we don't usually have access to the temperature profile. Instead it provides a means of assessing possible errors arising from using the tables "naively", and it leaves open the option of calculating different versions of the tables when circumstances might require them. -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars