Eccentricity is the shape of the Earth's orbit around the Sun. Its orbit is not perfectly circular, but rather slightly oval in shape and the degree of ovalness changes throughout time. Because it is oval in shape, its orbit changes the distance from the Earth to the Sun at different times of the year. Now, the Earth is the closest to the Sun in its orbit in January and farthest away in July (about 3 percent difference). Currently the Earth's orbit is only slightly oval—which means the eccentricity is low. When the eccentricity increases and the orbit becomes more oval, the difference could became 20 percent more energy reaching the Earth when it is closest to the Sun. This could have a significant effect on heating the atmosphere.
The Earth's eccentricity occurs in a cycle that has been well documented. Climatologists refer to this cycle as periodicity. The periodicity is 100,000 years—that is, the time necessary to change the orbit from a nearly circular one to a more elongated one. It also turns out that the Earth's ice ages also peak about one every 100,000 years and have for the past million years.
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