The cause of these nearly cyclic swings in temperature and the associated growth and retreat of great continental ice sheets was proposed by several scientists, notably by the Serbian mathematician Milutin Milanković in 1912. He recognized that the earth’s rotation axis precesses like a top and that its tilt with respect to the plane in which the earth orbits the sun also wobbles. Milanković also knew that the degree of elongation of the elliptical path of the earth’s orbit around the sun varies cyclically over time. All these factors affect the way sunlight is distributed around the world, even though they hardly affect the total amount of sunlight summed over the planet. He speculated—correctly it turns out—that ice ages are controlled by how much sunlight is received by the Arctic region during summer and set about calculating this value from the basic laws of physics that control the earth’s orbit and rotation. After years of hand calculation, Milanković produced a curve showing how ice ages should behave. At that time, data such as those used to produce Figure 4.2.1 did not exist, and so there was only rough agreement with what few data there were. But today we know that the great ice ages were caused by the cycles computed by Milanković, though there are gaps in our understanding of the details of how Earth’s climate responded to these.