This latest glaciation was initiated by a decrease in incoming solar radiation due to the elliptical nature of the Earth's orbit. Over the past 2.5 million years, variations in the axis of rotation and eccentricity of the Earth's orbit have caused many glaciations. These variations in the Earth's orbit are called Milankovitch orbital cycles after Milutin Milankovitch, an astronomer who first determined orbital variation frequencies and their influence on solar radiation received by the Earth. The tilt of the Earth's axis is currently 23.50. In the past this tilt, or obliquity of the Earth's axis, has varied from 24.50 to 21.50 on a cycle of 40,000 years.
Glacial conditions are most favorable when the tilt angle is lowest.
The Earth's axis also wobbles over time due to the gravitational pull of the Sun and Moon on the equatorial bulge of the Earth. After 900,000 years ago, glaciations became larger and lasted longer. Throughout the past 900,000 years climate has been dominated by the 100,000-year eccentricity cycle of the Earth's orbit around the sun. Incoming solar insolation has increased and decreased as the Earth's orbit has changed from elliptical to circular. The last several large glaciations were stimulated by low solar insolation during more circular orbits.