Magnetic declination

Question

i couldnt just figure out when i got to know that declination can be zero also. How can true and magnetic north ever align themselves in a straight line in any place? Also if a compass aligns in the direction of horizontal component of MF at that place does this mean that at any place horizontal component is directed towards the magnetic north ?? Maybe i am misinterpreting this idea of direction. here is what i think : If u look at a bar magnet's field lines not every tangent to the curve will pass through the north pole. So the compass placed at a point will align with the field but it wont point at the north pole always . please explain

Answer 1

Because the magnetic field does not align with the geographic axis, most places will show a variation between true north and magnetic north.

However since some areas have an east variation and some areas have a west variation, there must be a border between those areas where the variation just happens to be zero.

Answer 2

Also if a compass aligns in the direction of horizontal component of MF at that place does this mean that at any place horizontal component is directed towards the magnetic north ??

That's exactly what it means. That's the definition of magnetic north.

If u look at a bar magnet's field lines not every tangent to the curve will pass through the north pole. So the compass placed at a point will align with the field but it wont point at the north pole always.

That's true, but also irrelevant. A typical compass doesn't show you the direction of the tangent line- it shows you the projection of the direction of the magnetic field into the plane of the compass, whatever plane you're holding it in. Usually, one holds them parallel to the surface of the Earth. Neither the compass needle nor the magnetic field will typically point directly towards the actual north pole through Euclidean space, but that doesn't matter, because the direction of "North" that people care about for navigation on the surface of the Earth is defined in relation to the spherically curved surface of the Earth, in which "straight lines" are actually arcs of great circles. And the projection of the magnetic field into the plane of a compass held parallel to the surface of the Earth does point along a great circle that intercepts the pole, even though a straight line from that point will just shoot off into space somewhere.

If that same great circle also passes through the true geographic north pole, then the difference between them will be zero. As bowlofred noted, no matter what the difference between true and magnetic north is, that has to happen somewhere, along exactly one great circle. If the magnetic and spin axes are aligned, then it it happens everywhere.

The key is understanding the difference between "straight lines" on the surface of the Earth, vs. "straight lines" in space. in other words, yes, you are "misinterpreting this idea of direction". A compass does not point straight at the pole through space, but it does point straight at the magnetic pole along the surface of the Earth.

Answer 3

Simply: if the line from your position to the magnetic North also goes through the geographic North, your magnetic declination will be zero. This happens at every point along the great circle that includes magnetic North and the North Pole.

Note that in reality there are sufficient iron ore deposits in the earth to disturb the "ideal" picture above - see http://en.m.wikipedia.org/wiki/Magnetic_declination for maps of the actual magnetic deviation on the earth, including the locations of the agonic lines - where deviation is zero. These are not, in fact, great circles...