NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Multi-Moon line exercise in 2 parts
From: George Huxtable
Date: 2009 Aug 11, 00:36 +0100
From: George Huxtable
Date: 2009 Aug 11, 00:36 +0100
Peter Hakel wrote "So this "rapid-fire" procedure could be a viable backup if indeed only one celestial body is observable (like during the day) and you want a fix "now" rather than performing a running fix a few hours later. " Jeremy's observations were of the Moon. If the Moon is observed in the daytime, it can usually be crossed with the Sun. If at night, it can be crossed with a star. Either will provide what the navigator seeks; precision without an immense load of work in achieving it. He asked- "In the absence of a practical method of measuring the body's azimuth to sufficient accuracy, what else can one really do?" Simply wait. Until there's a decent change in azimuth. That's the time-honoured method of doing celestial navigation: the complete opposite of this "rapid-fire" method. Attempts to compress the time-span by taking so many observations, and discarding precision, are counter-productive. You can do far better by taking just two observations, either well-separated in time, or of separate objects. Better, though, would be three, to provide a cocked hat. Not thirty-three! Peter postulates special circumstances- "if indeed only one celestial body is observable (like during the day) and you want a fix "now" rather than performing a running fix a few hours later. " . Yes, then the method will give a result, providing impared accuracy for enhanced workload, if that's necessary and adequate, and no alternative exists. As long as the procedure is presented in that light, fair enough. But not if it's put forward as "this is the way to navigate". Peter continued- "Another method quoted here recently (post #9376) involved measuring dh/dt, the rate of change of a body of known azimuth - say by timing the rising or setting of the Sun disk at equinox. It is possible that there is some fundamental connection between that method and the rapid-fire procedure, since the latter also inherently includes dh/dt information. The rapid-fire procedure is certainly more available for practical use, since you don't need to know the azimuth and you can do it at any time provided that the sky is clear enough." Yes. A single altitude puts you somewhere on a circle around the GP of the body observed. At different positions around that circle, the rate of rise (or fall) differs, in the range zero to some maximum, which is never greater than 15 degrees per hour. In principle, if you could measure that rate with sufficient accuracy, it could tell you where you were on that circle. In normal altitude navigation, altitudes are measured, routinely, to within 1 part in 5000 of the maximum range of 0 to 90 degrees. To do similarly well by using rate-of-rise, you would have to measure that to within 1 part in 5000 also. To do that by timing the rising or setting of the Sun, which can be over in 2 minutes, would call for timing that to a fortieth of a second of time, and for establishing contact of the Sun's limbs with the sea horizon to within a third of a second of arc. It's completely impractical, just as establishing position around that circle by a measurement of azimuth is completely impractical. Only at sea, mind you. On land, measurement of azimuth can be highly precise. Jeremy has provided the list with useful real information by which we can assess the drawbacks of the proposed "rapid-fire" method. From his comments, he takes a realistic view of its limitations. So should we all. George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---