orbital eccentricity
/ ek-sen-TRISS-ih-tee /
How round is an orbit? A perfect circle, or an egg-shaped loop, or a long thin cigar? Eccentricity is the single number that answers this — a dial running from 0 to 1 that measures how far an ellipse departs from a circle. It is the difference between a moon on a gentle, near-circular path and a comet that plunges in close and then races far away.
Eccentricity e is defined so that e = 0 is a perfect circle, e between 0 and 1 is an ellipse (the closer to 1, the more stretched), e = 1 is a parabola, and e greater than 1 is a hyperbola — an unbound path that swings by once and leaves forever. Geometrically, e measures how far the focus (where the Sun sits) is offset from the center, as a fraction of the orbit's half-length. Earth's orbit is nearly circular at e ≈ 0.017; Mars is a noticeable e ≈ 0.093; Mercury, the most lopsided planet, has e ≈ 0.206; many comets exceed e ≈ 0.9.
Eccentricity controls the rhythm of an orbit: the higher it is, the more dramatically a body speeds up at its closest approach (perihelion) and dawdles at its farthest (aphelion), exactly as Kepler's equal-area law demands. It is also a fingerprint of history — the eccentricities of planets and exoplanets encode past gravitational tugs, migrations, and resonances, so an unusually eccentric exoplanet hints at a violent dynamical past.
Mercury's e ≈ 0.206 means it is about 46 million km from the Sun at perihelion but about 70 million km at aphelion — a swing of more than 50 percent in distance that an Earth-like near-circular orbit never sees.
A modest-looking eccentricity still means a large change in distance over one orbit.
Eccentricity is about shape, not tilt: a highly eccentric orbit can lie flat in the same plane as a circular one. And the seasons on Earth come mainly from axial tilt, not from our small orbital eccentricity — a frequent confusion.