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Question on Longitude

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  • Question on Longitude

    Hi Guys,

    I am having some trouble working this out....

    I get that I need to times 6nm x Cos (or 0.5..)

    But on my calculator I get an answer of 3.1305.

    So why is the answer given as per below 11.5? Surely 6nmw/0.5 = 3nm, which is 3 Minutes?

    Or have I completely misunderstood?

    Thanks in advance, having a brain fry here!
    Attached Files

  • #2
    P.S for some reason the image won't go any bigger. It's Question 7 of the General Nav Progress Test!


    • #3
      The departure is 6 NM. Departure = Change in longitude in minutes x cosine of the latitude.

      The change in longitude in minutes is therefore the departure DIVIDED by the cosine of the latitude (not multiplied by it).

      So 6NM Divided by Cos 58o33' (0.522) = 11.5 minutes

      11.5 minutes east of your start position = 173 degrees 48.5 minutes west

      Hope this helps


      • #4
        Thanks JJ, I knew it was me!

        Have you any advice for remembering when to divide or multiply? Or is it really down to remembering?



        • #5
          Draw it up as a triangle, similar to the Distance Speed Time triangle.
          Put Departure (nm) at the top of the triangle with ChLong (minutes) in the bottom left and Cos Lat at the bottom right.

          If you are looking for Departure, you multiply the two at the bottom against each other.
          If you need to find the ChLong (to calculate an unknown longitude from a known longitude) this will be equal to Departure (nm) divided by Cos Lat.
          If you need to find the cosine of the latitude (in order to find an unknown latitude) this will be equal to Departure (nm) divided by ChLong.

          NOTE: Remember that ChLong is stated in minutes so any change of longitude stated in degrees must be multiplied by 60 to convert to minutes.
          Best Regards,

          Tony Pike

          Give Sergei back his dignity......Simples!