Axis Tilt and Orbit Changes Recall from the seasons section in the third chapter that the Earth's orbit is elliptical and that it gets closest in early January. With an orbit eccentricity of 0.0167, Earth receives about 7% more solar energy in early January than at its aphelion point in early July. The Earth's orbit changes eccentricity on cycles of about 100,000 and about 400,000 years. At greatest eccentricity, the Earth can receive over 20% more solar energy at perihelion than at aphelion. Mars has a more elliptical orbit than the Earth and as a result its southern hemisphere winters are long and extreme while its northern hemisphere winters are short and milder. The coordinates section of the third chapter discusses the precession of the Earth's rotation axis that is caused by the Sun's gravity and the Moon's gravity pulling on the non-perfectly round Earth. The Earth bulges a bit at the equator and is flattened at the poles primarily because of its rapid spin. The Sun's and Moon's gravities cause the rotation axis to slowly wobble with a period
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