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Perpendicular to
plane of ecliptic
Rotational Vega North Star
velocity (A.D. 14,000) (today)
A B

To the Sun

FIGURE 16.10 Earth has a greater rotational velocity at the
equator and less toward the poles. As an object moves north or
south (A), it passes over land with a different rotational velocity,
which produces a deviation to the right in the Northern Hemisphere
(B) and to the left in the Southern Hemisphere. FIGURE 16.12 The slow, continuous precession of Earth’s
axis results in the North Pole pointing around a small circle over
a period of about 26,000 years.
the direction of movement in the Northern Hemisphere and to
the left in the Southern Hemisphere. The apparent deflection the occurrence of the spring equinox has been moving back-
caused by Earth’s rotation is called the Coriolis eff ect. The Co- ward (westward) through the zodiac constellations at about
riolis effect will explain Earth’s prevailing wind systems as well 1 degree every 72 years. Thus, after about 26,000 years, the spring
as the characteristic direction of wind in areas of high pressure equinox will have moved through all the constellations and will
and areas of low pressure (see chapter 23).
again approach the constellation of Aquarius for the next “age of
Aquarius” (Figure 16.12).
PRECESSION
If Earth were a perfect spherically shaped ball, its axis would 16.3 PLACE AND TIME
always point to the same reference point among the stars. The
reaction of Earth to the gravitational pull of the Moon and the The continuous rotation and revolution of Earth establishes
Sun on its equatorial bulge, however, results in a slow wob- an objective way to determine direction, location, and time on
bling of Earth as it turns on its axis. This slow wobble of Earth’s Earth. If Earth were an unmoving sphere, there would be no
axis, called precession, causes it to swing in a slow circle like side, end, or point to provide a referent for direction and loca-
the wobble of a spinning top (Figure 16.11). It takes Earth’s axis tion. Earth’s rotation, however, defines an axis of rotation, which
about 26,000 years to complete one turn, or wobble. Today, the serves as a reference point for determination of direction and
axis points very close to the North Star, Polaris, but is slowly location on the entire surface. Earth’s rotation and revolution
moving away to point to another star. In about 12,000 years, together define cycles, which define standards of time. The fol-
the star Vega will appear to be in the position above the North lowing describes how Earth’s movements are used to identify
Pole, and Vega will be the new North Star. The moving pole both place and time.
also causes changes over time in which particular signs of the
zodiac appear with the spring equinox. Because of precession, IDENTIFYING PLACE
A system of two straight lines can be used to identify a point, or
position, on a flat, two-dimensional surface. The position of the
letter X on a page, for example, can be identified by making a
Precession of axis line a certain number of measurement units from the top of the
page and a second line a certain number of measurement units
from the left side of the page. Where the two lines intersect will
identify the position of the letter X, which can be recorded or
communicated to another person (Figure 16.13).
A system of two straight lines can also be used to identify
a point, or position, on a sphere, except this time the lines are
circles. The reference point for a sphere is not as simple as in
the flat, two-dimensional case, however, since a sphere does
not have a top or side edge. Earth’s axis provides the north-
south reference point. The equator is a big circle around Earth
FIGURE 16.11 A spinning top wobbles as it spins, and the
axis of the top traces out a small circle. The wobbling of the axis is that is exactly halfway between the two ends, or poles, of the
called precession. rotational axis. An infinite number of circles are imagined to

16-7 CHAPTER 16 Earth in Space 411