 Eccentricity refers to the shape of the Earth's orbit around the Sun. The Earth doesn't travel around the Sun in a perfectly circular shape. It varies between a near-perfect circle and slightly elliptical. This variation occurs due to the gravitational pull of the two largest planets in the Solar System, Saturn and Jupiter. It takes around 100,000 years for the Earth's orbital path to cycle from circular to elliptical and back again. Eccentricity is the reason why our seasons are slightly different lengths. Currently Earth's eccentricity is near its most circular shape and is very slowly becoming more circular. In the Southern Hemisphere, winters are about four and a half days longer than summers and autumns are about three days longer than spring.