# eccentricity (orbit)

This page refers to eccentricity in astrodynamics. For other uses, see the disambiguation page eccentricity.

Examples of orbital trajectories with various eccentricities
In astrodynamics, under standard assumptions, any orbit must be of conic section shape. The eccentricity of this conic section, the orbit's eccentricity, is an important parameter of the orbit that defines its absolute shape. Eccentricity may be interpreted as a measure of how much this shape deviates from a circle.

Under standard assumptions eccentricity () is strictly defined for all circular, elliptic, parabolic and hyperbolic orbits and may take following values: For elliptical orbits, a simple proof shows that sin−1 yields the projection angle of a perfect circle to an ellipse of eccentricity . So to view the eccentricity of, say, the planet Mercury (0.2056), simply calculate the inverse sine to find the projection angle of 11.86 degrees. Then tilt any circular object (such as a coffee mug viewed from the top) by that angle and the apparent ellipse projected to your eye will be of that same eccentricity.

## Calculation

Eccentricity of an orbit can be calculated from orbital state vectors as a magnitude of eccentricity vector:
where: For elliptic orbits it can also be calculated from distance at periapsis and apoapsis:
:
where:
• is distance at periapsis (closest approach),
• is distance at apoapsis (farthest approach).

## Examples

For example, the eccentricity of the Earth's orbit today is 0.0167. Through time, the eccentricity of the Earth's orbit slowly changes from nearly 0 to almost 0.05 as a result of gravitational attractions between the planets (see graph [1]).

In other values, Mercury (with an eccentricity of 0.2056) holds the title as the largest value among the planets of the Solar System. Prior to the redefinition of its planetary status, the dwarf planet Pluto held this title with an eccentricity of about 0.248. The Moon also holds a notable value at 0.0554. For the values for all planets in one table, see Table of planets in the solar system.

Most of the solar system's asteroids have eccentricities between 0 and 0.35 with an average value of 0.17. [1] Their comparatively high eccentricities are probably due to the influence of Jupiter and to past collisions.

The eccentricity of comets is most often close to 1. Periodic comets have highly eccentric elliptical orbits, whose eccentricity will be just less than 1; Halley's Comet's elliptical orbit having a value of 0.967. Non-periodic comets follow near-parabolic orbits and thus have eccentricities very close to 1. Examples include Comet Hale-Bopp with a value of 0.995086 and Comet McNaught with a value of 1.000030. As Hale-Bopp's value is less than 1, its orbit is elliptical and so the comet will in fact return (in about 4380AD). Comet McNaught on the other hand has a hyperbolic orbit and so may leave the solar system indefinitely.

Planet Neptune's largest moon Triton has the smallest eccentricity of any known body in the solar system; it is as close to a perfect circle as can be currently measured.

## Climatic effect

Orbital mechanics require that the duration of the seasons be proportional to the area of the Earth's orbit swept between the solstices and equinoxes, so when the orbital eccentricity is extreme, the seasons that occur on the far side of the orbit (aphelion) can be substantially longer in duration. Today, northern hemisphere fall and winter occur at closest approach (perihelion), when the earth is moving at its maximum velocity. As a result, fall and winter are slightly shorter than spring and summer. In 2006, summer is 4.66 days longer than winter and spring is 2.9 days longer than fall [2]. Axial precession slowly changes the place in the Earth's orbit where the solstices and equinoxes occur. Over the next 10,000 years, northern hemisphere winters will become gradually longer and summers will become shorter. Any cooling effect, however, will be counteracted by the fact that the eccentricity of Earth's orbit will be almost halved, reducing the mean orbital radius and raising temperatures in both hemispheres closer to the mid-interglacial peak.

## References

1. ^ [2]
2. ^ [3]

Orbital mechanics or astrodynamics is the study of the motion of rockets and other spacecraft. The motion of these objects is determined by Newton's laws of motion and the law of universal gravitation.
Eccentricity may refer to:
• Eccentricity (behavior), unusual or odd behavior on the part of a person, as opposed to being "normal"
• Eccentricity (mathematics), a parameter associated with every conic section
• Eccentricity vector

Orbital mechanics or astrodynamics is the study of the motion of rockets and other spacecraft. The motion of these objects is determined by Newton's laws of motion and the law of universal gravitation.
A1: and are the only objects in the universe and thus influence of other objects is disregarded,
• A2: The mass of the orbiting body () is far smaller than central body (), i.e.
• ORBit is a CORBA compliant Object Request Broker (ORB). The current version is called ORBit2 and is compliant with CORBA version 2.4. It is developed under the GPL license and is used as middleware for the GNOME project.
conic section (or just conic) is a curve that can be formed by intersecting a cone (more precisely, a right circular conical surface) with a plane. The conic sections were named and studied as long ago as 200 BC, when Apollonius of Perga undertook a systematic study of their
eccentricity, denoted e or , is a parameter associated with every conic section. It can be thought of as a measure of how much the conic section deviates from being circular.

In particular,
• The eccentricity of a circle is zero.

A1: and are the only objects in the universe and thus influence of other objects is disregarded,
• A2: The mass of the orbiting body () is far smaller than central body (), i.e.
• For other meanings of the term "orbit", see orbit (disambiguation)

In astrodynamics or celestial mechanics a circular orbit is an elliptic orbit with the eccentricity equal to 0.
elliptic orbit can be computed from the Vis-viva equation as:
where:
• is standard gravitational parameter,
• is radial distance of orbiting body from central body,
• is length of semi-major axis.

In astrodynamics or celestial mechanics a parabolic trajectory is an orbit with the eccentricity equal to 1. When moving away from the source it is called an escape orbit, otherwise a capture orbit.
In astrodynamics or celestial mechanics a hyperbolic trajectory is an orbit with the eccentricity greater than 1. Under standard assumptions a body traveling along this trajectory will coast to infinity, arriving there with hyperbolic excess velocity relative to the central body.
ORBit is a CORBA compliant Object Request Broker (ORB). The current version is called ORBit2 and is compliant with CORBA version 2.4. It is developed under the GPL license and is used as middleware for the GNOME project.
For other meanings of the term "orbit", see orbit (disambiguation)

In astrodynamics or celestial mechanics a circular orbit is an elliptic orbit with the eccentricity equal to 0.
elliptic orbit can be computed from the Vis-viva equation as:
where:
• is standard gravitational parameter,
• is radial distance of orbiting body from central body,
• is length of semi-major axis.

In astrodynamics or celestial mechanics a parabolic trajectory is an orbit with the eccentricity equal to 1. When moving away from the source it is called an escape orbit, otherwise a capture orbit.
In astrodynamics or celestial mechanics a hyperbolic trajectory is an orbit with the eccentricity greater than 1. Under standard assumptions a body traveling along this trajectory will coast to infinity, arriving there with hyperbolic excess velocity relative to the central body.
ellipse (from the Greek ἔλλειψις, literally absence) is the locus of points on a plane where the sum of the distances from any point on the curve to two fixed points is constant.
ellipse (from the Greek ἔλλειψις, literally absence) is the locus of points on a plane where the sum of the distances from any point on the curve to two fixed points is constant.
ORBit is a CORBA compliant Object Request Broker (ORB). The current version is called ORBit2 and is compliant with CORBA version 2.4. It is developed under the GPL license and is used as middleware for the GNOME project.
In astrodynamics or celestial dynamics orbital state vectors (sometimes State Vectors) are vectors of position () and velocity () that together with their time (epoch) () uniquely determine the state of an orbiting body.

The magnitude of a mathematical object is its size: a property by which it can be larger or smaller than other objects of the same kind; in technical terms, an ordering of the class of objects to which
In astrodynamics the eccentricity vector of a conic section orbit is the vector pointing towards the periapsis and with magnitude equal to the orbit's scalar eccentricity. The magnitude is unitless.
In astrodynamics the eccentricity vector of a conic section orbit is the vector pointing towards the periapsis and with magnitude equal to the orbit's scalar eccentricity. The magnitude is unitless.
elliptic orbit can be computed from the Vis-viva equation as:
where:
• is standard gravitational parameter,
• is radial distance of orbiting body from central body,
• is length of semi-major axis.