eccentricity

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: zero for a circular orbit, between zero and one for an elliptical orbit, 1 for a parabolic trajectory, and greater than 1 for hyperbolic. These types of curves, termed conic sections, trace each path of a pair of bodies due to gravitational interactions between them. The eccentricity of an elliptical orbit (covering virtually all of what we think of as "orbits") is:

    ra - rp
e = ———————
    ra + rp

Where:

• e is the eccentricity.
• r_a is the orbiting body's orbital radius at apoapsis (i.e., its furthest distance from the pair of bodies' center of mass).
• r_p is the orbiting body's orbital radius at periapsis (i.e., its closest distance from the pair of bodies' center of mass).

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 with the varying distance between host 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.