Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Re: The pedant's rhumb-line.
    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 Huxtable 
    > 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

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site