### eccentricity

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

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
```

Where:

• e is the eccentricity
• ra is the radius at apoapsis (furthest distance)
• rp 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)
https://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)
epicycle
EXOSAT
exosphere
giant planet
gravity assist
Haumea
HD 80606 b
heartbeat star
heliometer
Jupiter
K2-18b
Kozai mechanism
Kuiper Belt (K Belt)
late heavy bombardment (LHB)
libration
Mars
mean anomaly
Mercury
moon
Neptune
Oort Cloud
orbital element
orbital resonance
Parker Solar Probe (PSP)
Planet Nine
planet formation
Pluto
precession of the equinoxes
rotation period
Saturn
Sedna
semi-major axis (a)
Comet Shoemaker-Levy 9
sidereal
solar circle
solar constant
solar time
Solar Orbiter (SolO)
synodic period
tidal heating
tidal Q
trans-Neptunian object (TNO)
Uranus
Venus

Index