True South

Discussion in 'Homework Help' started by rawanayyoub, Dec 30, 2012.

  1. rawanayyoub

    Thread Starter New Member

    Oct 9, 2012
    8
    0
    Hi all ,
    Please I need to know how we can determined true south practically ???


    Thanks in advance :)
    Rawan Ayyoub
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,140
    1,789
    1. Use a compass and apply the deviation in your local area.
    2. At "local noon" the sun will be directly overhead and to the South of your nadir in the northern hemisphere
     
  3. rawanayyoub

    Thread Starter New Member

    Oct 9, 2012
    8
    0
    aha Thanks.... , but what's meaning of " the deviation in your local area." ?? ,,,,, and please if you can explain method number 2

    with my deepest thanks
    Rawan Ayyoub
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,140
    1,789
    There is a correction between true north and magnetic north. It is called deviation. You can find this information on aeronautical charts among other places. The deviation might be positive or negative and you add or subtract it from the magnetic heading to get the true heading.

    When the sun is directly overhead in the Northern Hemisphere, you construct an imaginary arc from the sun to the horizon and that is true south. For other times of the day you can apply an angular correction. This is the principle on which a sundial is constructed.

    http://en.wikipedia.org/wiki/Sundial
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,360
    Plant a tall stick or pole into the ground. Hang a plumbline from the top of the pole to make sure the pole is perfectly vertical.

    On a sunny day make a mark on the ground where the shadow of the stick appears. Place a mark or stone on the ground where the tip of the shadow appears. (This must be done on level ground.)
    Do this many times over a couple of hours before midday and after midday.

    Find the shadow that is the shortest. This shadow will be aligned with true north and south.

    It will also tell you when was high noon.
     
    Last edited: Dec 30, 2012
  6. Papabravo

    Expert

    Feb 24, 2006
    10,140
    1,789
    Slight correction:
    Declination is the difference between true north and magnetic north.
    Deviation has to do with the presence of iron deposits in the area that perturb the Earth's magnetic field. As a pilot we always wrapped it into one number that we used for navigation. I must have done it right because I never got lost and I'm still here -- LOL
     
    Last edited: Dec 30, 2012
  7. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    Also true North is perpendicular to the shadow path on the ground at any time of day. You don't need an accurately vertical stick for this, you don't have to wait until noon, and you only need two points.
     
  8. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    look up

    angle of declination

    http://www.ehow.com/how_8255939_declination-angle.html
     
  10. WBahn

    Moderator

    Mar 31, 2012
    17,737
    4,789
    That's what the survival manuals say, everything from the Boy Scout stuff to the Department of Wildlife's Winter Survival Guide for Hunters. Even the Air Force's Survival Guide that is issued to pilots.

    All of these also show a diagram of finding south using a stick and a wristwatch. Interestingly, the diagrams of both in all three (of the ones that I have) are identical -- not similar, but copies of the exact same diagram.

    The only problem is that it is wrong.

    The shadow cast by the tip of something traces out a curve over the course of the day. I've verified this by direct experiment. I live at ~40N and the effect is pretty significant. Now, if I am only trying to get a quick estimate of what the general direction of south is, it works well enough for that.

    As for the watch and stick technique -- it is almost total hogwash. If you think about it as you move yourself from equator to pole, you quickly see that it is all but useless.
     
Loading...