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International Date Line

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  • International Date Line

    So this is the explenation I got for question 610098 (GNAV):

    "The earth rotates from West to East, therefore, time to the East is ahead of time to the West.

    When crossing the International Date Line from the Western to the Eastern Hemisphere we are going 'forward' in terms of time and 'backwards' in time when crossing from the Eastern to the Western Hemisphere. Therefore:

    Westerly - Add a day
    Easterly - Subtract a day"

    With which I agree except for the last statment... "Westerly - Add a day
    Easterly - Subtract a day."

    How can you add a day if you are going back and time? And the opposite, how can you subtract a day if you're going forward in time???


  • #2
    Re: International Date Line

    Think of it this way - the Western Hemisphere is behind the Eastern Hemisphere and the delineation is the International Date Line (which approxmates the 180° E/W Meridian).

    As you move in an easterly direction you advance in time.

    Let's say, start at 0400 Local Time on 15th June at 150° W.

    Moving Easterly at 15° per hour, on reaching 150° E you will have covered 300 degrees, 20 hours and the Local Time is midnight. Moving further East you will be moving into the next day, the 16th June. At the 180° E/W Meridian it is 0200 on 16th June and moving Easterly another 30 degrees into the Westerly Hemisphere (to 150° W), which is 'behind' the Eastern Hemisphere, another 2 hours will have elapsed to 0400 - but we already know that the time and date at 150° W is 0400 on 15th June. Hence, we must go back a day as we cross the IDL in an Easterly direction.

    I hope I've explained that OK.

    Last edited by Tony Pike; 03-09-2013, 12:34.
    Best Regards,

    Tony Pike

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


    • #3
      Re: International Date Line

      Perfect, thank you


      • #4
        Hi Tony, I have an objection over the wording of available choices . It states when you are crossing the date line on "Westerly Heading" So i assume that you are travelling from East to West , which means you will subtract and not add. If question states that you are crossing the international date Line from West to East then it does make a perfect sense. WHat is your opinion on this questions wording.?? Many thanks


        • #5
          I agree the wording can get a little confusing but think of it like this.
          If you travel from,say, 90W to 140W you are on a westerly heading. If you travel from 140W to 170W you are still on a westerly heading. If you now go across the dateline from 170W to 170E you are still on a westerly heading and going from the Western Hemisphere to the Eastern Hemisphere and adding a day as you go.
          Hope this helps.