Page 148 - Earth's Climate Past and Future

124 PART III • Orbital-Scale Climate Change

ε=0 ε=0.06 Circular motion (revolution)

Eccentricity
Wobbling motion
0.00 0.03 0.06 (precession)
0

Angle
of tilt
0.5
Myr ago 413,000 Spinning motion

(rotation)
years
1.0
FIGURE 7-8 Earth’s wobble In addition to its rapid (daily)
rotational spin and its slower (yearly) revolution around the
Sun, Earth wobbles slowly, like a top, with one full wobble
100,000 every 25,700 years.
years
1.5
Earth’s axis of rotation through a circular path, with one
full turn every 25,700 years. Today Earth rotates around
FIGURE 7-7 Long-term changes in eccentricity The an axis that points to the North Star (Polaris), but over
eccentricity ( ) of Earth’s orbit varies at periods of 100,000 time the wobbling motion causes the axis of rotation to
and 413,000 years. point to other celestial reference points (Figure 7-9).
Earth wobbles very slowly; it revolves 25,700 times
around the Sun and rotates almost 10 million times on
7-5 Precession of the Solstices and Equinoxes
around Earth’s Orbit its axis during the time it takes to complete just a single
wobble.
The positions of the solstices and equinoxes in relation to A second kind of precessional motion is known as
the eccentric orbit have not always been fixed at their pre- precession of the ellipse. In this case, the entire ellip-
sent locations (see Figure 7-2). Instead, they have slowly tically shaped orbit of the Earth rotates, with the long
shifted through time with respect to the eccentric orbit and short axes of the ellipse turning slowly in space
and the perihelion (close-pass) and aphelion (distant-pass) (Figure 7-10). This motion is even slower than the
positions. Although Hipparchus in ancient Greece first wobbling motion of axial precession.
noticed these changes, the French mathematician, scien- The combined effects of these two precessional
tist, and philosopher Jean Le Rond d’Alembert was the motions (wobbling of the axis and turning of the
first to understand them in the eighteenth century. ellipse) cause the solstices and equinoxes to move
The cause of these changes lies in a long-term wob- around Earth’s orbit, with one full 360° orbit around
bling similar to that of a top. Tops typically move with the Sun completed approximately every 22,000 years
three superimposed motions (Figure 7-8). They spin (Figure 7-11). This combined movement, called the
very rapidly (rotate) around a tilted axis. They also precession of the equinoxes, describes the absolute
revolve with a slower near-circular motion across the motion of the equinoxes and solstices in the larger ref-
surface on which they spin, with many spins (rotations) erence frame of the universe. It consists of a strong
for each complete revolution. Finally, tops also wobble, cycle near 23,000 years and a weaker one near 19,000
gradually leaning in different directions through time. years, with an average of one cycle every 21,700 years.
This wobbling motion in not caused by changes in the For the rest of this book, we will concentrate mainly on
amount by which the top leans (its angle of tilt), but the strong precession cycle near 23,000 years.
rather by changes in the direction in which it leans. The precession of the equinoxes involves complicated
Earth’s wobbling motion, called axial precession, is angular motions in three-dimensional space, and these
caused by the gravitational pull of the Sun and Moon on motions need to be reduced to a simple, easy-to-use
the slight bulge in Earth’s diameter at the equator. Axial mathematical form that can be plotted against time like
precession can also be visualized as a slow turning of the changes in tilt shown in Figure 7-4. To accomplish