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I How to find true north not magnetic north?

  1. Jun 28, 2018 #1
    How to find true north vs magnetic north? Online information says magnetic north moves around as much as 2.5 degrees. Lots of information online about true north 1 place save 5 degrees off magnetic, another place says 10 to 12 degrees, another place 10 to 25 degree, another place 15 to 30, another place says true north is 45 degrees from magnetic north. Another place says true north lines up perfect with the north star all calculations are taken from the north star.

    What I am trying to do is find out, when is the SUN is exactly straight up at my house. I used a compass to find magnetic north then hammer a steak in the ground when the suns shadow lines up with magnetic north sun it straight up with magnetic north but not with true north. Magnetic north shows sun is straight up a 1:12 pm at my house. Seems to me I read someone where magnet north vs true north is about 23 degrees but with a fluctuation of 2.5 degrees what does that really mean.

    Another place I read magnet north vs true north is at maximum angle at the north pole the closer you get to the equator the lower the angle. There is so much BS online these days no one knows what to believe.

    If I knew sun rise and sun set I could calculate the 1/2 point and know when sun is straight up. I do know sun rise was 5:03 am June 22 but never could find out what sun set was. I know from watching the sun it sets about 8:30 pm. Convert 8:30 to a 24 hr clock = 20:30. 5.03 + 20.30 = 25.33 / 2 = 13.34 convert back to 12 hr clock = 1.34 pm I have no clue how accurate this is. 1.34 pm is more accurate than 1:12 pm.

    My zip code is 37129 there may be a place to look up information by zip code for, sun rise, sun set, high noon but I have not found it?
     
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  3. Jun 28, 2018 #2

    .Scott

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    True north is easy.
    If you want a good approximation and you are in the Northern hemisphere, just look at Polaris, the North star. It is never more than 0.75 degrees away from due north (true north).

    If you want to be more precise, look at Polaris shortly after sunset and then again shortly before sunrise. You should try to get them as close to 12 hours apart as you can (easier to do in the winter). Then split the difference.

    Alternatively, you can use a GPS and follow a path where the latitude has increase and the longitude hasn't changed.

    ... or: wait for Polaris to be directly above or below the first star in the handle of the Big Dipper. At those times, Polaris will be due north.
     
  4. Jun 28, 2018 #3

    Dale

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  5. Jun 29, 2018 #4

    Vanadium 50

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    This will be easier still. Since you're north of the Tropic of Cancer (your zip code does not start with 987 or 8), then sun will never be directly overhead.
     
  6. Jun 29, 2018 #5
    You draw a circle on the ground. You put a straight-up stick in the center. The shadow will cross that circle before noon and after noon. Connecting those points will give you and east-west chord and perpendicular to that a north-south chord.
     
  7. Jun 29, 2018 #6

    .Scott

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    From the description, I think he was looking for "high noon".
     
  8. Jun 29, 2018 #7

    sophiecentaur

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    Why has no one suggested a simple magnetic compass and then look up the magnetic variation on an Ordnance Survey map or whatever is equivalent in the particular country? A magnetic compass is pretty convenient to use and the rest is numbers.
     
  9. Jun 29, 2018 #8
    I found this already. I even figured out my LN=35.8981 & LM=86.4561 but the calculator will not calculate this. Every time I click calculate it returns to the town already listed.

    Finally found the option to type in my LN=35.8981 & LM=86.4561 but it changes my numbers to another location. It tells me solar noon is 10:50 am I can look at the sun and see it is 2 hours from being straight up.














    ln=
     
  10. Jun 29, 2018 #9
    I predict the OP high noon for today, using longitude = 86.45610°.
    ------------------------------------------------------------------------------
    Knowing the following;
    Daylight Savings time = 1 hour = 15°
    Offset from standard meridian= 3.5439° ( east )
    Equation of time = 3m 29s slow = -.87083° ( for today 6/29 )
    -----------------------------------------------------------------------------------------------
    15° - 3.5439° + .87083° = 12.32693° = 49m 18s
    the OP high noon for today = 12:49:18 pm (hr:m:s)
    This agrees with NOAA within 3 sec.
     
  11. Jun 29, 2018 #10

    russ_watters

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    So, right; the question is oddly put(and contains the "straight up" error), so I think people are assuming constraints based on speculation.

    For me, the easiest way I can think of to find when the sun is due south is to add/subtract your angular distance from your nearest time zone center meridian.

    E.G., my longitude is 75deg, 16', 27", so local noon for me is 12:06 (EST).

    We could probably think of a dozen methods though.
     
  12. Jun 29, 2018 #11

    sophiecentaur

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    I can't find where the required accuracy is stated in this thread. The position of Polaris is near enough for Jazz and certainly is as good as you can hope for if you don't have some optical 'help' either to find it or to use it. You can improve by a factor of about ten with not much trouble.

    polarscope-classic-reticle.jpg
    Astronomical telescope mounts are aligned to point to the North Celestial Pole with a Polarscope. The polariscope is a small telescope with angular magnification such that small circle has a radius of 0.75°. You look at the Big Dipper with one eye and rotate the reticle so that the diargram of the Big Dipper is orientated the same way as the real thing (not overlaid; it's way outside the field of the scope) in the eye that's looking through the scope. This only needs a little practice. You then adjust the telescope mount so that polaris sits bang in the middle of the tiny circle and the + is at the CNP.
    That's not the most accurate method but other methods involve knowing your longitude and the exact time of day to rotate the reticle at the correct angle. This way uses the sky as its own clock. I was contemplating suggesting making one up yourself but the radius of that circle would be so small that you would need some magnification unless you have Elven Sight.
    Note - polaris doesn't appear where you would expect the 'pointers' to point because the polariscope is an inverting scope. The image of polaris in the scopeis diametrically opposite where it is in the sky.
    Acnowledgement: That diagram is what you see in a Skywatcher polariscope so I guess it could be copyrighted. The method is an improvement on the one quoted by @.Scott as it works whenever you can see polaris plus most of the Big Dipper.
     
  13. Jun 29, 2018 #12

    sophiecentaur

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    You would need to know your longitude for this, of course, and know about whether you need to add or subtract your angles. Astro navigation works 'upside down' if I remember correctly. (Jus' sayin' :smile:, as one who has frequently used reciprocal bearings in my navigation)
    I love the Polarscope method (above) because it does all that for you.
     
  14. Jun 29, 2018 #13

    russ_watters

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    Fortunately we all carry a device that can tell us.
    (of course, there is probably an app for telling local noon too)
    Gawd, I would hope most people can figure that out!
     
  15. Jun 29, 2018 #14

    sophiecentaur

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    You'd be surprised. It's as difficult as reverse parking in to a bay and that's beyond a lot of people. :wink:
    How many transatlantic family telephone calls have been made five hours early when they would have been five hours later? (Not me, of course!!)
    Edit: And, of course, that great Astronomical Thing in your profile pic gives it all away about you. :biggrin:
     
  16. Jun 29, 2018 #15

    tech99

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    To find True North exactly for surveying new microwave stations, I used a Sun sight taken with a theodolite. I then used Birdwood's Tables (probably no longer exist), which gave the Sun's azimuth for every second of every year.
    I notice that Stanford's Astro Navigation Tables, a small, current publication, give the time (near noon) when the Sun (or any navigational star or planet) is on the N-S line of the Greenwich Meridian, and this will probably enable N-S to be found elsewhere.
     
  17. Jun 29, 2018 #16

    sophiecentaur

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    What sort of angular precision do you have to work with? The Sun is very handy because it's there during your working day but it is not very co operative as it wanders all over the sky.
     
  18. Jun 29, 2018 #17

    Dale

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    In the drop down for the town select “Enter Lat/Long”
     
  19. Jun 30, 2018 #18
    https://www.esrl.noaa.gov/gmd/grad/solcalc/sunrise.html

    This link gave me wrong information using my zip code.

    I used my car GPS to find N35 52.802 W086 27.684

    I typed the GPS information into Google Maps and it shows me where I am standing holding the GPS in my driveway.

    Now that I know this is correct N35 52.802 W086 27.684 I type in into the above link and it tells me solar noon is 10:50am which is wrong by 2 hours. Sun is almost straight up now at 1 pm. I don't have time to set things up in 10 minutes to check high noon with the compass again. Last time I check the compass said high noon was 1.12pm which is only off by 22 minutes according to another persons solar noon calculations.

    Tomorrow I could set up the experiment in the yard again. I can measure the angle difference between the 12:50pm shadow vs 1:12pm shadow. Measuring the shadow is not 100% accurate but close enough I could be off by 1 minute. Once I know the angle between solar noon at magnetic noon I can always subtract from magnet noon to find solar noon. .
     
    Last edited: Jun 30, 2018
  20. Jun 30, 2018 #19

    Dale

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    Did you set your time zone and daylight savings correctly? It works for me.
     
  21. Jun 30, 2018 #20
    I set up a quick test, metal rod stuck in the ground at 45 deg angle. Time 22 minutes from 1:21 pm to 1:42 pm the sun shadow moved 6.5 degrees. OH wait I screws up I only timed it 21 minutes. OH well do the math. Math says shadow movement is 6.8 degrees for 22 minutes.

    I am still not 100% sure the difference between magnet noon and solar noon is 22 minutes?

    According to Sun Dial online information sun shadow movement gets bigger & bigger as the sun gets closer to the horizon.

    The best time to do this test again tomorrow would be from 12:50 pm to 1:12 pm from the experiment I just did sun movement should be slightly more than 6.8 degrees because sun is closer to 90 degrees..
     
    Last edited: Jun 30, 2018
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