In a two-body problem with inverse-square-law force, every orbit is of conic section shape, or part of a straight line. The eccentricity of this conic section, the orbit's eccentricity, is an important parameter of the orbit that defines its absolute shape, except that an orbit with eccentricity equal to 1 can be a parabola, a double half-line (which can be argued to be the shape of a parabola at zero scale), or a double line segment. Eccentricity may be interpreted as a measure of how much this shape deviates from a circle.
The eccentricity is given by
where E is the total orbital energy, is the angular momentum, m is the reduced mass. and the coefficient of the inverse-square law central force such as gravity or electrostatics in classical physics:
( is negative for an attractive force, positive for a repulsive one) (see also Kepler problem).
or in the case of a gravitational force:
where is the specific orbital energy (total energy divided by the reduced mass), the standard gravitational parameter based on the total mass, and the specific relative angular momentum (angular momentum divided by the reduced mass).
The eccentricity may take the following values:
- circular orbit:
- non-radial elliptic orbit:
- parabolic trajectory:
- radial trajectory:
- non-radial hyperbolic trajectory: , the orbit is one branch of the hyperbola.
Thus for the values of e not equal to 1 there is a one-to-one correspondence with the shape: for values of e from 0 to 1 the orbit's shape is an increasingly elongated (or flatter) ellipse; for values of e from 1 to infinity the orbit is a hyperbola branch making a total turn of 2 arccsc e, decreasing from 180 to 0 degrees. However, e = 1 if and only if and/or . Therefore this value of e corresponds to a parabola, a half-line described in the direction to the end or from the end (a degenerate parabola or hyperbola), or a line segment (a degenerate ellipse).
Radial trajectories are classified as elliptic, parabolic, or hyperbolic based on the energy of the orbit, not the eccentricity. Radial orbits have zero angular momentum and hence eccentricity equal to one. Keeping the energy constant and reducing the angular momentum, elliptic, parabolic, and hyperbolic orbits each tend to the corresponding type of radial trajectory while e tends to 1 (or in the parabolic case: remains 1).
For a repulsive force only the hyperbolic trajectory, including the radial version, is applicable.
For elliptical orbits, a simple proof shows that arcsin() yields the projection angle of a perfect circle to an ellipse of eccentricity . For example, to view the eccentricity of the planet Mercury (=0.2056), one must simply calculate the inverse sine to find the projection angle of 11.86 degrees. Next, 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.
- is radius at apoapsis (i.e., the farthest distance of the orbit to the center of mass of the system, which is a focus of the ellipse).
- is radius at periapsis (the closest distance).
The eccentricity of the Earth's orbit is currently about 0.0167, meaning that the Earth's orbit is nearly circular, the semiminor axis is 98.6% of the semimajor axis. Over thousands of years, the eccentricity of the Earth's orbit varies from nearly 0.0034 to almost 0.058 as a result of gravitational attractions among the planets (see graph).
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 0.0549. 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.  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, with eccentricities just below 1; Halley's Comet's elliptical orbit, for example, has 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 0.995 and Comet McNaught with a value of 1.000019. As Hale-Bopp's value is less than 1, its orbit is elliptical and so the comet will in fact return. 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 an eccentricity of 0.000016 which is the smallest eccentricity of any known body in the solar system; its orbit is as close to a perfect circle as can be currently measured.
The mean eccentricity of an object is the average eccentricity as a result of perturbations over a given time period. Neptune currently has an instant (current Epoch) eccentricity of 0.0113, but from 1800 AD to 2050 AD has a mean eccentricity of 0.00859.
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, in the northern hemisphere, fall and winter are slightly shorter than spring and summer. In 2006, summer was 4.66 days longer than winter and spring was 2.9 days longer than fall.
- See also: Error: Template must be given at least one article name 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
- See also: Error: Template must be given at least one article name, reducing the mean orbital radius and raising temperatures in both hemispheres closer to the mid-interglacial peak.
- ↑ A. Berger and M.F. Loutre (1991 (old, but published)). "Graph of the eccentricity of the Earth's orbit". Illinois State Museum (Insolation values for the climate of the last 10 million years). Retrieved on 2009-12-17.
- ↑ Asteroids
- ↑ 3.0 3.1 "JPL Small-Body Database Browser: C/1995 O1 (Hale-Bopp)" (2007-10-22 last obs). Retrieved on 2008-12-05.
- ↑ "JPL Small-Body Database Browser: C/2006 P1 (McNaught)" (2007-07-11 last obs). Retrieved on 2009-12-17.
- ↑ David R. Williams (22 January 2008). "Neptunian Satellite Fact Sheet". NASA. Retrieved on 2009-12-17.
- ↑ Williams, David R. (2007-11-29). "Neptune Fact Sheet". NASA. Retrieved on 2009-12-17.
- ↑ "Keplerian elements for 1800 AD to 2050 AD". JPL Solar System Dynamics. Retrieved on 2009-12-17.
- ↑ This information is concerning the summer of the year 2006 not the current year we are in now.
Prussing, John E., and Bruce A. Conway. Orbital Mechanicsc. New York: Oxford University Press, 1993.
- World of Physics: Eccentricity
- The NOAA page on Climate Forcing Data includes (calculated) data from Berger (1978), Berger and Loutre (1991) and Laskar et al. (2004) on Earth orbital variations, including eccentricity, over the last 50 million years and for the coming 20 million years
- The orbital simulations by Varadi, Ghil and Runnegar (2003) provide another, slightly different series for Earth orbital eccentricity, and also a series for orbital inclination
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