### eccentricity

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

The term **eccentricity** refers to an orbit's non-circularity,
and is more specifically used for a numerical quantification (*e*)
of the non-circularity, 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.
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,
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.

(*orbits,celestial mechanics,measure*)
**Further reading:**

http://en.wikipedia.org/wiki/Orbital_eccentricity

**Referenced by pages:**

apsis

black hole merger

Ceres

comet

corotation resonance (CR)

damocloid

DQ Tau

Earth

extreme mass ratio inspiral (EMRI)

EXOSAT

exosphere

giant planet

HD 80606 b

Hill radius

Jupiter

Kozai mechanism

Kuiper Belt (K Belt)

late heavy bombardment (LHB)

libration

Mars

mean anomaly

Mercury

moon

Neptune

orbital element

Planet Nine

planet formation

Pluto

precession

Laplace radius (r_{L})

Saturn

Sedna

semi-major axis (*a*)

sidereal

solar circle

solar constant

solar time

Solar Orbiter (SolO)

synodic period

tidal heating

tidal Q

trans-Neptunian object (TNO)

Uranus

Venus

Index