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

FIGURE 8-3 Extremes of tilt (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 8-4 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 of Earth’s orbit varies at periods of 100,000 and 413,000 years.

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.

3. Precession of solstices & equinoxes around Earth’s orbit

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 The angle between lines marking Earth’s perihelion axis and the vernal equinox (March 20) is called (A). The angle 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, sin, is the product of the sine wave function (sin) caused by precessional motion and the eccentricity of Earth’s orbit.

FIGURE 8-15 Long-term changes in precession The precessional index (sin) 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.

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.

Changes in insolation received on Earth

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).

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 23,000-year Ice-Volume Cycle”, Science 212 [1981]: 617-27.)

Insolation changes according to Caloric Season