Paleo Slide Set: The Ice Ages
Graphic of Earth's orbit adapted from Pisias and Imbrie [1986/1987].
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 Adhemar 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.

Photo Credits:
Thomas G. Andrews
NOAA Paleoclimatology Program