Viewed in the present, the tilted earth revolves around the sun on an elliptical path. The orientation of the axis remains fixed in space, producing changes in the distribution of solar
radiation over the course of the year. These changes in the pattern of radiation reaching earth's surface cause the succession of the seasons. The warm weather of summer comes to the northern hemisphere, for instance, because during these months the
northern hemisphere is tilted towards the sun (at the same time, the southern hemisphere experiences winter because it is tilted away from the sun).
Croll was strongly influenced by the findings of astronomers like Adhémar and another
Frenchman, U. Leverrier, who demonstrated that the earth's orbital geometry was not fixed over time. Croll argued that as the earth's orbit changed, the pattern of radiation received in various seasons changed too. These long-term variations in the
earth's orbit, he claimed, did much to explain the waxing and waning of global climate in the last several million years.
Thomas G. Andrews
NOAA Paleoclimatology Program
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