### eccentricity

**(***e*)
(degree to which an orbit is non-circular)

**Eccentricity** (*e*) is a number indicating the degree
to which an orbit is non-circular, 0 meaning circular,
between 0 and 1 being **elliptical**,
1 being parabolic, and greater than
1 being hyperbolic.

The eccentricity of an elliptical orbit is:

*r*_{a} - r_{p}
*e* = ———————
*r*_{a} + r_{p}

Where

*e* is the eccentricity
*r*_{a} is the radius at apoapsis (furthest distance)
*r*_{p} is the radius at **periapsis** (closest distance)

Earth's orbit has an eccentricity of 0.0167. The Moon's is 0.0549.
Halley's Comet's is 0.97.

Eccentricity can aid in the study of atmospheres of extra-solar planets,
i.e., bodies orbiting stars, because the changes in EM radiation from
the host star allow observation of the atmosphere at multiple
temperatures using light curve spectroscopic observations.

(*orbits,celestial mechanics,measure*)
http://en.wikipedia.org/wiki/Orbital_eccentricity

**Referenced by:**

apsis

black hole merger

DQ Tau

Earth

EXOSAT

HD 80606 b

Hill radius

Kozai mechanism

Kuiper Belt (K Belt)

late heavy bombardment (LHB)

libration

mean anomaly

Mercury

moon

orbital element

Planet Nine

planet formation

precession

Laplace radius (r_{L})

semi-major axis (*a*)

solar circle

solar time

synchronous orbit

synodic period

tidal heating

Venus

index