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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Irradiation
From: George Huxtable
Date: 2004 Nov 29, 10:49 +0000
From: George Huxtable
Date: 2004 Nov 29, 10:49 +0000
Alex is demonstrating, to himself and to us, the uncertainties of refraction at low altitudes. He tells us that his recent measurements of Sun semidiameter were taken, in one case "sun very low, just before sunset" , and on another day "few minutes before sunset", without saying what the Sun altitude actually was. But anyone can see, at times near sunrise and sunset, that the Sun's disc is often distorted, squashed or expanded in a vertical direction. And Alex mentioned a paper about refraction, in a posting labelled "dip" on 27 Nov, "their main conclusions seem to imply that the refraction near the horizon (and thus the dip) is uncertain to more than 1/2 of a degree." The only thing wrong with that quotation was Alex's interpolation "(and hence the dip)". The problem is due to horizontal layers of air at differing temperatures, through which the light has to pass. Those layers can be there at any time but they become obvious only when a distorted Sun disc shows them up. Trevor had written- >> The better demonstration of irradiation, for members of this list, >> might be to measure the semi-diameter of the Sun, under various conditions and Alex responded- >Actually, this was the main resaon for my questions: But it might have been better to qualify that suggestion- >> might be to measure the semi-diameter of the Sun, under various >>conditions, but NOT at low altitudes. If it's necessary to measure Sun semidiameter at low altitudes, an observation with the sextant horizontal (as Alex made) is likely to give better answers, as refraction affects altitudes rather than azimuths. If measured horizontal angles differ from tabulated values, then that discrepancy is likely to be due to a combination of instrumental error and the optical effect of irradiation. It is possible to calculate an allowance for vertical compression of the Sun near to the horizon, but this applies only to conditions of standard refraction, and can not account for the transient local distortions which can have such an important effect, so is not worth much. The vertical compression can change from minute to minute, as the image of the Sun's disc rises or sinks through these air-layers. In comparing predicted Sun semidiameter (from the Nautical Almanac tabulations, presumably) with his observations, a significant source of scatter may be in those tabulations, which are given to the nearest 0.1 arc-minute. The semidiameter is of course known to a much higher precision, and given to a hundredth of an arc-second in the Astronomical Almanac, if Alex is seeking the ultimate. I agree with all three of Alex's conclusions. George. ================================================================ contact George Huxtable by email at george@huxtable.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================