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Sphere March 20 or 21
June 21 or 22 December 22 or 23
NP NP
23.5°N
(7,900 mi) Direct sunlight
Oblate
spheroid 23.5°S
12,756 km (7,926 mi) 23.5° September 22 or 23 23.5°
12,714 km FIGURE 16.5 The consistent tilt and orientation of Earth’s axis

as it moves around its orbit is the cause of the seasons. The North
Pole is pointing toward the Sun during the summer solstice and
away from the Sun during the winter solstice.

FIGURE 16.4 Earth has an irregular, slightly lopsided, slightly
pear-shaped form. In general, it is considered to have the shape of
an oblate spheroid, departing from a perfect sphere as shown here. than it would be if the orbit were a circle. This total difference
of about 5 million km (about 3 million mi) results in a January
Sun with an apparent diameter that is 3 percent larger than the
shape of Earth is a slightly pear-shaped, slightly lopsided oblate July Sun, and Earth as a whole receives about 6 percent more
spheroid (Figure 16.4). All the elevations and depressions are less solar energy in January. The effect of being closer to the Sun is
than 85 m (about 280 ft), however, which is practically negligible much less than the effect of some other relationships, and win-
compared to the size of Earth. Thus, Earth is very close to, but ter occurs in the Northern Hemisphere when Earth is closest to
not exactly, an oblate spheroid. The significance of this shape will the Sun. Likewise, summer occurs in the Northern Hemisphere
become apparent when Earth’s motions are discussed next. when the Sun is at its greatest distance from Earth (Figure 16.5).
The important directional relationships that override
the effect of Earth’s distance from the Sun involve the daily
rotation, or spinning, of Earth around an imaginary line
16.2 MOTIONS OF EARTH
through the geographic poles called Earth’s axis. The important
Ancient civilizations had a fairly accurate understanding of the directional relationships are a constant inclination of Earth’s
size and shape of Earth but had difficulty accepting the idea axis to the plane of the ecliptic and a constant orientation of the
that Earth moves. The geocentric theory of a motionless Earth axis to the stars. The inclination of Earth’s axis to the plane of
with the Sun, Moon, planets, and stars circling it was discussed the ecliptic is about 66.5° (or 23.5° from a line perpendicular to
in chapter 15. Ancient people had difficulty with anything but the plane). This relationship between the plane of Earth’s orbit
a motionless Earth for at least two reasons: (1) they could not and the tilt of its axis is considered to be the same day after day
sense any motion of Earth, and (2) they had ideas about being throughout the year, even though small changes do occur in the
at the center of a universe that was created for them. Thus, it inclination over time. Likewise, the orientation of Earth’s axis
was not until the 1700s that the concept of an Earth in motion to the stars is considered to be the same throughout the year as
became generally accepted. Today, Earth is understood to move Earth moves through its orbit. Again, small changes do occur
a number of different ways, seven of which were identified in in the orientation over time. Thus, in general, the axis points in
the introduction to this chapter. Three of these motions are the same direction, remaining essentially parallel to its position
independent of motions of the Sun and the galaxy. These are during any day of the year. The essentially constant orientation
(1) a yearly revolution around the Sun, (2) a daily rotation on its and inclination of the axis result in the axis pointing toward the
axis, and (3) a slow, clockwise wobble of its axis. Sun as Earth moves in one part of its orbit, then pointing away
from the Sun six months later. The generally constant inclination
and orientation of the axis, together with Earth’s rotation
REVOLUTION and revolution, combines to produce three related effects:
Earth moves constantly around the Sun in a slightly elliptical (1) days and nights that vary in length, (2) changing seasons,
orbit that requires an average of one year for one complete circuit. and (3) climates that vary with latitude.
The movement around the Sun is called a revolution, and all Figure 16.5 shows how the North Pole points toward
points of Earth’s orbit lie in the plane of the ecliptic. The aver- the Sun on June 21 or 22, then away from the Sun on De-
age distance between Earth and the Sun is about 150 million km cember 22 or 23 as it maintains its orientation to the stars.
(about 93 million mi). When the North Pole is pointed toward the Sun, it receives
Earth’s orbit is slightly elliptical, so it moves with a speed sunlight for a full 24 hours, and the South Pole is in Earth’s
that varies. It moves fastest when it is closer to the Sun in January shadow for a full 24 hours. This is summer in the Northern
and slowest when it is farthest away from the Sun in early July. Hemisphere with the longest daylight periods and the Sun
Earth is about 2.5 million km (about 1.5 million mi) closer to the at its maximum noon height in the sky. Six months later, on
Sun in January and about the same distance farther away in July December 22 or 23, the orientation is reversed with winter in

408 CHAPTER 16 Earth in Space 16-4