NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sextant Accuracy and anomalous dip.
From: George Huxtable
Date: 2003 Mar 18, 17:17 +0000
From: George Huxtable
Date: 2003 Mar 18, 17:17 +0000
Arthur Pearson has made an interesting response to my posting on anomalous dip, confirming it to be an important matter from his own observations.Jared Sherman is surprised that such an effect could be routinely ignored. Arthur quotes Bowditch as follows- Bowditch also states that "The effect is greater as the temperature >difference increases." He notes that studies attempting to derive a >correction factor based on the sea-air temperature difference "differ >considerably". However, he summarizes the range of results and reports >"the average of these [studies] is about 0.16' per degree Fahrenheit >(0.28' per degree Celsius). Thus, the correction is about one-sixth of >a minute per degree Fahrenheit, or one minute per each six degrees." It's easy to see why there might be such conflict between observers as Bowditch reports. We are considering the bending of a light-ray which skims past the wave-tops at a tangent to the sea surface and then rises to reach the height-of-eye of the observer, so it's never more than a few feet above the sea. This ray gets curved according to the vertical temperature gradient. How can we guess what that temperature gradient might be? Mariners tried to allow for it by measuring the air temperature at the level of the observer's viewpoint (A), and drawing a bucketful of seawater to measure the water surface temperature (B). The temperature of the air just above the water surface will presumably be somewhere between A and B, but it's likely to be greatly affected by low-level air turbulence from the wind, and by disturbance of the sea-surface, which will both affect the temperature gradient. The gradient will presumably be greatest under glassy-calm conditions, which is when a mirage (an extreme case of anomalous dip) is likely to be seen at the horizon, if there are any objects beyond the horizon to show them up. Anomalous dip is also likely to occur when air, heated by a tropical Sun shining on desert sands, blows across a nearby stretch of water. Also above coral shoals, when awash and heated by the Sun. These may be the reasons why the Red Sea has always been regarded as a haunt of anomalous dips. So it's going to be hard to find a trustworthy rule that preducts value for dip under a wide range of climatic conditions. In the past, I suspect, many a duff sextant measurement has been blamed on "anomalous dip", which provided a convenient scapegoat. But it was never easy to assess the dip when out of sight of land, because there was no independent way to discover a ship's true position, in the ocean (how different things are now!). Fred Hebard added, about Bowditch 1966- > One interesting point was that >measurements of three or four bodies more-or-less equidistant in >azimuth would cancel the deleterious effects of anomalous dip. Another >was that backsights of an object combined with normal sights would >allow one to estimate the magnitude of anomalous dip. This is, of course quite true. However, anomalous dip effects are presumably driven by the heat of the Sun to a great extent, and probably affect Sun altitudes much more than stars. (and we have only one Sun in the sky, and dip may change over the day}. It's another argument for using stars for position, rather than the Sun. The "backsight" proposal is fair enough, but with a normal sextant this method can only be used when the Sun is more than 60 degrees up. When it is, you can take its altitude in the normal way, then turn round and measure it up from the opposite horizon, when it will be above-and-behind your head. The difference between the sum of these two altitudes and 180 deg will be twice the dip. ==================== Thanks to Fred for providing data of his land-based observations, which any sextant observer would be proud of. It would be interesting to compare those figures with what Fred could get out of shore-based observations up from a horizon, or those taken in real-life from a small boat. It's notable that much of the scatter in Fred's positions occurs in his altitudes of the Moon. Is Fred taking all the minor corrections for the Moon into account, such as augmentation of semidiameter, reduction of parallax, I wonder? Normally, one would only bother with such matters when seeking the ultimate precision of a lunar, but Fred is achieving such remarkably consistent answers that it may be worthwhile taking these minor matters into account. George Huxtable. ================================================================ 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. ================================================================