NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: The pedant's rhumb-line.
From: Dave Weilacher
Date: 2002 Oct 10, 12:53 -0700
From: Dave Weilacher
Date: 2002 Oct 10, 12:53 -0700
Well. Crap. Although this piece was entertaining, educational, and thought provoking, it will have everyone on this listed specifying "shortest" rhumb-line where just rhumb-line used to do :-} On Thu, 10 Oct 2002 20:31:54 +0100 George Huxtablewrote: > Every now and again, rhumb-lines appear as a > topic on this list. > > Most list-members are aware that wherever you > start from on the Earth, with > whatever course, if you keep following a rhumb > line (constant course), you > will end up spiralling into one or other of the > poles. You will have made a > number (perhaps an infinite number?) of turns, > in longitude, to get there, > but will have travelled (in general) a finite > distance. > > Navigators frequently ask for the rhumb-line > course from A to B. Here I > wish to display my pedantry by insisting that > there is not just one > rhumb-line course between A and B, but many > (perhaps an infinite number?). > The one required by the navigator is the > SHORTEST rhumb-line course. > > This can be illustrated by a simple example. > Take a navigator on the > equator at position A, long = 0 degrees. What > is his rhumb-line course and > distance to a destination B, lat 10 deg North, > long 0 deg.? The obvious > answer is a course of zero degrees, which > happens to be identical with the > great-circle in this case. But what if he sets > off from A with a course of > about 88.4 degrees, that is, slightly North of > due East, and holds that > course? Well, that's a rhumb line, also, which > will also take him to B (or > it would if no continents got in the way), but > he has to go Eastwards, > right round the world, to get there. Similarly, > a course of about 271.6 > degrees will also take him round the world to > B, but Westwards. > > He could set off with a course even closer to > due East, roughly 89.2 > degrees, and if he sticks to that while making > two whole circuits of the > Earth it will take him to B. Again, there's > another corresponding rhum line > Westwards. Other courses, progessively closer > still to due East or West, > would take him to B after 3, 4, or more > circuits of the Earth, right up to > infinity. > > What's the practical importance of this? None > whatsoever. I am just > pointing out that these alternative, but quite > impractical ways of getting > from A to B are all genuine rhumb lines between > A and B, and each fully > meets the definition of a rhumb line. There is > no unique rhumb line. The > only rhumb line that's of any practical > significance, however, is the > shortest rhumb line. > > Just something to ponder over on a quiet watch. > > George Huxtable. > > > ------------------------------ > > george@huxtable.u-net.com > George Huxtable, 1 Sandy Lane, Southmoor, > Abingdon, Oxon OX13 5HX, UK. > Tel. 01865 820222 or (int.) +44 1865 820222. > ------------------------------ > Dave Weilacher .US Coast Guard licensed captain . #889968 .ASA certified sailing and celestial . navigation instructor #990800 .IBM AS400 RPG contract programmer