# NavList:

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

**Re: Refraction at the horizon.**

**From:**George Huxtable

**Date:**2008 Mar 18, 17:17 -0000

Marcel has let us watch his reasoning develop, which was interesting. But to explain a total refraction of only 16.3', as observed on one occasion from Mauna Kea, Hawaii, at 4205 m. altitude, in terms of the over-island refraction, is difficult, and it really needs no calculation to show that. Assume that the refraction over the ocean, on the way down to the horizon, is close to34', corresponding to the standard atmosphere And then add on another lesser, amount, to corresponding to the 100 miles or so path length over oceanic waters. I don't know how much to add for that, but those that are familiar with refraction integrals might offer some values for a standard atmosphere, where the integration up from the horizon has been truncated at certain heights, at say 1000 metre intervals. Anyway, an expected oceanic refraction to that altitude would be in the range of 40' to 50', at a guess. Then, after an over-island path of only 30 miles or so, the light arrives at Mauna Kea, by which point the total refraction was only 16.3'. If so, that would have to be explained by a highly NEGATIVE refraction over that 30 miles! That means the light would have to be bending concave-upwards, the opposite way to the usual, and by an enormous amount. That implies that the air density has to be INCREASING, and considerably so, as the altitude increases. The laws of physics tell us that air pressure can only decrease as the altitude increases, because there is less weight of air above pressing it down. And for the density to increase with altitude, requires a decreasing temperature, and by much more than the amount required to compensate for the pressure change. An unfeasibly unstable state of affairs! So something is wrong. What might that be? One possibility is a simple cock-up, in the sunset timing or in its calculation. Another might be that the over-ocean part of the refraction, on that evening, differed greatly from the expected value, to an extent that some pundits on this list would find hard to accept. ========================== In an earlier message (4717) Marcel wrote- "I investigated some time ago how the refraction at large zenith distance depend on changes in temperature and pressure at the observer and found at that time that those dependencies are quite exponential." Marcel, I have searched for some meaning in that sentence, but remain baffled by it. What on earth does "quite exponential" imply? A naive view is that the quantity (refractive index - 1), which defines refractive bend, is almost exactly proportional to air density. That is, to pressure, to the power +1, and to temperature, to the power -1. Is Marcel saying that isn't so? What does "quite exponential" mean in this context? George contact George Huxtable at george---.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To unsubscribe, email NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---