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°
Distance from top 90 N
of page 60 N
°
°
40 N

X
20 N
°

Distance from left
side of page

FIGURE 16.13 Any location on a flat, two-dimensional
surface is easily identified with two references from two edges. This
technique does not work on a motionless sphere because there are FIGURE 16.15 If you could see to Earth’s center, you would
no reference points. see that latitudes run from 0° at the equator north to 90° at the
North Pole (or to 90° south at the South Pole).
90°
80°
60° Parallels 60° 40° 40° 60°
North of equator Equator West 20° 0° 20°
40°
East
20°
longitude
longitude
(0°)

FIGURE 16.14 A circle that is parallel to the equator is used FIGURE 16.16 Meridians run pole to pole and perpendicular
to specify a position north or south of the equator. A few of the to the parallels, providing a reference for specifying east and west
possibilities are illustrated here. directions.

run around Earth parallel to the equator as shown in Figure 16.14.
The east- and west-running parallel circles are called paral- 180°
lels. Each parallel is the same distance between the equator 120°
and one of the poles all the way around Earth. The distance
from the equator to a point on a parallel is called the latitude West 90°
of that point. Latitude tells you how far north or south a point
is from the equator by telling you the parallel on which the
point is located. The distance is measured northward from East
the equator (which is 0°) to the North Pole (90° north) or 0°
southward from the equator (0°) to the South Pole (90° south)
(Figure 16.15). If you are somewhere at a latitude of 35° north, Prime meridian
you are somewhere on Earth on the 35° latitude line north of
the equator. FIGURE 16.17 If you could see inside Earth, you would see
Since a parallel is a circle, a location of 40°N latitude could 360° around the equator and 180° of longitude east and west of
be anyplace on that circle around Earth. To identify a location, the prime meridian.
you need another line, this time one that runs pole to pole and
perpendicular to the parallels. These north-south running arcs
that intersect at both poles are called meridians (Figure 16.16). the Greenwich Observatory near London, England, and this
There is no naturally occurring, identifiable meridian that can meridian is called the prime meridian. The distance from the
be used as a point of reference such as the equator serves for prime meridian east or west is called the longitude. The degrees
parallels, so one is identified as the referent by international of longitude of a point on a parallel are measured to the east or
agreement. The referent meridian is the one that passes through to the west from the prime meridian up to 180° (Figure 16.17).

412 CHAPTER 16 Earth in Space 16-8