Page 144 - Earth's Climate Past and Future

120 PART III • Orbital-Scale Climate Change

Earth’s Orbit Today lution around the Sun, Earth maintains a constant angle
of tilt (23.5°) and a constant direction of this tilt in space.
The geometry of Earth’s present solar orbit is the start- When the northern or southern hemisphere is tilted
ing point for understanding past changes in Earth-Sun directly toward the Sun, it receives the more direct radia-
geometry. Much of our knowledge of Earth’s orbit dates tion of summer. When it tilts directly away from the Sun,
back to investigations in the seventeenth century by the it receives the less direct radiation of winter. But at both
astronomer Johannes Kepler. The larger frame of refer- times and at all times of year it keeps the same 23.5° tilt.
ence for understanding Earth’s present orbit is the plane If we switch back to our Earthbound perspective, we
in which it moves around the Sun, the plane of the see the overhead Sun appearing to move back and forth
ecliptic (Figure 7-1). through the year between the north tropic (Cancer) at
23.5°N and the south tropic (Capricorn) at 23.5°S. But
7-1 Earth’s Tilted Axis of Rotation and the Seasons again, this apparent movement is actually the result of
Two fundamental motions describe today’s orbit. First, Earth’s revolution around the Sun with a constant 23.5°
Earth spins on its axis once every day. One result is the tilt. Earth’s 23.5° tilt also defines the 66.5° latitude of
daily “rising and setting” of the Sun, but of course that the Arctic and Antarctic circles: 90° – 23.5° = 66.5°.
description is inaccurate. Days and nights are caused by Because of the 23.5° tilt away from the Sun in northern
Earth’s rotational spin, which carries different regions winter, no sunlight reaches latitudes poleward of 66.5°
of Earth’s surface into and out of the Sun’s direct radia- on the shortest winter day (winter solstice).
tion every 24 hours. Midway between the extremes of the winter and sum-
Earth rotates around an axis (or line) that passes mer solstices, during intermediate positions in Earth’s
through its poles (see Figure 7-1). This axis is tilted at revolution around the Sun, the lengths of night and
an angle of 23.5°, called Earth’s “obliquity,” or tilt. day become equal in each hemisphere at the equinoxes
This tilt angle can be visualized in either of two ways: (which means “equal nights”—that is, nights equal in
(1) as the angle Earth’s axis of rotation makes with a line length to days). Again, Earth’s tilt angle remains at 23.5°
perpendicular to the plane of the ecliptic or (2) as the during the equinoxes, and its direction of tilt in space
angle that a plane passing through Earth’s equator stays the same. The only factor that changes is Earth’s
makes with the plane of the ecliptic. position in respect to the Sun. The two equinoxes and
The second basic motion in Earth’s present orbit is two solstices are handy reference points for describing
its once-a-year revolution around the Sun. This motion distinctive features of its orbit.
results in seasonal shifts between long summer days,
when the Sun rises high in the sky and delivers stronger 7-2 Earth’s Eccentric Orbit: Distance between
radiation, and short winter days, when the Sun stays Earth and Sun
low in the sky and delivers weaker radiation. These sea-
sonal differences culminate at the summer and winter Up to this point, everything that has been described
solstices, which mark the longest and shortest days of would be true whether Earth’s orbit was perfectly circu-
the year (June 21 and December 21 in the northern lar or not. But Earth’s actual orbit (Figure 7-2) is not
hemisphere, the reverse in the southern hemisphere). a perfect circle: it has a slightly eccentric or elliptical
If we move outside our Earthbound perspective, we shape. The noncircular shape of Earth’s orbit is the
find that the cause of the seasons, the solstices, and the result of the gravitational pull of other planets on Earth
changes in length of day and angle of incoming solar as it moves through space.
radiation actually lies in the changing position of the tilted Basic geometry shows that ellipses have two focal
Earth with respect to the Sun. During each yearly revo- points rather than the single focus (center) of a circle. In
Earth’s case, the Sun lies at one of the two focal points
in its elliptical orbit, as required by the physical laws of
North Pole gravitation. The other focus is empty (see Figure 7-2).
23.5˚ Earth’s distance from the Sun changes according to
its position in this elliptical orbit. Not surprisingly, these
Plane of Earth's orbit changes in Earth-Sun distance affect the amount of solar
Equator
23.5˚ radiation Earth receives, especially at two extreme posi-
tions in the orbit. The position in which Earth is closest
South Pole to the Sun is called perihelion (the “close pass” position,
from the Greek meaning “near the Sun”), while the
FIGURE 7-1 Earth’s tilt Earth’s rotational (spin) axis is position farthest from the Sun is called aphelion (the
currently tilted at an angle of 23.5°away from a line “distant pass” position, from the Greek meaning “away
perpendicular to the plane of its orbit around the Sun. from the Sun”). On average, Earth lies 155.5 million