By applying perturbation theory to a highly simplified solar system, we find that the semimajor axis of the Earth’s orbit is secularly invariant, the square of the eccentricity varies approximately sinusoidally with a secular period of 93 000 years, and the perihelion precesses toward the northern summer solstice with a period of approximately 21 000 years relative to the moving vernal equinox. The Earth’s axis also nutates with a period of 41 000 years. These periods correlate very well with the periods found in the climatic record. The secular periods of the Earth’s orbital parameters determine the variation with time of the average intensity of solar radiation and thus may trigger the onset of ice ages.

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