Direction of the magnetic field of the Earth

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So, I have had a bit of a look around and can't come up with a concrete answer, so I thought I'd post my curiosity on here!

What is the direction that a compass points in 3 dimensional space, and how is this affected by proximity to North?

to clarify what I mean: We all know that compasses point north, but this is usually on a single axis, perpendicular to Gravity. if we had a compass which was free to rotate in 3 axes, would it point directly through the ground towards the north pole, or would it continue to be parallel to the earths surface? would this change as the compass was moved closer to the pole? I am assuming that it would point straight down at the pole.

Thanks!
 

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  • #3
phinds
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Have you ever Googled a map of the Earth's magnetic field? While I believe that most such make the probably unwarranted assumption that the Magnetic North Pole is on the surface of the Earth, even if you posit that it is below the surface you will still see that there is no way that a magnetic field line could run directly from the North Pole to a point on the surface where you are holding a compass (unless that point was directly above one of the poles)
 
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  • #5
tech99
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The downward angle is called the Angle of Dip.
Compasses are optimised for the area where they will be used by altering the balance a little, so they do not tend to point downward.
 
  • #6
phinds
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The downward angle is called the Angle of Dip.
Compasses are optimised for the area where they will be used by altering the balance a little, so they do not tend to point downward.
Very interesting. I didn't know that. Thanks for posting.

I see from a quick look on the Internet that this is actually an issue since if the dip is too great and is not accommodated, the needle can rub against the casing and give poor readings.
 
  • #7
tech99
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Very interesting. I didn't know that. Thanks for posting.

I see from a quick look on the Internet that this is actually an issue since if the dip is too great and is not accommodated, the needle can rub against the casing and give poor readings.
When I was at school we measured angle of dip using a vertical compass called a dip circle.
 
  • #8
phinds
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I also note that the one internet article I read states that the dip is zero at the equator. Technically, that can't be quite right since the poles are not on a line that is perpendicular to the equatorial circle. It's probably close enough for government work though.
 
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The dip comes up in geology. As iron-rich lavas cool below, crystals (free to rotate in three dimensions) to the local magnetic field. When the rocks are completely cooled, the dips are locked in place and record the latitude at which the igneous rocks cooled. This is part of the evidence for plate tectonic theory.
 
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The downward angle is called the Angle of Dip.
Inclination.
 
  • #12
gleem
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You are aware that the compass does not usually point directly north (to the earths rotational axis) but varies according to location (Lat and Long) around the world. In the US it varies by a total of more than 40 degrees from the east coast to the west. This variation is called the magnetic declination. At the east cost the needle points NNW and at the west coast it points NNE from true north. The magnetic declination also change a small but significant amount each year. The declination and its variation are recorded on the compass rose(s) of navigational charts for the area that the chart covers.
 
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This just in from the morning newspaper. The Earth's magnetic field intensity is weakening and magnetic North is moving towards Siberia at 34 miles/year. All of which makes a case for relying on GPS rather than compasses.
 
  • #15
OmCheeto
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@dlgoff pointed me to a very nice NOAA tool a few years back:
Magnetic Field Calculators

According to that thing, the magnetic field lines where I live, 45.5°N & 122.5°W, go through my brain, and enter the earth just 2 feet in front of me.

Not quite vertical, but still, not as I ever pictured it. (Until dl pointed it out, of course.)
 
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  • #16
phinds
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... the magnetic field lines where I live, 45.5°N & 122.5°W, go through my brain, and enter the earth just 2 feet in front of me.
You're lucky. They don't seem to leave my brain at all :confused:
 
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