Page 149 - Earth's Climate Past and Future

CHAPTER 7 • Astronomical Control of Solar Radiation 125

25,700
years North Star Shift of perihelion
(Polaris)

Spin axis
today
N

Precession
of Earth's Equator
spin axis
(wobble)

S

FIGURE 7-10 Precession of the ellipse The elliptical shape
of Earth’s orbit slowly precesses in space so that the major and
FIGURE 7-9 Precession of Earth’s axis Earth’s slow
minor axes of the ellipse slowly shift through time. (Adapted
wobbling motion causes its rotational axis to point in different
from N. Pisias and J. Imbrie, “Orbital Geometry, CO , and
directions through time, sometimes (as today) toward the 2
Pleistocene Climate,” Oceanus 29 [1986–87]: 43–49.)
North Star, Polaris, but at other times toward other stars.
(Adapted from J. Imbrie and K. P. Imbrie, Ice Ages: Solving the
Mystery [Short Hills, NJ: Enslow, 1979].).
Return to modern
position after 23,000
March 20 years
h 20
20
Marc
Marc
h
Today
December 21
June 21
this goal, we make use of two basic geometric characteris-
September 22
tics of precessional motion.
5,750 years June 21
The first characteristic has to do with the angular from now
h
h
Marc
Marc
form of Earth’s motion with respect to the Sun. We September 22 March
20
20
define ω (omega) as the angle between two imaginary
lines (Figure 7-12A): (1) a line connecting the Sun to December 21
Earth’s position at perihelion (its closest pass to the Sun) 11,500 years September 22
from now
and (2) a line connecting the Sun to Earth’s position at
December 21 June 21
the March 20 equinox. The first line is tied to the ellip-
tical shape of Earth’s orbit and the second to the varying
Marc
Marc h 20
March 20
20
h
positions of the seasons within the orbit. As a result, the
16,725 years December 21
slow change in the angle ω is a measure of Earth’s wob- from now
bling motion—the very slow changes in the positions of
h
March 20
Marc
Marc h 20 September 22
20
the seasons with respect to the elliptical orbit.
The changing angle ω slowly sweeps out a 360° June 21
arc, starting at 0° (where the March 20 equinox coin-
cides with the perihelion position), increasing to 90°, FIGURE 7-11 Precession of the equinoxes Earth’s wobble
then to 180° (where the March 20 equinox occurs on and the slow turning of its elliptical orbit combine to produce
the other side of the orbit, coincident with the aphelion the precession of the equinoxes. Both the solstices and
position), later to 270°, and finally to 360°, at which equinoxes move slowly around the eccentric orbit in cycles of
point the cycle is complete and the angle returns to 0° 23,000 years. (Adapted from J. Imbrie and K. P. Imbrie, Ice Ages:
(Figure 7-12B). Solving the Mystery [Short Hills, NJ: Enslow, 1979].)