### 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

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

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