Eccentricity, is the change in the shape of Earth's orbit from a more circular shape to a more elliptical shape. Earth's orbit is an ellipse, and eccentricity measures how circular/elliptical the orbit is. It is one of the Milankovitch cycles, a three part group that incluence earth's climate through the variation of the Earth's orbit. Eccentricity takes about 100,000 years to complete a whole period and return back to the orbit's original shape. Changes in eccentricity affect the distribution of solar radiation received by Earth, inturn affecting climate patterns. Currently, Earth's orbit is characterized by low eccentricity, this is one of the reasons for the Earth's stable climate in the millenia.