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Great Circle Sailings: Initial Course and Distance

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  • Great Circle Sailings: Initial Course and Distance

    Hi all,

    Studying for my mates exams which are coming up later this year.
    Ive run in to a really frustrating problem, and I cant seem to figure out what Im not understanding here.

    Im working on Great Circle sailings calculations,

    "Determine the great circle initial course from LAT 37°-12.6'S, LONG 073°-58.0'W to LAT 10°-33.0'S, LONG 142°-08.0'E."

    I can work out the entire problem, no issue, but I cant seem to remember the last step.
    For this problem, after working it out I end up with an Initial Course of 42.7°, Im under the impression that this should be notated as N 42.7° E.
    This is because, if you go from the initial long and lat, to the final long and lat, you are going North, and in the East direction.
    That would give me a final course of 42.7°.

    However, the answer is actually an initial course of S 42.7° W? And the actual final course would be around 223°?

    I dont understand why the initial course would be notated south of west, when the vessel is going to be going NE.

    Can someone clear this up for me?
    For reference, an example problem would be USCG #452.

    Thanks.

  • #2
    It has been a long time since I did a GC calculation but I’m pretty sure if you are in the Southern Hemisphere there is no way your initial course would be Northeast. You always start heading towards the pole, not away from it. Maybe you are suffering from classroom induced errors, have you got a chart you can find to plot it on to give you an overview of the route?


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    • #3
      You're heading west though. Going east would be the long way round.

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