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Earth's orbit when it is closest to the Sun is the longitude of perihelion (presently
about 250 ). About 5,500 years ago, the seasons rotated 90 and the longitude of
perihelion was about 160 . About 11,000 years ago, the longitude of perihelion
was about 70 .
9.2.2 Variability of the orbital plane
M&M pointed out that, although the orbit of the Earth lies in a plane called the
''ecliptic'', this is not a good reference plane for paleoclimate because it changes
with time. It is perturbed by torques from Jupiter, Venus, and Saturn. When we
refer to the orbital plane, we must state the year that we are referring to. It
requires two angles to specify the inclination of the Earth's orbital plane relative
to a reference plane fixed in space. One is the angle between the orbital plane and
the reference plane and the other specifies the direction of the tilted orbital plane.
These angles were given by M&M as shown in Figure 9.5 . Over the past 600,000
years, the inclination of the Earth's orbital plane in space has wobbled by about
0.5 with a period that varied from about 50,000 to 100,000 years.
As M&M pointed out, little attention was paid to this variation because NH
insolation does not depend directly on this parameter and, according to the
standard astronomical theory, such insolation is the only physical parameter that
affects climate. M&M speculated that this variation could play a role in ice age
cycles.
Figure 9.5. Variability of the tilt of the Earth's orbital plane.
 
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