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Paleo Slide Set: The Ice Ages
Precession of Earth's orbit adapted from Pisias and Imbrie [1986/1987]
Like a spinning top, the earth's orbit wobbles so that over the course of a precessional cycle, the North Pole traces a circle in space. This wobble causes the precession of the equinoxes. Croll adapted part of his theory from Adhémar, who demonstrated that the cycle of precession takes about 22,000 years to complete. As shown in this figure, the position of the equinoxes and solstices shifts slowly around the earth's elliptical orbit. Precession changes the date at which the earth reaches its perihelion serving to amplify or dampen seasonal climatic variability. For example, the earth currently reaches its perihelion on January 3, close to the Northern Hemisphere's winter solstice. This timing of the perihelion and Northern Hemisphere's winter solstice reduces seasonal differences in insolation in the Northern Hemisphere because the hemisphere is closer to the sun in winter and hence relatively warmer. On the other hand, the earth is further away from the sun and relatively cooler during the Northern Hemisphere's summer, reaching its aphelion on July 5. However, 11,000 years ago, the reverse was true: the earth reached its perihelion during the northern summer, increasing the seasonal variability of earth's climate.

Photo Credits:
Thomas G. Andrews
NOAA Paleoclimatology Program
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Last Modified: 12 October 2001

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