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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Basics of computing sunrise/sunset
From: Marcel Tschudin
Date: 2009 Jun 27, 15:48 +0300
From: Marcel Tschudin
Date: 2009 Jun 27, 15:48 +0300
OK, I give it a try. Frank questioned whether his hypothetical model of a series of balloon soundings along the line of sight would allow an improved calculation of the refraction near the horizon and Bill B. questions whether this would help with thermal inversion/abnormal lapse rates causing refraction anomalies at my visible horizon? First I should mention that the abnormal refraction phenomena depend on the viewing angle; they appear near the astronomical horizon, i.e. when looking horizontally. If the position of the eyes is considerably above or below the the large temperature gradient they may - depending on the height difference - actually not even notice that there exists somewhere an abnormal layer, this because the angle of incidence is too large. It is thus important from where I observe e.g. the sun near the horizon. If the observation is done from a boat with the eyes a few meters above the sea, the apparent horizon is fairly close to the astronomical horizon; the dip can therefore be subjected to anomalous refraction phenomena but the sun still well above the horizon not. The measured angular distance between sun and horizon is erroneous because of the wrong position of the horizon (dip) not because of the sun's position. If, in the other case, the sunset is observed from a high point above the sea the viewing angle of the horizon is already too large (too distant from the astronomical horizon) to be disturbed by anomalous atmospheric layers. If in this case the sun is near the astronomical horizon and the position of the eyes close to e.g. an inversion the image of the sun is "disturbed". The wrong result obtained from measuring the distance between sun and horizon is in this case due to the wrong position of the sun and not the position of the dip. If both, sun and apparent horizon, are close to the astronomical horizon both can also be affected by anomalous atmospheric layers. A word on weather balloons which measure pressure, temperature and relative humidity of the atmosphere. These measurements are only available at certain selected heights, which, depending on the situation, may have been selected too coarse to catch certain anomalous features. Imagine a "S"-shaped vertical temperature profile which may eventually only have been measured with two arbitrary selected data points; depending where those two data points are one may obtain "any" temperature gradient. I also intend to use weather balloon data, but from those I try to obtain from statistical analyses "typical" values for a certain location at a given local time for the various height layers of the atmosphere. If you are more interested in refraction: have a look at the over 100 Web-pages which Andrew T. Young dedicated to this subject. There are two entries into this labyrinth, the alphabetic one http://mintaka.sdsu.edu/GF/bibliog/alphindex.html and the tables of content http://mintaka.sdsu.edu/GF/bibliog/toc.html Marcel --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---