Milankovitch theory is named after Serbian astronomer Milutin Milankovitch. He theorized that the severity of the seasons and the Earth's climate in general can be determined by careful measurement of irregularities in the Earth's rotation and orbit. The Earth's axis of rotation is tilted, which produces the different seasons in the northern and southern hemispheres. But this axis tilt can vary, or "wobble," by up to 3 degrees over the course of ~40,000 years. The other factor is the Earth's orbit around the sun. It is not perfectly circular but elliptical instead, meaning that certain times of the year the planet is closer to the sun than at other times. This orbital shape also varies on a 22,000 year cycle. Milankovitch theorized that the coordination of these movements could be timed to determine when the seasons would be more severe and lead to general planetary climate trends, including ice ages. Incidentally, an ice age is not expected within the next 50,000 to 100,000 years.