Eccentricity of the Earth's orbit around the Sun. The fluctuation between a nearly circular and elliptical orbit drives cyclic changes in the Earth's environment, including the global carbon cycle.

The maximum eccentricity possible is 1, which forms not a closed orbit but a parabola, meaning the object will never complete a full orbit but instead be flung off into space.) The difference ...

Some orbits are ever so slightly elliptical (like a somewhat squashed circle, or, in astronomical terms, an orbit with low eccentricity), whilst some appear rather flattened (highly eccentric orbits).