Transcript 投影片 1

Chapter 8
Astronomical Control of Solar Radiation
1. Changes in Earth’s axial tilt through time
2. Changes in Earth’s Eccentric Orbit through time
3. Precession of solstices and equinoxes around
Earth’s orbit
Earth’s Orbit Today
FIGURE 8-1 Earth’s tilt Earth’s
rotational (spin) axis is currently tilted
at an angle of 23.50 away from a line
perpendicular to the plane of its orbit
around the Sun.
FIGURE 8-2 Earth’s eccentric orbit Earth’s orbit around the Sun is slightly
elliptical. Earth is most distant from the sun at aphelion, on July 4, just after the
June 21 solstice, and closest to the Sun at perihelion, on January 3, just after
the December 21 solstice. (Modified from J. Imbrie and K.P. Imbrie, Ice Ages:
Solving the Mystery [Short Hills, N.J.: Enslow, 1979].)
1. Changes in Earth’s axial tilt through time
(A) If Earth’s
orbit were circular and its axis had no tilt, solar
radiation would not change through the year and
there would be no seasons. (B) For a 900 tilt, the
poles would alternate seasonally between
conditions of day-long darkness and day-long
direct overhead Sun. (Adapted from J. Imbrie and
K.P. Imbrie, Ice Ages: Solving the Mystery [Short
Hills, N.J.: Enslow, 1979].)
FIGURE 8-3 Extremes of tilt
FIGURE 8-4 Long-term changes
in tilt Changes in the tilt of
Earth’s axis have occurred on a
regular 41,000-year cycle.
FIGURE 8-5 Effects of increased tilt on polar regions Increased tilt brings more
solar radiation to the two summer-season poles and less radiation to the two
winter-season poles.
2. Changes in Earth’s Eccentric Orbit through time
FIGURE 8-6 Eccentricity of an ellipse
The eccentricity of an ellipse is related
to half the lengths of its longer (major)
and shorter (minor) axes.
FIGURE 8-7 Long-term changes in eccentricity
The eccentricity (e) of Earth’s orbit varies at
periods of 100,000 and 413,000 years.
3. Precession of solstices & equinoxes around Earth’s orbit
FIGURE 8-8 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 every 25,700 years.
FIGURE 8-9 Precession of Earth’s axis
Earth’s slow wobbling motion causes its
rotational axis to point in different
directions through time, sometimes (as
today) toward the 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,
N.J.: Enslow, 1979].)
25,700 years
FIGURE 8-10 Precession of the ellipse
The elliptical shape of Earth’s orbit
slowly processes in space, so that the
major and minor axes of the ellipse
slowly shift through time. (Adapted
from N. Pisias and J. Imbrie, “Orbital
Geometry, CO2, and Pleistocene
Climate”, Oceanus 29 [1986-87]: 43-49.)
23,000 years (19,000)
FIGURE 8-11
Precession of the
equinoxes Earth’s
wobble and the slow
turning of its elliptical
orbit combine to
produce the
precession of the
equinoxes. Both the
solstices and
equinoxes move
slowly around the
eccentric orbit in
cycles that take 23,000
years. (Adapted from J.
Imbrie and K.P. Imbrie,
Ice Ages: Solving the
Mystery [Short Hills,
N.J.: Enslow, 1979].)
FIGURE 8-12 Precession and
the angle w The angle between
lines marking Earth’s perihelion
axis and the vernal equinox
(March 20) is called w (A). The
angle w increases from 00 to
3600 with each full 23,000-year
cycle of precession (B).
FIGURE 8-13 Extreme solstice positions Slow precessional changes in the attitude
(direction) of Earth’s spin axis produce changes in the distance between Earth and
Sun as the summer and winter solstices move into the extreme (perihelion and
aphelion) positions in Earth’s eccentric orbit. (Modified from W.F. Ruddiman and A.
McIntyre, “Oceanic Mechanisms for Amplification of the 23,000-year Ice-Volume
Cycle”, Science 212 [1981]: 617-27.)
FIGURE 8-14 The precessional index The precessional index, esinw,
is the product of the sine wave function (esinw) caused by
precessional motion and the eccentricity (e) of Earth’s orbit.
FIGURE 8-15 Long-term
changes in precession The
precessional index (esinw)
changes mainly at a cycle of
23,000 years. The amplitude of
this cycle is modulated at the
eccentricity periods of 100,000
and 413,000 years.
Changes in insolation received on Earth
FIGURE 8-16 June and
December insolation variations
June and December monthly
insolation values show the
prevalence of precessional
changes at low and middle
latitudes and the presence of tilt
changes at higher latitudes.
Cycles of tilt and precession are
indicated by T and P. The
double arrows indicate
variations of 30 W/m2 for these
signals.
FIGURE 8-17 Opposing seasonal
insolation trends Seasonal insolation
trends move in opposite directions in
winter and in summer at any location.
For precession, these opposing
trends cancel each other out when
they are added to calculate an
annual average.
FIGURE 8-18
Phasing of insolation
maxima and minima
Tilt causes in-phase
changes for polar
regions of both
hemispheres in their
respective summer
and winter seasons
(A). Precession
causes out-of-phase
changes between
hemispheres for their
summer and winter
seasons (B).
FIGURE 8-19 Family of monthly precession curves Because all seasons
change position (precess) around Earth’s orbit, each season (and month)
has its own insolation trend through time. Monthly insolation curves are
offset by slightly less than 2000 years (23,000 years divided by 12 months).
Insolation changes according to Caloric Season
FIGURE 8-20 Caloric season
insolation anomalies Plots of
insolation anomalies for the
summer and winter caloric
half-year show a larger
influence of tilt in relation to
precession at higher latitudes
than do the monthly
anomalies. ( Adapted from W.F.
Ruddiman and A. McIntyre,
“Oceanic Mechanisms for
Amplification of the 23,000year Ice-Volume Cycle”,
Science 212 [1981]: 617-27.)