Earth Orbital Eccentricity

Kepler needed Tycho Brahe's Mars observations, accurate to two arcminutes, to detect that orbits are ellipses at all; with Earth's eccentricity of only 0.0167, the orbit drawn to scale is indistinguishable from a circle by eye — the Sun simply sits 2.5 million km off centre. The consequence is not the seasons, which come from axial tilt, but a 6.8 per cent swing in incoming sunlight between January perihelion and July aphelion, currently mild for the northern hemisphere's summer.

Over long times the value is anything but constant. Jupiter's and Saturn's perturbations drive it between about 0.0034 and 0.058 on roughly 100 000- and 400 000-year cycles, and Milanković showed that this beat, combined with the tilt and precession cycles, paces the ice ages — a prediction confirmed in the 1970s by oxygen-isotope records in deep-sea cores.