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CHAPTER 7 • Astronomical Control of Solar Radiation 121

Equinox September 22 equinox, than in the short part of the
March 20 orbit, between September 22 and March 20. The greater
length of the interval from March 20 to September 22
tends to compensate for the fact that Earth is farther
from the Sun on this part of the orbit and thus is receiv-
Solstice Empty ing less solar radiation.
June 21 Perihelion
focus
153 million km
July 4 January 3
Aphelion Long-Term Changes in Earth’s Orbit
158 million km Sun at Solstice
one focus December 21 Astronomers have known for centuries that Earth’s
orbit around the Sun is not fixed over long intervals
of time. Instead, it varies in a regular (cyclic) way
because of the mass gravitational attractions among
Equinox Earth, its moon, the Sun, and the other planets and
September 22 their moons. These changing gravitational attrac-
tions cause cyclic variations in Earth’s angle of tilt, its
FIGURE 7-2 Earth’s eccentric orbit Earth’s orbit around eccentricity of orbit, and the relative position of
the Sun is slightly elliptical. Earth is most distant from the sun
the solstices and equinoxes around its elliptical orbit
at aphelion, on July 4, just after the June 21 solstice, and
(Box 7–1).
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, NJ: Enslow, 1979].) 7-3 Changes in Earth’s Axial Tilt through Time
If we assume for simplicity that Earth has a perfectly cir-
cular orbit around the Sun, we can examine two hypo-
kilometers from the Sun, but the distance ranges thetical cases that show the most extreme differences in
between 153 million kilometers at perihelion and 158 tilt. For both cases, we look at the summer and winter
million at aphelion. This difference is equivalent to a solstices, the two seasonal extremes in Earth’s orbit.
total range of variation of slightly more than 3% around For the first case, Earth’s axis is not tilted at all
the mean value. (Figure 7-3A). Incoming solar radiation is directed
Earth is now in the perihelion position (closest to the straight at the equator throughout the year, and it
Sun) on January 3, near the time of the December 21 always passes by the poles at a 90° angle. Without any
winter solstice in the northern hemisphere and summer tilt, no seasonal changes occur in the amount of solar
solstice in the southern hemisphere (see Figure 7-2).
The fact that the close-pass position occurs in January
causes winter radiation in the northern hemisphere and N N
summer radiation in the southern hemisphere to be
slightly stronger than they would be in a perfectly circu-
Eq Eq
lar orbit.
Conversely, Earth lies farthest from the Sun on S S
July 4, near the time of the June 21 summer solstice in A No tilt
the northern hemisphere and winter solstice in the
southern hemisphere. The occurrence of this distant-
pass position in July makes summer radiation in the Eq Eq
northern hemisphere and winter radiation in the south- S N S N
ern hemisphere slightly weaker than they would be in a
circular orbit.
The effect of Earth’s elliptical orbit on its seasons is
small, enhancing or reducing the intensity of radiation B 90˚ tilt
received by just a few percent. Remember that the main FIGURE 7-3 Extremes of tilt (A) If Earth’s orbit were
cause of the seasons is the direction of tilt of Earth’s axis circular and its axis had no tilt, solar radiation would not
in its orbit around the Sun (see Figure 7-1). change through the year and there would be no seasons. (B)
Another consequence of Earth’s eccentric orbit is For a 90°tilt, the poles would alternate seasonally between
that the time intervals between the two equinoxes are conditions of day-long darkness and day-long direct overhead
not exactly equal: there are seven more days in the long Sun. (Adapted from J. Imbrie and K. P. Imbrie, Ice Ages: Solving the
part of the orbit, between the March 20 equinox and the Mystery [Short Hills, NJ: Enslow, 1979].)