How to find true north not magnetic north?

  • Thread starter gary350
  • Start date
  • #1
216
35
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?
 

Answers and Replies

  • #2
.Scott
Science Advisor
Homework Helper
2,948
1,219
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
Vanadium 50
Staff Emeritus
Science Advisor
Education Advisor
2021 Award
28,214
12,849
This will be easier:

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.
 
  • Like
Likes russ_watters
  • #5
270
62
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.
 
  • Like
Likes berkeman
  • #6
.Scott
Science Advisor
Homework Helper
2,948
1,219
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.
From the description, I think he was looking for "high noon".
 
  • #7
sophiecentaur
Science Advisor
Gold Member
27,258
5,952
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.
 
  • #8
216
35

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=
 
  • #9
270
62
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.
 
  • #10
russ_watters
Mentor
21,501
8,544
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.
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.
 
  • #11
sophiecentaur
Science Advisor
Gold Member
27,258
5,952
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.
 

Attachments

  • polarscope-classic-reticle.jpg
    polarscope-classic-reticle.jpg
    42.6 KB · Views: 675
  • #12
sophiecentaur
Science Advisor
Gold Member
27,258
5,952
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.
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.
 
  • #13
russ_watters
Mentor
21,501
8,544
You would need to know your longitude for this, of course...
Fortunately we all carry a device that can tell us.
(of course, there is probably an app for telling local noon too)
and know about whether you need to add or subtract your angles.
Gawd, I would hope most people can figure that out!
 
  • #14
sophiecentaur
Science Advisor
Gold Member
27,258
5,952
Gawd, I would hope most people can figure that out!
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:
 
  • #15
tech99
Science Advisor
Gold Member
2,447
1,024
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.
 
  • Like
Likes russ_watters
  • #16
sophiecentaur
Science Advisor
Gold Member
27,258
5,952
To find True North exactly for surveying new microwave stations, I used a Sun sight taken with a theodolite.
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.
 
  • #17
Dale
Mentor
Insights Author
2021 Award
32,844
10,034
Every time I click calculate it returns to the town already listed.
In the drop down for the town select “Enter Lat/Long”
 
  • #18
216
35
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:
  • #19
Dale
Mentor
Insights Author
2021 Award
32,844
10,034
which is wrong by 2 hours.
Did you set your time zone and daylight savings correctly? It works for me.
 
  • #20
216
35
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:
  • #21
Dale
Mentor
Insights Author
2021 Award
32,844
10,034
Hopefully sometime before tomorrow you can get the noaa calculator working :-)
 
  • #22
216
35
Hopefully sometime before tomorrow you can get the noaa calculator working :-)

I finally figured out how to use noaa. These numbers are correct for GPS & Google Maps but not for noaa N35 52.802 W086 27.684

N & W & 0 need to be dropped. The . needs to be moved 2 places to the left for noaa.

N35 52.802 = 35.52802 = Longitude

W086 27.684 = 86.27684 = Latitude

DST has to be clicked

Do not change time zone number -6

Magnetic noon = 1:12 pm

Solar noon = 1:18 50 pm

Shadow movement angle from magnetic noon to solar noon is 6.8 degrees
 
  • Like
Likes Dale
  • #23
russ_watters
Mentor
21,501
8,544
I finally figured out how to use noaa. These numbers are correct for GPS & Google Maps but not for noaa N35 52.802 W086 27.684

N & W & 0 need to be dropped. The . needs to be moved 2 places to the left for noaa.

N35 52.802 = 35.52802 = Longitude

W086 27.684 = 86.27684 = Latitude

DST has to be clicked

Do not change time zone number -6

Magnetic noon = 1:12 pm

Solar noon = 1:18 50 pm

Shadow movement angle from magnetic noon to solar noon is 6.8 degrees
I get 12:46pm CDT by my previous calculation. The link tells me 12:50 CDT. As a cross-check, "the equation of time" of 4 minutes is the deviation, which matches the difference between the calculations (I didn't realize the deviation due to Earth's orbit was that much).

So I think you are doing at least two things wrong:
1. When separated by one space each, the position measurements are degrees and minutes. NOAA wants degrees, minutes and seconds. You have the degrees and minutes already, and just need to convert the seconds by multiplying the decimal of minutes by 60. Or just omit seconds because it makes very little difference.

2. I don't know what the other error could be. The date maybe? Since the center of your time zone is 90 degrees, you are east of it, which means local noon is usually earlier, not later than what your clock tells you (DST adjusted).
 
  • #24
216
35
I should check magnet north again. I have learned a very good expensive compass will give a screw up reading in town it is so sensitive it picks up nails in the yard, metal car in the driveway 50 ft away, metal in the house, metal utility in the yard, chain link fence around the yard, etc. A cheap $1 toy compass is more accurate in town because near by metal things do not effect it.
 
  • #25
Vanadium 50
Staff Emeritus
Science Advisor
Education Advisor
2021 Award
28,214
12,849
A cheap $1 toy compass is more accurate in town because near by metal things do not effect it.

That's not possible. A compass measures a magnetic field. There is one magnetic field at each point in space, not multiple ones depending on whether nearby objects "count" or not.
 
  • #26
tech99
Science Advisor
Gold Member
2,447
1,024
You can point the hour hand of a watch at the Sun, then split the angle between the hour hand and 12 o'clock, and that is True South.
 
  • #27
216
35
That's not possible. A compass measures a magnetic field. There is one magnetic field at each point in space, not multiple ones depending on whether nearby objects "count" or not.

Metal attracts magnets. Compass needle is a magnet. Car is metal, chain link fence is metal, nails are metal. The cheapo compass is a magnet it does not point in the direction of metal. The expensive compass is a better magnet it will find everything metal in the yard & house you can not get a accurate reading unless your out in the wilderness where there is no metal. Buy a compass see for yourself.
 
Last edited:
  • #28
Vanadium 50
Staff Emeritus
Science Advisor
Education Advisor
2021 Award
28,214
12,849
None of what you wrote is correct.
 
  • #29
728
169
You can point the hour hand of a watch at the Sun, then split the angle between the hour hand and 12 o'clock, and that is True South.

Hour hand? All I see are numbers.

Cheers
 
  • Like
Likes sophiecentaur
  • #30
phyzguy
Science Advisor
5,012
1,995
  • Like
Likes anorlunda
  • #31
sophiecentaur
Science Advisor
Gold Member
27,258
5,952
You can point the hour hand of a watch at the Sun, then split the angle between the hour hand and 12 o'clock, and that is True South.
Actually, it isn't unless you happen to be in the centre of a time zone. Your watch shows Universal time (plus or minus the appropriate hours offset) so there can be up to 180/24 = 7.5 degrees of error with that method. Near enough if you're lost out on the moors but not if you are trying to build a house or draw an accurate map.
 
  • #33
Vanadium 50
Staff Emeritus
Science Advisor
Education Advisor
2021 Award
28,214
12,849
You were the one who claimed that some compasses measure different magnetic fields than others. Apart from going against several centuries experience with magnetism, it is contrary to all good sense and logic. So it seems to me the burden is on you to prove your iconoclastic and heterodoxical beliefs, not mine. (However, before you go down that path, I recommend you review the PF Rules on personal theories. Wouldn't want to have it removed as crackpottery, would we?)

A compass is a device to measure the direction of the local magnetic field. The local magnetic field has one value, with one direction. It does not have multiple values depending on the imputed presence or absence of various nearby objects. That would be like having two thermometers, one that tells you the temperature of the room with the air conditioning on and another with the air conditioning off. Instead, a thermometer measures the actual temperature in the actual room, and not the imagined temperature in some speculative room. A compass does the same thing - it measures the real magnetic field, and not some imaginary field for some counterfactual arrangement of nearby objects.
 
  • Like
Likes berkeman and Doc Al
  • #34
berkeman
Mentor
62,520
13,318
Compass needle is a magnet.
True, but a weak and very small one. Unless you get real close to some ferrous material, the magnetic attraction between the compass needle and the object will have basically no effect.

But it's true that ferrous objects do deflect the Earth's magnetic field nearby. That's why it's hard to find a good place to mount a compass in a typical car made of steel.

https://i2.wp.com/mumetal.co.uk/wp-content/uploads/2013/10/Picture1.jpg
Picture1.jpg
 

Attachments

  • Picture1.jpg
    Picture1.jpg
    19.6 KB · Views: 358
  • #35
Nugatory
Mentor
13,900
7,344
Metal attracts magnets. Compass needle is a magnet. Car is metal, chain link fence is metal, nails are metal. The cheapo compass is a magnet it does not point in the direction of metal. The expensive compass is a better magnet it will find everything metal in the yard & house you can not get a accurate reading unless your out in the wilderness where there is no metal. Buy a compass see for yourself.
This isn't quite right. A compass points in the direction of the magnetic field at wherever the compass is located. Nearby metal objects do affect the direction that the compass points, but not because they attract the needle; they change the direction the magnetic field in their vicinity points.

The difference in readings between a cheap compass and an expensive one has nothing to do with one being better than the other at finding nearby metal. They're both reading the exact same magnetic field because there's only one magnetic field at each point in space; the expensive one is just doing it better (more accurately and more repeatably).

And all of this reminds me... we just replaced some stuff in the console of the boat, and the added metal is affecting the local magnetic field differently than what had been there.... so in the next few days I have to slightly adjust the compass housing so that it reads zero degrees when I'm going due north. Because it's the 21st century I'm not going to use Polaris to determine north, I'm going to use the GPS - head out to open water, drive due north according to the GPS, adjust the compass so it agrees. And why am I messing with a compass at all when I have GPS? Long before I had a GPS I would get through the fog by following a compass heading from buoy to buoy and I want the backup in case the GPS dies on me.
 
  • Like
Likes Dale, anorlunda and berkeman

Related Threads on How to find true north not magnetic north?

  • Last Post
Replies
18
Views
2K
Replies
15
Views
881
Replies
12
Views
3K
Replies
10
Views
26K
  • Last Post
Replies
6
Views
808
Replies
2
Views
2K
Replies
9
Views
3K
Replies
5
Views
1K
Replies
11
Views
54K
Top