Milankovitch cycles

Milankovitch cycles describes the collective effects of changes in the Earth's movements on its climate over thousands of years. The term is named after Serbian geophysicist and astronomer Milutin Milanković. In the 1920s, he theorized that variations in eccentricity, axial tilt, and precession of the Earth's orbit resulted in cyclical variation in the solar radiation (insolation) reaching the Earth, and that this orbital forcing strongly influenced climatic patterns on Earth.

The Earth's orbit varies between nearly circular and mildly elliptical (its eccentricity varies). When the orbit is more elongated, there is more variation in the distance between the Earth and the Sun, and in the amount of solar radiation, at different times in the year. In addition, the rotational tilt of the Earth (its obliquity), which causes the seasons as the Earth revolves around the Sun, changes slightly. A greater tilt makes the seasons more extreme. Finally, the direction in the fixed stars pointed to by the Earth's axis changes (axial precession), while the Earth's elliptical orbit around the Sun rotates (apsidal precession). The combined effect of the two precessions is a cycle in which proximity to the Sun occurs during different astronomical seasons. If the Earth is closer to the Sun while the northern or southern hemisphere is tilted toward the Sun (is in summer), then both effects work together to heat that hemisphere. If the Earth is further from the Sun during summer, the greater distance slightly reduces the heat of summer.

Similar astronomical theories had been advanced in the 19th century by Joseph Adhemar, James Croll and others, but verification was difficult because there was no reliably dated evidence, and because it was unclear which periods were important.

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