Laboratory Manual for Introductory Geology 4e

But which north is north? No, this isn’t a silly question. A compass needle points
to the magnetic north pole, the point where the lines of force from the Earth’s
magnetic field enter the planet (FIG. 8.9a). The true or geographic north pole
is the north end of the Earth’s rotational axis. The two “north poles” are not
located in the same place, as shown in Figures 8.9a and 8.9b, so which do we
use in discussing direction? As its name suggests, directions are given relative to
true north.
In some places, as FIGURE 8.9b shows, compass needles (red arrows) point to
a direction west of a line pointing to true north (black arrows), but in other places,
compass needles point in a direction east of true north. The angular difference
between true north and magnetic north is called magnetic declination. There are
still other places where a compass needle pointing toward magnetic north also
points toward true north (white arrows)—in other words, where magnetic declination
is zero. FIGURE 8.9c shows how magnetic declination varies in the contiguous
United States. Everywhere along the heavy blue line, declination is zero. Areas west
of that line have east declination, and areas east of that line have west declination,
with the declination increasing with distance from the zero line.
FIGURE 8.9 Magnetic north, true north, and magnetic declination.
True north pole
Magnetic north pole
East
declination
N
True north pole
Magnetic
north pole
West
declination
Equator
(Latitude 5 0)
Magnetic south pole
True south pole
(a) Magnetic and true north and south poles.
S
(b) Magnetic declination is the angle between true north and
magnetic north.
East declination
West declination
158
108
58
08
258
2108 2158
2158
158
2108
108
58
08
258
(c) Magnetic declination map for the contiguous United States.
210 CHAPTER 8 STUDYING THE EARTH’S LANDFORMS: MAPS AND OTHER TOOLS