Earth and Its Orbit - The Science of Navigation - page 26

In figure 1.13 we see a much-simplified geomagnetic field structure,
which nevertheless shows two features that are important to us. First, the
field is not aligned exactly north-south. That is, magnetic north is not the
same as true north. In fact, it moves. In 2005, the location of magnetic
north was approximately at 82\ 54% north, 114\ 24% west. It is moving from
northern Canada toward Siberia at a rate of 60 km yr -1 and accelerating.
The angular di√erence between magnetic north and true north is called
declination by geophysicists and variation by navigators. It is a great prob-
lem for navigation because it means that a compass does not point north-
ward; the error may be small in equatorial regions, but it can be very large
near the poles. For this reason, navigators have for centuries measured the
magnetic field of the earth so that calculations can be made to compensate
for the compass variation error. 23
The second feature of the geomagnetic field that is apparent in figure
1.13 is the inclination angle ; this is the angle that the field lines make with
the surface of the earth. Near the equator, the inclination is 0\; moving
toward the magnetic poles we see that the inclination increases.
We met Edmond Halley earlier in this chapter. He was one of the first sci-
entists to investigate the variation of the earth's magnetic field, and under-
took a sea voyage over the North and South Atlantic in order to measure it.
His magnetic map was the first to use contour lines to indicate regions of
constant magnetic variation ( isogonic lines). He published his book New
and Correct Chart Shewing the Variations of the Compass in 1701. Subsequent
measurements over the centuries revealed that the variations are changing,
and doing so quite significantly. Not only does the location of magnetic
north vary, but so does magnetic south—and magnetic south is not even
diametrically opposite magnetic north. Also, inclination angles vary with
time, for any fixed location on the earth's surface. The intensity as well as
the direction of the field varies; it has been weakening for the last century
or so. Because of these continuous and substantial changes, it became
necessary to establish a worldwide network of stations to monitor the geo-
magnetic field. Such a network was first set up in the nineteenth century,
and today there are 170 magnetic observatories spread across the globe. 24
Mo√att (1993), and Rikitake (1958). The website of the U.S. Geological Survey (USGS)
provides a clear, nontechnical explanation.
23. The problems that magnetic variations cause when one is navigating toward the true
North Pole are made plain in Avery (2009).
24. For Halley's measurements of magnetic variation, see Cook (1998, pp. 131, 270-71,
283). Halley also produced tide charts (pp. 288-89).