this axis at right angles to it (fig. 2-3). A great circle
drawn through the poles is called a meridian, and an
infinite number of great circles maybe constructed in this
manna. Each meridian is divided into four quadrants by
the equator and the poles. Since a circle is divided into
360 degrees, each quadrant contains 90 degrees.
Take a point on one of these meridians 30 degrees
north of the equator. Through this point passes a plane
perpendicular to the north-south axis. This plane will
be parallel to the plane of the equator, as shown in
figure 2-3, and will intersect the earth in a small circle
called a parallel or parallel of latitude. This particular
parallel of latitude is called 30°N, and every point on
this parallel will be at 30°N. Parallels can be
constructed at any desired latitude.
The equator is the great circle midway between
the poles. The parallels of latitude are small circles
constructed with reference to the equator. The
angular distance measured on a meridian north or
south of the equator is known as latitude and forms
one component of the coordinate system.
LONGITUDE.— The latitude of a point can be
shown as 20°N or 20°S of the equator, but there is no
way of telling whether one point is east or west of
another. This is resolved by the use of the other
component of the coordinate system—longitude.
Longitude is the measurement of this east-west
distance.
There is not a natural starting point for numbering
longitude. With latitude, the starting point is the
equator. This problem was solved by selecting an
arbitrary starting point. Many places had been used,
but when the English speaking people began to make
charts, they chose the meridian through their principal
observatory in Greenwich, England. This meridian
has now been adopted by most other countries as the
starting point. This Greenwich meridian is sometimes
called the prime meridian or first meridian, though
actually it is the zero meridian. Longitude is counted
east or west from this meridian through 180 degrees.
The Greenwich meridian is the 0-degree meridian on
one side of the earth and the 180th meridian after
crossing the poles (180 degrees east or west of the
0-degree meridian).
If a globe has the circles of latitude and longitude
drawn on it according to the principles described, and
the latitude and longitude of a certain place have been
determined, this point can be located on the globe in its
proper position (fig. 2-4). In this way, a globe can be
formed that resembles a small-scale copy of the earth.
Latitude is measured in degrees up to 90, and
longitude is expressed in degrees up to 180. The total
number of degrees in any one circle cannot exceed
360. A degree (°) of arc may be subdivided into
smaller units by dividing each degree into 60 minutes
(’) of arc. Each minute can be divided into 60 seconds
(“) of arc.
Measurement may also be made in
degrees, minutes, and tenths of minutes.
Figure 2-3.-Planes of the earth.
2-4
