How to find true north not magnetic north?

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In summary, online information says magnetic north moves around as much as 2.5 degrees, but lots of information online about true north claim it is somewhere between 10 and 25 degrees off magnetic north. There is no one accepted answer to this question.
  • #36
Vanadium 50 said:
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.

I am not claiming a compass can measure a magnetic field. A claim a cheap toy compass does not have a good strong pointer needle magnet like a much better quality more expensive compass. Directions that come with a compass says, keep it away from metal objects to get an accurate reading. Sometimes wind moves my TV antenna then I have to reset it. I have noticed the toy compass works best to aim the TV antenna in the correct direction because the toy compass is not attracted to the metal antenna tower from 5 ft away. I have to walk 20 ft away from the antenna using the good quality compass. The good quality compass is effected by all the metal objected in the yard, patio furniture, cars in the driveway, garden tools leaning up against the tree, tiller in the garden, lawn mower in the shed, chain link fence around the yard. I have noticed the good quality compass can be used to find nails in the sheet rock wall once I find a nail I know there are other studs in the wall all 16" apart this helps me hang pictures on the wall for my wife. The toy compass is a pain in the butt trying to find nails in the wall but when either of the compass get close to a nail the needle points right at the metal nail.

Do this experiment. Buy a stack of magnets here. https://www.ebay.com/itm/Super-Strong-N35-Round-Disc-Neodymium-Mini-Fridge-Magnets-Rare-Earth-New/173384504513?var=&hash=item285e8594c1 put the stack on a float in a plastic tray of water in the yard away form all metal. The float boat will rotate to North south direction. Now move a piece of metal near the plastic tray the float boat will move toward the metal. Compass needle does the same thing.
 
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  • #37
Vanadium 50 said:
None of what you wrote is correct.
I'm not sure what you referred to here because you didn't actually quote one of ~Gary's posts. I am not sure that he has been putting across what he really means. It could be muddled rather than plain wrong. - could be a language thing? (@gary350 ?)

But it is true to say that finding magnetic North is affected by nearby objects and their actual orientation. Ships (even humble boats. sometimes) have various arrangements round a compass binnacle which attempt to correct for the iron in the vessel. There are permanent magnetic effects, varying electromagnetic effects and there are effects of the susceptance of the ship parts on the Earth's field. See this link.
On top of that, doing it properly involves using a 'Deviation Table' which is a set of deviations from the ship's apparent magnetic N and the magnetic N on the chart. You 'swing the ship' over 360° and take readings every few degrees. and comparing with a known geographical bearing. If you want a good magnetic bearing, you correct with the deviation table and then you apply the Variation, which is on every chart. That tells you the variation on a certain date with a yearly rate figure to correct further.
Something I have not understood is how a Fluxgate compass is reckoned to be better behaved than a magnetic needle. Also, why do they tell you to mount a fluxgate sensor low down in the ship? That could be very near an iron keel.
 
  • #38
Nugatory said:
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

Back in Days Of Old, the ship's compass was placed in something called a binnacle. It had two nearby iron correcting spheres to compensate for all the ferrous material on the boat. Add (or remove) ferrous material and "simply" readjust.
 
  • #39
sophiecentaur said:
But it is true to say that finding magnetic North is affected by nearby objects and their actual orientation.

Sure. But it's not true that a cheap compass is not affected by the nearby objects and only the Earth's magnetic field. (The claim "A cheap $1 toy compass is more accurate in town because near by metal things do not effect it.") As mentioned, the magnetic field is what it is, and nearby objects can change it, but change it they do. A cheap compass does not ignore these changes.
 
  • #40
Vanadium 50 said:
Back in Days Of Old, the ship's compass was placed in something called a binnacle. It had two nearby iron correcting spheres to compensate for all the ferrous material on the boat. Add (or remove) ferrous material and "simply" readjust.
Yes, and your scare-quotes around the word "simply" are well-taken - it's trickier than it sounds.
 
  • #41
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.
 
  • #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.
 
  • #43
gary350 said:
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.
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.

gary350 said:
a cheap toy compass does not have a good strong pointer needle magnet like a much better quality more expensive compass
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.
 
  • #44
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.
 
  • #45
gary350 said:
Now move a piece of metal near the plastic tray the float boat will move toward the metal.
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.
 
  • #46
gary350 said:
The goal was to find solar noon buy knowing where magnet noon is.
Whatever is "magnet Noon"?
 
  • #47
anorlunda said:
By the way, a ships binnacle is hardly an obsolete term.
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.
 
  • #48
gary350 said:
What I am trying to do is find out, when is the SUN is exactly straight up at my house.

gary350 said:
The goal was to find solar noon buy knowing where magnet noon is.

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:
Helios said:
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.

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.
 
  • #49
Vanadium 50 said:
Sure. But it's not true that a cheap compass is not affected by the nearby objects and only the Earth's magnetic field. (The claim "A cheap $1 toy compass is more accurate in town because near by metal things do not effect it.") As mentioned, the magnetic field is what it is, and nearby objects can change it, but change it they do. A cheap compass does not ignore these changes.

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.net/v/t1.0-9/36688576_2336260109725061_6862444676205510656_n.jpg?_nc_cat=0&oh=f590686a6ee74ac7262ebb06b786eb04&oe=5BA149DE
 
  • #50
gary350 said:
It was only more accurate in my yard because it was not effected by all the metal in the yard.

Repeating a wrong thing does not make it right.
 
  • #51
anorlunda said:
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.

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 Earth's 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.net/v/t1.0-9/36688576_2336260109725061_6862444676205510656_n.jpg?_nc_cat=0&oh=f590686a6ee74ac7262ebb06b786eb04&oe=5BA149DE
 
  • #52
gary350 said:
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.

Whoa. You're going off the deep end of complexity. What are you trying to accomplish? I thought it was aligning a sundial.
 
  • #53
gary350 said:
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.
Vanadium 50 said:
Repeating a wrong thing does not make it right.

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
 
  • #54
Tom.G said:
An insanely strong magnet from a disc drive head positioner versus a medium size paper clip magnetized from it.
And you believe that expensive compasses are made from insanely strong magnets?
 
  • #55
Dale said:
And you believe that expensive compasses are made from insanely strong magnets?
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.
gary350 said:
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.
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.
 
  • #56
Compasses are not stud finders.
Compasses are not made with superstrong magnets that can magnetize the surrounding objects.
"Much maligned" is for a reason.
 
  • #57
Tom.G said:
Not necessarily. Just pointing out that a much maligned condition as stated by the OP can, in fact, occur.
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.
 
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  • #58
Nugatory said:
And why am I messing with a compass at all when I have GPS?
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).
 
  • #59
I would say that we are getting off-topic except that the topic of this whole thread is very unclear.

sophiecentaur said:
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).
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.
 
  • #60
anorlunda said:
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.
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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  • #61
Maybe I should make it clear that I have 4 compasses. 3 are low cost boy scout compasses. All 3 boy scout compasses appear to be identical and made by the same company except that 1 is green color the other 2 are black. I paid $2, $3 & $5 for the boy scout compasses so I am calling these cheap or low cost compasses. Needle sticks on the $2 green compass. The dial sticks on the $3 black compass. The $5 compass works best of the 3. I also have a more expensive compass that cost $20 but maybe the price should not be considered as making it be a better compass than the $5 compass. The $20 needle moves very smooth and the dial rotates very nice much better than the $5 compass. The more expensive compass has a stronger magnet field on the needle that is why it is more sensitive to metal objects near by. The $20 compass is hard to use to adjust my TV antenna because I have to stand too far away from the TV antenna to make adjustments. There you have it the details of my 4 compasses. I consider the $2 & $3 compass to be basically worthless. If I am lost in the wilderness I would like to have the $20 compass as long as I am not lost near a large iron ore deposit in the earth.
 
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  • #62
gary350 said:
I consider the $2 & $3 compass to be basically worthless.
Do not rely on any magnetic compass, at any price. There is too much variation, local field or magnetic storm.

You read your Lat/Long with GPS earlier. Now go to a point as far away that you can see from the first site. Read the GPS there. Go back and check the first point. You now have two points on the WGS84 ellipsoid. Use navigation software to find the bearing from the first point to the second, remote point. Offset that bearing from that survey line to find the N-S meridian.
 
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  • #63
tech99 said:
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.
But since I'm South of the equator, it gives me North.
That will only give an approximation of the North-South meridian line. It depends on your longitude in the time zone and a seasonal correction using the Analemma.
That sundial correction oscillates between –14min 15sec and +16min 25sec.
See; https://en.wikipedia.org/wiki/Analemma
https://en.wikipedia.org/wiki/Equation_of_time
 
  • #64
gary350 said:
The more expensive compass has a stronger magnet field on the needle that is why it is more sensitive to metal objects near by.
I doubt this is correct. I suspect that most of the difference in sensitivity is due to the better bearings
 
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  • #65
@gary350 , please answer the question. What are you trying to accomplish?
 
  • #66
Dale said:
I doubt this is correct. I suspect that most of the difference in sensitivity is due to the better bearings

That is true the $20 does have a better needle movement. If I go out into the center of the front yard where there is no fence and no metal with 2 compasses the needle point at each other. If I put both compasses on the ground and move them apart soon Earth magnetic field becomes stronger than the other compass needle and they both stop pointing at each other and point at magnetic north.
 
  • #67
gary350 said:
If I put both compasses on the ground and move them apart soon Earth magnetic field becomes stronger than the other compass needle and they both stop pointing at each other and point at magnetic north.
When close, they should align, but with the North pole of one pointing to the South pole of the other.
When far apart, they do not both point to magnetic North, they will always influence each other.

There is no point buying a magnetic compass if you want to know true North.

QUESTION; Why do you need to know true North?
 
  • #68
I think this thread has run its course.

Thread closed.
 
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