Milankovich cycles are cyclical changes in the rotation and orbit of our planet that influence the amount of solar radiation striking different parts of Earth at different times of year and have been correlated with climatic effects. There are three cycles: changes in the eccentricity of Earth's orbit, with a period of about 100,000 yr, that alter the distance between Earth and the Sun at aphelion and perihelion; variations in the tilt of Earth's rotational axis (obliquity of the ecliptic), with a period of about 40,000 yr; and a wobble in the angle by which the axis of Earth's rotation is tilted with respect to the orbital plane, altering the seasons at which aphelion and perihelion occur (precession of the equinoxes), with a period of about 25,800 yr. They are named after the Serbian mathematician, Milutin Milankovitch, who explained how these orbital cycles cause the advance and retreat of the polar ice caps. Although they are named after Milankovitch, he was not the first to link orbital cycles to climate. Adhemar (1842) and Croll (1875) were two of the earliest.