Magnetic meridian

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Field line display of the entire geomagnetic field filling the near-earth space
Meridian representation of the Earth's near-surface magnetic field. The magnetic meridians are drawn as red, the geographical meridians and circles of latitude as blue lines. The angles between the magnetic and geographic meridians correspond to the local declination .

A magnetic meridian of the earth's magnetic field indicates the direction in which the horizontal component of the magnetic field points. This is also the direction in which a compass sets itself .

Earth's magnetic field

The field lines of the earth's magnetic field emerge from the earth's surface at a more or less steep angle (angle of inclination ) at all locations south of the magnetic equator (not just at the pole) , sometimes swinging far outward through the near-earth space and occur in the northern hemisphere at a similar angle back into the earth's surface. The local direction of a field line therefore has a horizontal and a vertical component. For compass navigation, only the horizontal component is relevant. It is described by the direction of the magnetic meridian.

Magnetic meridian as a plane

A magnetic needle that is suspended in its center of gravity in such a way that it can swing freely in both the horizontal and vertical directions will adjust itself to be parallel to the local field lines (i.e. in particular be inclined ). If a vertical plane is chosen to contain the magnetic needle, the orientation of that plane will reflect the horizontal component of both the needle orientation and the field line direction, while being independent of the inclination of the needle. The formal definition of the magnetic meridian is:

A vertical plane through the magnetic axis of a free-swinging but stopped magnetic needle is called the magnetic meridian of the observation site.

The magnetic meridian is not understood here as a line, but as a plane. If the geographical meridian is also defined as a plane (namely as the meridional plane containing the observation site and the two geographical poles), the magnetic declination is simply the angle between these two meridian planes.

Viewing it as a level has the advantage of a simpler mathematical treatment and a simpler expression. It allows one to speak briefly of the horizontal component of the magnetic field without having to worry about the vertical component. Instead of “aligning a needle in such a way that the horizontal component of its alignment is parallel to the horizontal component of the field lines, whereby their inclination is arbitrary”, one speaks more briefly of “bringing the needle into the magnetic meridian”.

Compass needles are not completely free to move, but so that they always lie horizontally. They therefore only react to the horizontal component of the earth's magnetic field and come to rest in the same meridian plane as a freely moving needle. The fact that both are then with different inclinations in this plane is irrelevant for the intended direction display and is appropriately described by the term meridian plane, which is also independent of the inclination.

Magnetic meridian as a line

However, it is also common to understand both the magnetic and the geographic meridian as lines that result as a projection of the relevant vertical planes onto the earth's surface. These lines can then be displayed on maps. Such a map does not show the field lines, but lines which have the same direction at every point as the horizontal component of the local field lines near the surface.

Magnetic meridians and field lines

The magnetic meridians must be carefully distinguished from the magnetic field lines. The meridians describe the direction of the near-surface magnetic field and thus only the respective near-surface sections of the local field lines. The sections of the field lines in the interior of the earth or far in space are of no interest to the compass user and are also not represented by the meridian lines.

The meridians describe the horizontal component of the course of the field lines and ignore their vertical component. The meridians themselves - understood as surfaces - have no defined vertical component or - understood as lines - run horizontally by definition.

All meridians start at the Antarctic magnetic pole and end at the Arctic magnetic pole . In between, they run along the surface of the earth as a line, the course of which follows the direction indicated by compasses. The field lines, on the other hand, emerge from the earth's surface distributed over the southern hemisphere (particularly close to the magnetic pole) and enter the earth's surface distributed over the northern hemisphere.

Meridian representations of the earth's magnetic field

  • View at 0 ° longitude

  • View at 90 ° longitude

  • View of 180 ° longitude

  • View at −90 ° longitude

  • View of the arctic magnetic pole

  • View of the Antarctic magnetic pole

The meridian representations of the earth's magnetic field reveal the local compass directions and thus the course of the horizontal component of the field. They show the irregularities of the field and demonstrate that compasses generally point neither to the geographic nor exactly to the magnetic north pole.

Web links

Individual evidence

  1. ↑ a b c J. Heussi: Textbook of Geodesy. FA Brockhaus, Leipzig 1861, p. 138 ( Google Books )
  2. ↑ a b J.H. Nelson, L. Hurwitz, DG Knapp: Magnetism of the Earth. US Department of Commerce - Coast and Geodetic Survey, Publication 40-1, United States Government Printing Office , Washington 1962, p. 5 ( PDF 7.1 MB )
  3. ↑ CF Gauß, W. Weber: Atlas of Earth Magnetism - designed according to the elements of the theory. Weidmann'sche Buchhandlung, Leipzig 1840, p. 7 ( Google Books )