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: Bruce Stark
Date: 2003 Mar 19, 14:20 EST
From: Bruce Stark
Date: 2003 Mar 19, 14:20 EST
Even if a boat could be made as steady as a lighthouse foundation, altitudes taken from it would not be as reliable as those taken from a ship. Or so it seems to me. I believe that the nearer the navigator's eye is to the water, the more anomalous refraction will displace the horizon. To simplify the logic behind this idea, forget about normal refraction. Suppose that under normal conditions light would come to your eye from the horizon in a straight line. To simplify things further, suppose anomalous refraction occurs at a particular point along the line, making an angle at that point. So the navigator will see the horizon at that point, rather than at its true place. Air near the water is the main breeding ground of anomalous refraction in the dip. A navigator on the bridge of a ship will be a long way from the point where anomalous refraction has bent the line. The angle at his eye, between the true horizon and the point where he sees it, will be small. A navigator on a small boat, on the other hand, will be close to that point. Consequently the angle at his eye, between the true horizon and the point where he sees it, will be larger. I hope List members will overlook the simplistic way this is presented and examine the question in their own minds, as well as in the light of experience. Bruce