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Anomalous dip. was: [NAV-L] Testing pocket sextant.
From: George Huxtable
Date: 2006 Jun 16, 00:53 +0100
From: George Huxtable
Date: 2006 Jun 16, 00:53 +0100
Alex mentioned the paper I recommended- > "I've just received an offprint of a new article by Andrew T Young, of | > the Astronomy Deparment, San Diego State University, "Understanding | > Astronomical Refraction", which has recently appeared in the journal | > "The Observatory"(Vol. 126, no. 1191, pp. 82-115, 2006 April.)" and asked- | Have you seen the paper? Is it available on the web? Yes, I've kindly been sent a reprint. I must be on his refraction mailing-list, having discussed a lot of details about refraction with him in the past. I don't know whether it's on the web. I can no longer find Andy Young's email address at SDSU, but you could try asking them. I have always found him to be a most helpful character. In my opinion, his paper is the sort of thing you might want to keep in printed form, rather than as web ephemera, but I take a somewhat old-fashioned attitude toward such things. I get the picture that to some (here I exclude Alex) if it isn't available online then it doesn't truly exist. Anyway, now I consider myself somewhat better informed by Andy's lucid exposition, and can try to comment further about Alex's problems with dip; if dip really is the underlying reason for his sextant discrepancies. Imagine that in the Kielefjord, on the day Alex was observing, there was a temperature inversion in the air over the surface of the water. Here we are considering just the lower few feet, between the level of the water surface and Alex's height of eye; probably just the lower couple of metres, depending on Alex's height and how far up the beach he was standing. If in that region the temperature gradient, with increasing height, was as great as -0.115 degrees C per metre, that is sufficient to bend light downwards, towards the water surface, so that it's curvature exactly matches the curvature of the surface. In that case, light would be "trapped" into following the water surface. In that case the visible horizon, the boundary between sea and sky, would appear to be exactly horizontal, no matter what your height of eye. So the actual dip under thise conditions would not be the text-book value that Alex took corresponding to his height of eye, but zero instead. Wouldn't that, on its own, account for most of Alex's observed discrepancy? If the gradient were higher still, that would give rise to a reversed dip. Note that we are talking here about the temperature at the water surface being only a quarter-degree or so cooler that it is at eye level, which doesn't seem to be a great deal. However, that gradient is a lot greater ( and in the opposite direction) than the value taken for the Standard Atmosphere, which is only +.0065 degrees C per metre. But there's nothing unphysical or unfeasible about a gradient of - 0.115 degrees C per metre. If the air is cooler below, as it is in such an inversion, then that is a stable state of affairs, and air convection doesn't act to stir things up. So, according to Young, there's no limit to the gradient in such inversions, and "... rates exceeding a degree a meter are common. An inversion gradient of 20 degrees per metre has been measured directly ..." So how can such a temperature inversion near sea-level come about? Consider a land-mass near the water, such as happens in the Red Sea (and the Keilefjord). The worst situation is apparently caused over desert sand, and you can sea why. When the Sun shines directly on sand, it can get so hot that it's painful to walk on, the reason being that all those grains separated by air, just making point contact with grains below, act as a good insulator, so heat can't conduct down into the earth. The high local temperature, close to the surface, causes the air layer in contct with it to be efficiently heated. Conversely, at night, the surface of sand cools down very quickly. Black volcanic sands would presumably absorb Sun energy even more effectively and heat the air above them more. But it's not necessary to invoke desert sands. Any land surface will heat more quickly in daytime, and cool more quickly at night, than the local sea. In the sea, turbulence causes mixing between the upper layers, making any water-mass an effective heat-sink, with a temperature that changes little, and slowly. Now we have a picture, of air being warmed in the daytime over adjacent land, then a light breeze carrying it or drifting it over the surface of the cooler water, so that lower layers of the air, in contact with that water, are somewhat cooler than the rest, and the resulting temperature gradient gives rise to anomalous dip. Alex reports his measurements as being in fine weather, daytime, taken over a sea-body that's surrounded by land. That seems like perfect conditions for upsetting the dip. The moral might be that sextant observations should be taken, not near land, but out at sea, where there's no local source of warm air. Does any of that seem plausible? Please note that I am no atmospheric-scientist, but just doing my best to make a few logical deductions from the evidence that Young has provided. George contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.