The extent to which the Earth's orbit around the Sun departs from a perfect circle.
1. The value of eccentricity (e) is defined as follows:
(1) for circular orbits: e = 0,
(2) for elliptical orbits: 0 < e <1,
(3) for parabolic trajectories: e = 1, and
(4) for hyperbolic trajectories: e > 1.
2. The eccentricity of the Earth's orbit is currently about 0.0167, which means that it is nearly circular. It changes over a 100,000 year cycle (see Milankovitch cycles) varying from nearly 0.0034 to almost 0.058.