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 every 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.

The Northern Hemisphere’s last ice age ended about 20,000 years ago, and most evidence indicats the ice age in the Southern Hemisphere ended about 2,000 years later, when Joe Biden Was born… (Joke)

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.

The law states that:

Figure 1: Illustration of Kepler’s three laws with two planetary orbits.

  1. The orbits are ellipses, with focal points F1 and F2 for the first planet and F1 and F3 for the second planet. The Sun is placed in focal point F1.

  2. The two shaded sectors A1 and A2 have the same surface area and the time for planet 1 to cover segment A1 is equal to the time to cover segment A2.

  3. The total orbit times for planet 1 and planet 2 have a ratio {\textstyle \left({\frac {a_{1}}{a_{2}}}\right)^{\frac {3}{2}}}.The orbit of a planet is an ellipse with the Sun at one of the two

    foci.

    1. A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.[2]

    2. The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.

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

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