the ends of the magnet. This is where the highest
concentration of magnetic lines of force exist.
However, the earth's magnetic poles are not at
the geographic poles, nor are they antipodal
(opposite) to each other.
GREAT CIRCLES AND
SMALL CIRCLES
The intersection of a sphere and a plane is a
circle. The intersection is a great circle if the plane
passes through the center of the sphere. It will be
a small circle if it does not.
PARALLELS AND MERIDIANS
Look at figure 7-2. Here, the earth's equator
is a great circle. If a second plane (fig. 7-3) passes
through the earth parallel to the equator, its
intersection is a small circle. Small circles don't
always have planes perpendicular to the polar axis.
However, if they are perpendicular, then all points
on the small circle are equidistant from the
equator; that is, the circles are parallel to
the equator. Such small circles, together with
the equator, are PARALLELS. They provide
Figure 7-4.-Great circle through the poles form meridians.
one component of a system of geographical
Now, suppose that planes pass through the
same as an address at the corner of Fourteenth
earth's poles (fig. 7-4). Such planes contain the
Street and Seventh Avenue. You are just using
axis, and since they also contain the center, they
different names for identifying the parallels and
form great circles at the surface. Great circles
meridians.
through the poles of the earth are MERIDIANS.
The circumference of a circle is divided into
All meridians are perpendicular to the equator.
360 units. This unit is the degree. It is the same
Meridians form the second part of a system of
unit you use to measure an angle. In figure 7-5,
geographical coordinates commonly used by
navigators.
LATITUDE AND LONGITUDE
You can identify any point on earth by the
intersection of a parallel and a meridian. It is the
Figure 7-5.-Latitude of M is angle QOM or arc QM.
Figure 7-3.-The plane of a parallel is parallel to the equator.