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change over time. A complete cycle takes about 22,000 years and is termed the
precession of the equinoxes (Figure 9.10). It has the effect of changing the relative
warmth of winter and summer between the two hemispheres. Aphelion in the northern
hemisphere will produce cooler winters but the summer perihelion should give warmer
summers, increasing the seasonal difference in temperature. We also find that the degree
of ellipticity of Earth's orbit changes through time over a cycle of about 96,000 years;
this phenomenon is known is the eccentricity of the orbit . At times the orbit is almost
circular and there is little difference in input between perihelion and aphelion; 47,500
years later the orbit is at its most elliptical, with a strong difference between perihelion
and aphelion. This variation affects the amount of solar radiation intercepted by Earth by
a small amount.
The final source of variation in the distribution of solar inputs is the changes in the tilt
of Earth's axis of rotation. Although, at present, the tilt is about 23·5°, it can range from
21·8° to 24·4°. This means that the precise latitude of our tropics will shift slightly. When
the axis has a greater
Figure 9.10 (a) Geometry of the sun-Earth system, showing
the factors causing variation in radiation receipt by the
Earth. (b) Changes in eccentricity, tilt and precession for the
last 250,000 years and the next 100,000 years.
Source: After Imbrie and Imbrie (1979).
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