Magnetic declination
The magnetic declination (or magnetic variation) at any point on the earth is an angle that must be added or subtracted in converting between two kinds of directional information:
For points in the Northern Hemisphere, these are usually described as magnetic north and true north respectively. (In the Southern Hemisphere, visualizing the underlying physics and the practical calculations would be clearer with magnetic south and true south substituted.)
"True" directions
True north and south are of course the local directions to the respective geographic poles. (More precisely, these are horizontal directions, along great circles, toward the poles; the real directions to the poles, along straight lines, point into the ground at angles to the earth's surface.) The geographic poles are defined by astronomical observations, and reflect the rotation of the earth (experienced roughly as the progress of day and night): the earth's axis is the line connecting the geographic poles, and every other point on the earth's surface traces, roughly daily, a circle whose center lies on that axis.
Where compasses point
Magnetic north and south, on the other hand, are widely misunderstood. The statement is often made that magnetic north is the direction to the North Magnetic Pole. This is, in most places, fairly close to being true. Of course, as with geographic poles, the direction of interest is almost always a horizontal direction. But the horizontal direction in question is that of the needle of a good compass, which nearly always differs measurably from the horizontal direction to the nearer magnetic pole.
The popular idea of "a huge body of magnetized material inside the earth" (and the picture of it as a symmetrical body) encourage this picture. In fact, flows of electrical charges in molten minerals produce the magnetic field, and their deviation from the "big bar magnet" picture is not simply failing to maintain a neat symmetrical pattern. In fact, any overall pattern of flow is secondary to flows that are largely up and down rather than horizontal. These separate flows coordinate to a substantial extent, so that there is an overall roughly north-south magnetic field, but the nearest flows contribute most strongly to the field sensed by a compass at the earth's surface; the horizontal direction of these fields are nearly always near the horizontal direction from one magnetic pole to another, but usually a little off to left or right. (What is true in the popular picture is that following a compass will eventually lead to a magnetic pole, but it will do so because the mis-aiming cancels out overall, as the traveller follows a probably curved and perhaps meandering path to the magnetic pole.)
Theory
Change of declination in time and space
Magnetic declination varies both from place to place, and with the passage of time.
In most areas, the spatial variation reflects the irregularities of the flows deep in the earth; in some areas, small or large, iron ore or magnetite in the earth's crust may contribute strongly to the delination.
The time variation reflects changes in the deep flows: a flow becoming stronger or weaker, changing direction, or shifting its location. In each case, such a change is likely to contribute to shift in the location of at least one of the magnetic poles, unless its effect on that pole is cancelled by the effect of a change to a flow in another part of the earth's interior.
Stating the declination
There are three major ways of stating the declination for a given place:
- In a diagram
- On some maps intended for wilderness or navigational use, including the topographic maps of the U.S. Geological Survey (USGS), a diagram shows the relationship between magnetic north in the area concerned (with an arrow marked "MN") and true north (a vertical line with a five-pointed star at its top), with a label near the angle between the MN arrow and the vertical line, stating the size of the declination and of that angle, in degrees, mils, or both. (On USGS maps, the diagram is near the lower left hand corner, and the information labelled "GN" in the same diagram is irrelevant to this discussion.)
- As the numeric size of the angle between magnetic and true north, and the direction from true north to magnetic north.
- For instance, "10° W" would indicate that magnetic north lies 10 degrees counter-clockwise from true north.
- As the signed number of degrees, where a positive angle indicates clockwise from true north and a negative counter-clockwise.
- For instance, "-10°" would indicate the same as the "10° W" just discussed.
Learning the declination for an area
Most use of declination is in conjunction with a map; as stated, that map may state the declination. If not,
- Lines of equal declination (isoclines) are given on aeronautical and shipping maps.
- A prediction of the current magnetic declination for a given location (based on an world-wide National Geophysical Data Center, a division of the National Oceanic and Atmospheric Administration of the United States. (At first glance, one would of course rather have the real delination off a map than a prediction. However, the map is guaranteed to be months or years out of date; each model were built with all the information available to the map makers at the start of the five-year period it is prepared for, reflects a highly predictable rate of change, and will usually be more accurate than a map, and almost never less accurate.)
Using the declination
Adjustable compasses
A magnetic compass points to magnetic north. Modern navigational compasses usually include a "baseplate" marked with a compass rose and a scale of degrees; some include a
