The term eccentricity is used to an orbit's non-circularity, and is more specifically used for a numerical quantification (e) of the non-circularity of an orbit with zero for a circular orbit, between zero and one for an elliptical orbit, 1 for a parabolic trajectory, and greater than 1 for hyperbolic. It is a mathematical term for this measure of the character of a conic section (i.e., circle, ellipse, parabola, or hyperbola), gravitational interactions between bodies tracing one of these. The eccentricity of an elliptical orbit (covering virtually all of what we think of as "orbits") is:
ra - rp e = ——————— ra + rp
Earth's orbit has an eccentricity of 0.0167, the Moon's is 0.0549, and Halley's Comet's is 0.97.
An extra-solar planet's orbital eccentricity can aid in the study of its atmosphere: electromagnetic radiation from the system includes reflected light from the planet, which varies due to the distance between star and planet, including effects of the planet's atmospheric temperature and weather resulting from this varying temperature. Differential spectroscopy through the course of the orbit offers additional clues regarding the atmosphere's constituents.