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SRH
2012-Jul-05, 02:06 AM
If I was standing on the surface of the Earth, with a compass held parallel with the surface, which direction would the compass needle point at:
1. the geomagnetic (rotational) north pole?
2. the magnetic north pole?
3. the geomagnetic (rotational) south pole?
4. the magnetic south pole?
5. the equator?

If I was standing on the surface of the Earth, with a compass held upright, or perpendicular to the surface (like I was going to shake somebody's hand), which direction would the compass needle point at:
6. the geomagnetic (rotational) north pole?
7. the magnetic north pole?
8. the geomagnetic (rotational) south pole?
9. the magnetic south pole?
10. the equator?

I have heard many different answers, so please let me know if you are 100% sure.


Here are my guesses, but this is probably wrong:

1. N would point toward the magnetic north pole
2. needle would spin around
3. S would point toward the magnetic south pole
4. needle would spin around
5. N would point West
6. N would point down
7. N would point up
8. N would point down (S point up)
9. N would point up
10. N would point up

Thank you very much!

Jeff Root
2012-Jul-05, 12:30 PM
You used the term "geomagnetic" incorrectly if you mean
to refer to the rotational pole. "Geomagnetic" just means
"having to do with Earth's magnetic field". So it has no
connection to the rotational poles.

A regular compass near or at the magnetic north pole
would have the north-pointing end pulled down, so that
it couldn't turn freely and wouldn't work if held level. The
same compass at or near the south pole obviously would
have the south-pointing end pulled down.

A compass which is designed to work in the vertical plane
is called a "dip needle compass".

A dip needle compass at the north magnetic pole would,
just like the regular compass, have its north-pointing end
pulled down, but it would be able to turn freely in the
vertical direction and operate correctly.

The north-pointing end of a regular compass at the
equator would point toward the north magnetic pole,
not west! Where did THAT come from??

A dip needle compass at the equator would point
horizontally, not vertically.

So you have some correct guesses, some backwards
guesses, and two that are off by 90 degrees:

1. N would point toward the magnetic north pole (correct)
2. N would be pulled down (ambiguous)
3. S would point toward the magnetic south pole (correct)
4. S would be pulled down (ambiguous)
5. N would point toward the magnetic north pole (not west!)
6. N would point downish and toward the magnetic north pole
7. N would point down (not up!)
8. N would point upish (not down!) and S would point downish
(not up!) and toward the magnetic south pole
9. N would point up (correct)
10. N would point toward the magnetic north pole (not up!)

-- Jeff, in Minneapolis

.

ToSeek
2012-Jul-05, 02:33 PM
Strictly speaking, the compass would attempt to line up with the local magnetic field lines, which on the Earth's surface follow curved paths between the magnetic poles that when projected onto the Earth's surface point toward the poles with sufficient accuracy for most practical purposes.

profloater
2012-Jul-05, 02:38 PM
have just read "forgotton footprints" about antarctic exporation and you question was very relevant, they were trying to locate the s magnetic pole by observing a dip compass (as explained above) because they knew a regular ship's compass becomes very sluggish at high lattitudes and navigation difficult. Obviously they could not sail anywhere near the S pole but they did not know that for sure at the early attempts. The magnetic field becomes vertical at the surface at the magnetic pole which moves around relative to the spin pole.

JustAFriend
2012-Jul-06, 01:09 AM
dont forget that any magnetic rock/mountains can cause a compass needle to shift away from magnetic north.
(happens more often than you'd think)

pilots used to carry maps of magnetic variances so they wouldn't be surprised if they went off course.

Jens
2012-Jul-06, 07:06 AM
1. N would point toward the magnetic north pole
3. S would point toward the magnetic south pole


The fact that you write these differently hints that you may have a misconception. "N pointing toward the magnetic north pole" is the same as "S pointing toward the magnetic south pole." It's just as correct to say that S is repelled by magnetic north. It's not like the north magnetic pole is something that pulls the north needle of the compass. As ToSeek said, the whole magnet is trying to line itself up. So that if you cut the needle in half, both pieces would still try to line up in the same way.

SRH
2012-Aug-19, 02:02 PM
You used the term "geomagnetic" incorrectly if you mean
to refer to the rotational pole. "Geomagnetic" just means
"having to do with Earth's magnetic field". So it has no
connection to the rotational poles.

A regular compass near or at the magnetic north pole
would have the north-pointing end pulled down, so that
it couldn't turn freely and wouldn't work if held level. The
same compass at or near the south pole obviously would
have the south-pointing end pulled down.

A compass which is designed to work in the vertical plane
is called a "dip needle compass".

A dip needle compass at the north magnetic pole would,
just like the regular compass, have its north-pointing end
pulled down, but it would be able to turn freely in the
vertical direction and operate correctly.

The north-pointing end of a regular compass at the
equator would point toward the north magnetic pole,
not west! Where did THAT come from??

A dip needle compass at the equator would point
horizontally, not vertically.

So you have some correct guesses, some backwards
guesses, and two that are off by 90 degrees:

1. N would point toward the magnetic north pole (correct)
2. N would be pulled down (ambiguous)
3. S would point toward the magnetic south pole (correct)
4. S would be pulled down (ambiguous)
5. N would point toward the magnetic north pole (not west!)
6. N would point downish and toward the magnetic north pole
7. N would point down (not up!)
8. N would point upish (not down!) and S would point downish
(not up!) and toward the magnetic south pole
9. N would point up (correct)
10. N would point toward the magnetic north pole (not up!)

-- Jeff, in Minneapolis

.

Thanks Jeff!

When you asked me: "The north-pointing end of a regular compass at the
equator would point toward the north magnetic pole,
not west! Where did THAT come from??"

I figured out what originally made me think of this...
http://en.wikipedia.org/wiki/Ionosphere#Anomalies_to_the_.22ideal.22_model

Under Equatorial Anomaly:
"Within approximately 20 degrees of the magnetic equator, is the equatorial anomaly. It is the occurrence of a trough of concentrated ionization in the F2 layer. The Earth's magnetic field lines are horizontal at the magnetic equator. Solar heating and tidal oscillations in the lower ionosphere move plasma up and across the magnetic field lines. This sets up a sheet of electric current in the E region which, with the horizontal magnetic field, forces ionization up into the F layer, concentrating at 20 degrees from the magnetic equator. This phenomenon is known as the equatorial fountain."
Under Equatorial Electrojet:
"At the magnetic dip equator, where the geomagnetic field is horizontal, this electric field results in an enhanced eastward current flow within 3 degrees of the magnetic equator, known as the equatorial electrojet."

I think this may have to do with with the magnetic-dip compass as you were describing. But could you please explain further?
I am struggling with why the compass would point North if the magnetic field lines are horizontal. Thanks so much!

Hornblower
2012-Aug-19, 02:10 PM
Thanks Jeff!

When you asked me: "The north-pointing end of a regular compass at the
equator would point toward the north magnetic pole,
not west! Where did THAT come from??"

I figured out what originally made me think of this...
http://en.wikipedia.org/wiki/Ionosphere#Anomalies_to_the_.22ideal.22_model

Under Equatorial Anomaly:
"Within approximately 20 degrees of the magnetic equator, is the equatorial anomaly. It is the occurrence of a trough of concentrated ionization in the F2 layer. The Earth's magnetic field lines are horizontal at the magnetic equator. Solar heating and tidal oscillations in the lower ionosphere move plasma up and across the magnetic field lines. This sets up a sheet of electric current in the E region which, with the horizontal magnetic field, forces ionization up into the F layer, concentrating at 20 degrees from the magnetic equator. This phenomenon is known as the equatorial fountain."
Under Equatorial Electrojet:
"At the magnetic dip equator, where the geomagnetic field is horizontal, this electric field results in an enhanced eastward current flow within 3 degrees of the magnetic equator, known as the equatorial electrojet."

I think this may have to do with with the magnetic-dip compass as you were describing. But could you please explain further?
I am struggling with why the compass would point North if the magnetic field lines are horizontal. Thanks so much!

My bold. The magnetic field lines are oriented north and south at the equator, and are horizontal in which case an ordinary compass is at its best.

SRH
2012-Aug-19, 02:15 PM
So "horizontal" in this case means parallel with the Earth's surface and does not mean parallel with the equator?

Thanks!

Ara Pacis
2012-Aug-19, 07:19 PM
So "horizontal" in this case means parallel with the Earth's surface and does not mean parallel with the equator?

Thanks!

Think of them like meridians, they converge at the poles and are perpendicular to the equator.