NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Don Seltzer
Date: 2013 Jun 14, 06:44 -0700
Frank Reed wrote:
>Jacques, you wrote:
"equal altitude before and after the meridian alitude"
Yes, that is the simplest form of it. It can be significantly improved upon, and that's what I teach in my "Easy Intro" course. A single pair of altitudes centered about noon is subject to significant errors. Because the variation of altitudes around noon is slow, it's much better to have four or more altitudes on either side of noon. Fior example, starting 15 or 20 minutes before noon, shoot four altitudes. Then shoot four altitudes close to noon. Then shoot four more approaching 15 to 20 minutes after noon. In practice, this is nearly dientical to saying "shoot a dozen altitudes, at leisure, during the 30-45 minute period centered on noon. The you plot the altitudes versus GMT (UT) on common graph paper. The best estomate of the "smoothed" peak gives the meridian altitude. Then the graph can be folded in half to get the best alignment of the before noon and after noon sights to yield the axis of symmetry. That axis is LAT = 12:00:00. Read off the GMt at that axis, add/subtract equation of time, and then you have longitude. There is a catch, of course. If the vessel is in motion, the sights are skewed. And in fact, the changing declination of the Sun is equivalent to vessel motion so even if the observer is motionless, the sights are skewed. There is a simple correction of this which can be done graphically or by a table lookup. But if this correction is ignored, the error can be very large.
-FER
>
In early 19th century texts such as Moore, Bowditch, and MacKay, I find descriptions of finding local apparent time by equal altitudes, in which a pair of sights are taken several hours before and after noon, when the altitude of the sun is changing at a more rapid rate. The math is trivial compared to that required for a time sight, yet the method is suggested not as a method of determining longitude at sea, but as a means of checking the accuracy of the chronometer while in a harbor with a known longitude.
Was the equal altitudes method used at any time in the 19th century for determining longitude?
Don Seltzer
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