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I How to find true north not magnetic north?

  1. Jul 6, 2018 #41

    Dale

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    Why are we using a compass at all. I thought that the goal was to find when local solar noon was. That can be simply looked up on the net on multiple websites which have been provided.

    @gary350 can you remind us what the goal is? To my understanding a magnetic compass was not essential.
     
  2. Jul 6, 2018 #42
    I just learned something new. Assume your standing on the sun looking at earth with true north straight up to the north star and magnet north to the right. 6 hrs later earth has rotated 90 degrees and magnet north is pointing straight at the sun. Another 6 hrs later magnetic north is pointing to the left. 6 hrs later magnetic north is pointing away from the sun. Depending on where you live on earth the angle between magnet north and true north will be different at solar noon for you and your geographical location. There are 6 time zones from east Canada to Alaska if magnetic north pointing straight at the sun in Alaska at the exact same time magnet north is point to the right in Canada. But 3 months later as earth rotates around the sun magnet noon is now rotated 90 degrees to the west. If I take a magnet noon reading with a compass and homemade type sun dial rod stuck in the ground at 45 degree angle north the angle between solar noon and magnet noon will never be the same from day to day. I probably will not notice changes from day to day but changes from month to month will be easy to detect. I was hoping to use magnetic noon to find solar noon by checking the angle of the shadow to true north but it is changing every day. 2 times every year solar noon and magnet noon will be at the same time. 3 months later and 9 months later magnet noon will be as far as it gets from solar noon.

    The goal was to find solar noon buy knowing where magnet noon is. Lots of people suggested use noaa but so far I have not been able to know if the answer I get is correct. From what I just learned magnet noon changes every day so it makes no sense to use it to find solar noon.

    I never thought about it until now but if you drive your car from east Canada to Alaska through 6 times zones you have driven 1/4 of the way around earth.
     
  3. Jul 6, 2018 #43

    Dale

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    If you go to the north pole and walk around it then you have walked all the way around the earth. The closer to either pole you go, the shorter distance it is.

    I suspect that the difference between a cheap and a good compass is more in the bearing than in the strength of the magnet. Cheap bearings get "stuck" so the resulting compass is less accurate.
     
  4. Jul 6, 2018 #44

    anorlunda

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    When boating on New York Canals, I used my magnetic autopilot. I had to stay alert because when passing under low bridges, the autopilot would lurch as much as 40 degrees. It was interesting because it only happened with ancient bridges, not modern ones. I'm not sure if the old bridges had residual fields left from when they were manufactured, or if they acquired it in their century of service. A reasonable guess is that one of the specifications for modern structural steel is that it be demagnetized before leaving the factory.

    By the way, a ships binnacle is hardly an obsolete term. Nor are the metal balls used to adjust compasses obsolete. What makes compasses expensive has more to do with gimbals and damping than accuracy. As the boat rolls and pitches and yaws in rough seas, it is most annoying to have the compass heading swing also. Also interesting, is that the same difference between cheap and expensive GPS marine instruments. The cheap ones show near-instantaneous values, the expensive ones suppress periodic oscillations by active digital filtering.

    As @sophiecentaur said, a marine compass, regardless of how accurate the claim, must use a deviation card which is calibration by experiment.

    I'm afraid we're getting off-topic, but this stuff is interesting. The best answer to the OP was given by @Helios in #5.
     
  5. Jul 6, 2018 #45

    Mark44

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    It depends on the kind of metal. If you put a piece of aluminum or any other nonferrous metal near the magnets, they won't be attracted to that metal.
    I believe that this misconception about metal is just one of the things you said in post #27 that Vanadium 50 referred to when he said that none of the things you said were correct.
     
  6. Jul 6, 2018 #46

    sophiecentaur

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    Whatever is "magnet Noon"?
     
  7. Jul 6, 2018 #47

    sophiecentaur

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    When everything else has stopped working and they've jury rigged a piece of canvas to the radio mast, they can still tell where North is.
     
  8. Jul 6, 2018 #48

    anorlunda

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    You are contradicting yourself and confusing the posters with your conflicting goals.

    To find true north, there is no need to even mention magnetic north. Use the method @Helios gave:
    Or, if your goal is to set up a sundial in the Northern Hemishphere, it is more than accurate enough to point to Polaris, The North Star.
     
  9. Jul 7, 2018 #49
    It was only more accurate in my yard because it was not effected by all the metal in the yard. Your right the high price compass is best if your hiking in the wilderness.

    https://scontent-atl3-1.xx.fbcdn.ne...=f590686a6ee74ac7262ebb06b786eb04&oe=5BA149DE
     
  10. Jul 7, 2018 #50

    Vanadium 50

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    Repeating a wrong thing does not make it right.
     
  11. Jul 7, 2018 #51
    Yes the goal is to find solar noon but at the time of this post I figure the easy way to do that is check the angle from the magnet noon shadow. But now i know magnet noon changes every day so my original idea will not work. Using noaa and GPS reading I am not sure the answer is correct. Solar noon will not be the same every day either. Earth orbits sun every 365 days, 5 hrs, 48 min, 46 seconds. On leap year longest day of the year is June 21, the next year June 21, the next year June 22, the next year June 22, after leap year June 21 again. Every year earths orbit is off 1/4 day plus 1 min 14 seconds. We make up the 1/4 day with leap year every 4th year but we never make up the 1 min 14 seconds. In about 54 years earth orbit is off 1 hour and about 1296 years earth orbit is off 1 day. Now I see only way to know for sure where solar noon is every day do the math every day I can not find solar noon with the clock or magnetic noon. Online information says, people navigated earth for 100s of years by using the North star and other stars. Funny thing 1977 scientist published a paper that said, The north star is the only star in the northern sky. Scientists thought they had learned something but the Bible written 2000 years earlier says, The north star is the only star in the northern sky. LOL I don't know how to find solar noon with the north star I can't even see the north star during the day. Online says, find solar midnight to know where solar noon is. This was all very educational I never knew solar noon moves every day.

    https://scontent-atl3-1.xx.fbcdn.ne...=f590686a6ee74ac7262ebb06b786eb04&oe=5BA149DE
     
  12. Jul 7, 2018 #52

    anorlunda

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    Whoa. You're going off the deep end of complexity. What are you trying to accomplish? I thought it was aligning a sundial.
     
  13. Jul 7, 2018 #53

    Tom.G

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    I just ran the experiment. An insanely strong magnet from a disc drive head positioner versus a medium size paper clip magnetized from it.

    Suspended the magnet with about 3 feet of sewing thread and got it settled pointing North. Then moved it near the refrigerator and it rotated about 45° when a foot away. The paper clip had to approach to about two inches before rotating the same amount.

    Conjecture: the field from a compass pointer should be about the same strength as the Earth magnetic field for minimum interference from nearby ferrous objects.

    Cheers,
    Tom
     
  14. Jul 8, 2018 #54

    Dale

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    And you believe that expensive compasses are made from insanely strong magnets?
     
  15. Jul 8, 2018 #55

    Tom.G

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    Not necessarily. Just pointing out that a much maligned condition as stated by the OP can, in fact, occur. I presume that at least some compass manufactures would be aware of the condition, even if other folks are not.
    Consider those "Stud Finders" you can by in the loacal hardware store. Some are just a suspended magnet that point to the nails holding the wallboard to the studs. Same idea, on a smaller scale and range.
     
  16. Jul 9, 2018 #56

    Vanadium 50

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    Compasses are not stud finders.
    Compasses are not made with superstrong magnets that can magnetize the surrounding objects.
    "Much maligned" is for a reason.
     
  17. Jul 9, 2018 #57

    Dale

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    But not with compasses, regardless of the price of the compass. The condition stated by the OP does not occur. The condition was not regarding magnets of varying strengths, but compasses of varying price.
     
    Last edited: Jul 9, 2018
  18. Jul 9, 2018 #58

    sophiecentaur

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    If you are sailing, the compass heading can be at least as useful as the COG (Course O~ver Ground), which is what the GPS tells you (after a finite delay).
     
  19. Jul 9, 2018 #59

    anorlunda

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    I would say that we are getting off-topic except that the topic of this whole thread is very unclear.

    Not always true. Consider the limiting case of a vessel adrift in a current. GPS tells you COG. Compass heading may be anything 001-360 degrees. The compass tells which direction the bow points, but that is irrelevant to motion in that case.

    When fresh water sailors first encounter salt water tidal currents, they can be seen to mistakenly believe that motion of their boat is in the direction the bow is pointing.
     
  20. Jul 9, 2018 #60

    sophiecentaur

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    I used the word “can” and seeing the effect of leeway and tide is very useful at times. Bearing, Heading and COG all have their place. The good old magnetic compass should be the last thing you chuck over the side (except perhaps for the Aeropress coffee maker).
     
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