The Serbian astronomer Milutin Milankovitch (1879–1958) developed a theory that explained climatic variations in astrophysical terms. He was particularly concerned with the origin of an ice age during the Pleistocene. Through observations of the stars, Milankovitch found that the basic elements that govern the earth's orbit around the Sun are not constant. First, he noticed that the eccentricity of the elliptical path of the earth's revolution around the Sun changes with cycles of roughly 100,000 and 400,000 years. Second, he found that the obliquity, that is the angle of the earth's spin axis with the plane of its eccentric orbit changes with a frequency of roughly 41,000 years between 22 and 25 degrees. Third, he took into consideration that the earth's axis of revolution behaves like the spin axis of a top that is winding down. The spin axis traces a circle on the celestial sphere over a period of approximately 22,000 years...