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True North vs Magnetic North

  1. Aug 3, 2017 #1
    I learned this in college 48 years go, problem is I have forgotten more than I ever knew. I know the answer is not very hard like 20 degrees but the internet is pissing me off. I looked this up and I get FAKE stuff like advertisements. Novels with many pages to read that never tell the answer. Formulas to calculate the answer for lots of things but what I want to know. I give up I am tired of looking. I know magnet north moves around but there is a average place where it usually is? I am trying to position solar panels so they get the same amount of sun in the morning as they do in the evening. I have a 1/4" dowel rod stuck straight up in the yard the sun shadow lines up the compus needle at 1:10 PM. This is magnetic north not true north. Setting solar panels at true north can not be that hard.
     
  2. jcsd
  3. Aug 3, 2017 #2
    Apparently it isn't that easy.... I'm curious to know, too.
     
  4. Aug 3, 2017 #3
    433px-North_Magnetic_Poles.svg.png

    The magnetic North is less than 10 degrees from the true North. And yes, while it moves around, the effect is so little that it should be negligible for your purposes. The only ones affected by the difference and movement to my knowledge are airports that are very far North. When the magnetic North moves around, they have to relabel runways, since their label indicates the magnetic North.

    EDIT:

    220px-Magnetic_North_Pole_Positions_2015.svg.png

    That said, historically it used to be much further South, beyond 20 degrees off. But, for the next few decades it will be very close to true North.
     
  5. Aug 3, 2017 #4

    OCR

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  6. Aug 3, 2017 #5

    Janus

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    What you need to do is look up the magnetic declination for where you live. This is the difference between true North and the direction a compass needle will point.
    For instance, the magnetic declination for where I live in Portland, OR is +15 degrees 20 min.
    Here's a site that will give you that info.
    http://www.magnetic-declination.com/
     
  7. Aug 4, 2017 #6
    If you are trying to determine the direction of true north in order to position your solar panels – or, better still, your new home - there is a method for determining that direction that is far simpler than fumbling with a magnetic compass. Here’s how:

    First, consult your local newspaper or weather blog for the times of sunrise and sunset in your area. Then, calculate the time that lies midway between sunrise and sunset. That time is called solar noon. Example: If sunrise occurs at 7:14 a.m. and sunset at 6:28 p.m. on a given day, then 12:51 p.m. is the time of solar noon on that day.


    Suspend a plumb bob or similar weighted object above the ground. At the moment of solar noon, note where the Sun casts the shadow of the plumb bob line. The shadow points towards true north. The direction 180 degrees opposite points towards true south. That is, solar south.


    And, of course, the mirror case applies for Southern Hemisphere blog visitors who wish to determine the direction of solar north.


    It is much simpler to use this method - would you not agree?


    Source: Solar Remodeling, a Sunset book
     
  8. Aug 4, 2017 #7
    I was thinking that would be the best way but I never watch TV and do not get a New paper. Maybe I can fine sun rise and sun set online for my zip code. I already have a plum bob set up.
     
  9. Aug 4, 2017 #8

    jim mcnamara

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  10. Aug 5, 2017 #9
    Get the times of sunrise and sunset for your city by using the following website:
    https://www.timeanddate.com/
    It is a superb free almanac.
     
  11. Aug 5, 2017 #10

    jim hardy

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    This .gov site doesn't try to sell you something

    If you type in your US zipcode and hit "Calculate" it calculates the variation/declination (two names for it) for you and shows a map.
    https://www.ngdc.noaa.gov/geomag-web/


    "Add Westerly Variation "

    in other words , if variation is West X degrees, ADD X to your compass reading to get the truth.

    Last place i lived i had to add about 6 degrees

    WesterlyVariation.jpg

    right now it's about zero here in Arkansas
    fifty years ago it was about zero there.

    old jim
     
  12. Aug 5, 2017 #11
    Here is another method to find true North that should be accurate and easy. Locate the North Star (Polaris) in the night sky. Drive two stakes in the ground several meters apart that line up with the star. Of course, if you are on a rooftop, you wouldn't drive stakes to make a pointer, but you could simply mark the roof with a suitable tool.
     
  13. Aug 6, 2017 #12

    russ_watters

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    It's true north, if you do it right. You need to know what time is your "local noon". Time zones are 15 degrees or 1 hour wide/apart, so depending on where in the time zone you live, you might get an answer that is off by +-7.5 degrees. What you do is find your longitude on a map or with a GPS that reads position and subtract from the nearest time zone center (75 degrees, 90 degrees, 105 degrees, etc.). Divide the difference by 15 and multiply by 60 to find the time added or subtracted from what you see on your watch (er....minus the hour for daylight savings time too) to find the time of local noon.

    I'd prefer the Polaris method though.
     
  14. Aug 6, 2017 #13
    Ok, here's a question based on my own post above:

    In 1859, magnetic North was around Cambridge Bay in Canada. Does that mean that a sailor in Iceland had to deal with something like a deviation of 45 degrees from true North?
     
  15. Aug 8, 2017 #14
    You almost got it.

    Here's what I learned in an outdoor skills and survival class about navigating without a compass. It was just about as long ago as you were in college, and it stuck with me ever since -- and has proven useful now and then. You don't need to look up sunrise or sunset or anything.
    1. Get a stick, a couple of feet tall (taller the better as long as it's stiff enough to hold still) and drive it into the ground so that it stands up. It doesn't have to be straight up. You're interested only in the top of the stick.
    2. The stick will cast a shadow. Mark on the ground where the shadow of the top of the stick falls.
    3. Wait several minutes, up to an hour, but you can get a decent reading after 10 minutes or so.
    4. Put another mark on the ground where the shadow of the top of the stick falls.
    5. Draw a line between the two marks.
    6. Because the sun moves from east to west, shadows move in the opposite direction; therefore, your first mark will be on the west end of the line, and your second mark will be the east end.
    7. Ninety degrees from this line is true north. Voila!
    You can also use a board with a hole in it, and see where the sunlight through the hole hits the ground. The important thing is the stick or the board cannot move during the course of this measurement.
     
    Last edited: Aug 8, 2017
  16. Aug 8, 2017 #15

    FactChecker

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    I never realized how fast magnetic North moved till I saw your post.
     
  17. Aug 8, 2017 #16

    FactChecker

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    Stars and a clock would be much better, or at least help them to calibrate their magnetic devices. Ocean navigation was a big motivator in the development of clocks.
     
  18. Aug 8, 2017 #17

    Janus

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    I've got at National Geographic map of Glacier National Park. It gives the magnetic deviation at the time of printing (2009) and the approximate change in deviation per year to keep it up to date.
     
  19. Aug 8, 2017 #18
    Or, you can go to https://skyvector.com/ to look at up-to-date pilot's navigational charts, which show magnetic north deviations as isogonic lines. You have to zoom in to find them. They're purplish in color and are labeled with degrees and minutes, like this screenshot showing a region southeast of Dallas, TX:

    WAC_chart.jpg

    I still think my suggestion about using the motion of the tip of a shadow to track a west-east line is an easier way of finding true north wherever you are. Or at night, Polaris (the North Star) is extremely close to true north also.
     
  20. Aug 8, 2017 #19

    phyzguy

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    This method ignores the equation of time. Depending what time of year you do it, you could be as much as 15 minutes off. Better to take local noon as halfway between local sunrise and local sunset as suggested by Flash Kellam, or use Polaris, as suggested by others.

    This method doesn't really work unless the sun is directly overhead, like on the equator at one of the equinoxes, for example. Elsewhere, if you do it near local noon, it will work, but if you know when local noon is, you can just take the direction of the shadow as north, as discussed elsewhere. If you try your method near sunrise or sunset, the line you described will point nowhere near north. Try it and see.
     
  21. Aug 8, 2017 #20
    I have tried it many times. You have a rather large window (an hour or so) before and after local noon where the shadow movement method gives very good results. You get perfect results on or near the equinoxes, when shadows move in a straight line from sunrise to sunset.

    But even on the worst possible days (the solstices), doing the exercise when the sun is approximately near its zenith still gives good results. You don't need to know when local noon is, you just have to be reasonably close. If you want perfection near the solstice, you can get a good west-east line by marking the shadow any time before noon, marking intervals as you observe the shadow get shorter, and then stop marking when the shadow is the same length as the original mark. The shadow traces a hyperbolic path with the apex pointing at your stick, so you just go for symmetry of the hyperbola to get your west-east line. And the point where the shadow is shortest would be local noon, I suppose.

    For the purpose of the OP, to line up solar panels, it doesn't have to be exact.
     
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