<P> The Earth's orbit approximates an ellipse . Eccentricity measures the departure of this ellipse from circularity . The shape of the Earth's orbit varies between nearly circular (with the lowest eccentricity of 0.000055) and mildly elliptical (highest eccentricity of 0.0679) Its geometric or logarithmic mean is 0.0019 . The major component of these variations occurs with a period of 413,000 years (eccentricity variation of ± 0.012). Other components have 95,000 - year and 125,000 - year cycles (with a beat period of 400,000 years). They loosely combine into a 100,000 - year cycle (variation of − 0.03 to + 0.02). The present eccentricity is 0.017 and decreasing . </P> <P> Eccentricity varies primarily due to the gravitational pull of Jupiter and Saturn . However, the semi-major axis of the orbital ellipse remains unchanged; according to perturbation theory, which computes the evolution of the orbit, the semi-major axis is invariant . The orbital period (the length of a sidereal year) is also invariant, because according to Kepler's third law, it is determined by the semi-major axis . </P> <P> The semi-major axis is a constant . Therefore, when Earth's orbit becomes more eccentric, the semi-minor axis shortens . This increases the magnitude of seasonal changes . </P> <P> The relative increase in solar irradiation at closest approach to the Sun (perihelion) compared to the irradiation at the furthest distance (aphelion) is slightly larger than four times the eccentricity . For Earth's current orbital eccentricity, incoming solar radiation varies by about 6.8%, while the distance from the Sun currently varies by only 3.4% (5.1 million km). Perihelion presently occurs around January 3, while aphelion is around July 4 . When the orbit is at its most eccentric, the amount of solar radiation at perihelion will be about 23% more than at aphelion . However, the Earth's eccentricity is always so small that the variation in solar irradiation is a minor factor in seasonal climate variation, compared to axial tilt and even compared to the relative ease of heating the larger land masses of the northern hemisphere . </P>

Which of the following is not part of the milankovitch theory