Cyclical changes in the rotation and orbit of the Earth that Milankovitch correlated to climatic effects. There are three cycles: changes in the eccentricity of the Earth’s orbit, altering the distance between Earth and the Sun at aphelion and perihelion, with a period of about 100 000 years; variations in the tilt of the Earth’s rotational axis (obliquity of the ecliptic), with a period of about 40 000 years; and a movement (wobble) in the angle by which the axis of the Earth’s rotation is tilted in respect of the orbital plane, altering the seasons at which aphelion and perihelion occur (precession of the equinoxes), with a period of about 21 000 years. Climatic changes associated with Milankovitch cycles may be recorded in cyclic sedimentation.
Milankovitch cycles
There are three interacting, astronomical cycles in the Earth’s orbit around the sun: in the shape of the elliptical orbit (c.95 000 years); in the axis of rotation (c.42 000 years); and in the date of perihelion (the time of year when the Earth is closest to the sun, about 21 000 years) (M. Milankovitch 1930). There is also a 100 ka cycle; see Berger (1978) J. Atmos. Scis 35.
The medium-scale cycles are poorly known: Campbell et al. (1998) Geology 26 recognize up to eight cycles, but cannot explain their origins. Ware (1995) Fisheries Oceanog. 4 demonstrates four short-term cycles: 2–3 years, 5–7 years (El Niño–Southern Oscillation), 20–25 years (bidecadal oscillation), and a poorly resolved, very low-frequency oscillation with a 50–75-year frequency. See Harris (2005) PPG 29, 2 for an excellent summary. Imbrie and Imbrie (1980) Science 207 show that Milankovitch cycles satisfactorily explain the major fluctuations in ocean temperature.