Excentricidad orbital

Amount by which an orbit deviates from circlar

Este artículo trata sobre la excentricidad en astrodinámica. Para otros usos, consulte Excentricidad (desambiguación) .

Una órbita de Kepler elíptica, parabólica e hiperbólica :
  elíptica (excentricidad = 0,7)
  parabólico (excentricidad = 1)
  órbita hiperbólica (excentricidad = 1,3)

Órbita elíptica por excentricidad
  0.0  ·   0,2  ·   0,4  ·   0,6  ·   0,8

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Tipos de órbitas de dos cuerpos , por excentricidad Órbita circular Órbita elíptica Transferencia de órbita ( Órbita de transferencia de Hohmann Órbita de transferencia bielíptica ) Órbita parabólica Órbita hiperbólica Órbita radial Órbita decadente
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v t mi

La excentricidad orbital de un objeto astronómico es un parámetro adimensional que determina la cantidad en la que su órbita alrededor de otro cuerpo se desvía de un círculo perfecto . Un valor de 0 es una órbita circular, los valores entre 0 y 1 forman una órbita elíptica , 1 es una órbita de escape parabólica y mayor que 1 es una hipérbola . El término deriva su nombre de los parámetros de las secciones cónicas , ya que cada órbita de Kepler es una sección cónica. Normalmente se utiliza para el problema aislado de dos cuerpos , pero existen extensiones para los objetos que siguen unLa roseta de Klemperer orbita a través de la galaxia.

Definición

e = 0,5

Órbitas en un sistema de dos cuerpos para dos valores de excentricidad, e. (NB: + es baricentro )

En un problema de dos cuerpos con fuerza de ley del cuadrado inverso, cada órbita es una órbita de Kepler. La excentricidad de esta órbita de Kepler es un número no negativo que define su forma.

La excentricidad puede tomar los siguientes valores:

• órbita circular : e = 0
• órbita elíptica : 0 < e <1 (ver elipse )
• trayectoria parabólica : e = 1 (ver parábola )
• trayectoria hiperbólica : e > 1 (ver hipérbola )

La excentricidad e viene dada por

${\displaystyle e={\sqrt {1+{\frac {2EL^{2}}{m_{\text{red}}\alpha ^{2}}}}}}$

donde E es la energía orbital total , L es el momento angular , m _rojo es la masa reducida y α el coeficiente de la fuerza central de la ley del inverso del cuadrado , como la gravedad o la electrostática en la física clásica :

${\displaystyle F={\frac {\alpha }{r^{2}}}}$

( α es negativo para una fuerza atractiva, positivo para una repulsiva; ver también el problema de Kepler )

o en el caso de una fuerza gravitacional:

${\displaystyle e={\sqrt {1+{\frac {2\varepsilon h^{2}}{\mu ^{2}}}}}}$

donde ε es la energía orbital específica ( energía total dividida por la masa reducida), μ el parámetro gravitacional estándar basado en la masa total y h el momento angular relativo específico ( momento angular dividido por la masa reducida).

Para valores de e de 0 a 1, la forma de la órbita es una elipse cada vez más alargada (o más plana); para valores de e de 1 a infinito, la órbita es una hipérbola rama haciendo un giro total de 2 arccsc e , disminuyendo de 180 a 0 grados. El caso límite entre una elipse y una hipérbola, cuando e es igual a 1, es una parábola.

Las trayectorias radiales se clasifican en elípticas, parabólicas o hiperbólicas según la energía de la órbita, no la excentricidad. Las órbitas radiales tienen momento angular cero y, por lo tanto, una excentricidad igual a uno. Manteniendo la energía constante y reduciendo el momento angular, las órbitas elípticas, parabólicas e hiperbólicas tienden cada una al tipo correspondiente de trayectoria radial, mientras que e tiende a 1 (o en el caso parabólico, sigue siendo 1).

Para una fuerza repulsiva solo es aplicable la trayectoria hiperbólica, incluida la versión radial.

Para órbitas elípticas, una demostración simple muestra que arcsin ( ⁠) produce el ángulo de proyección de un círculo perfecto a una elipse de excentricidad e . Por ejemplo, para ver la excentricidad del planeta Mercurio ( e = 0,2056), simplemente se debe calcular el seno inverso para encontrar el ángulo de proyección de 11,86 grados. A continuación, incline cualquier objeto circular (como una taza de café vista desde arriba) en ese ángulo y la elipse aparente proyectada en su ojo será de la misma excentricidad.${\displaystyle e}$

Etimología

La palabra "excentricidad" proviene del latín medieval eccentricus , derivado del griego ἔκκεντρος ekkentros "fuera del centro", desde ἐκ- ek- , "fuera de" + κέντρον kentron "centro". "Excéntrico" apareció por primera vez en inglés en 1551, con la definición "... un círculo en el que la tierra, el sol, etc. se desvía de su centro". ^{[ cita requerida ]} Cinco años después, en 1556, se había desarrollado una forma adjetiva de la palabra.

Cálculo

La excentricidad de una órbita se puede calcular a partir de los vectores de estado orbital como la magnitud del vector de excentricidad :

${\displaystyle e=\left|\mathbf {e} \right|}$

dónde:

• e es el vector de excentricidad ( "vector de Hamilton" ).

Para órbitas elípticas también puede calcularse a partir de la periapsis y apoapsis desde y donde una es la longitud del semieje mayor , el-geométrico promedio y distancia de tiempo de la media.${\displaystyle \,r_{\text{p}}=a\,(1-e)\,}$${\displaystyle \,r_{\text{a}}=a\,(1+e)\,,}$

${\displaystyle {\begin{aligned}e&={\frac {r_{\text{a}}-r_{\text{p}}}{r_{\text{a}}+r_{\text{p}}}}\\\,\\&={\frac {r_{\text{a}}/r_{\text{p}}-1}{r_{\text{a}}/r_{\text{p}}+1}}\\\,\\&=1-{\frac {2}{\;{\frac {r_{\text{a}}}{r_{\text{p}}}}+1\;}}\end{aligned}}}$

dónde:

• r _a es el radio en apoapsis (también conocido como "apofocus", "afelio", "apogeo", es decir, la distancia más lejana de la órbita al centro de masa del sistema, que es un foco de la elipse).
• r _p es el radio en la periapsis (también conocido como "perifocus", etc., la distancia más cercana).

La excentricidad de una órbita elíptica también se puede utilizar para obtener la relación entre el radio de periapsis y el radio de apoapsis :

${\displaystyle {\frac {r_{\text{a}}}{r_{\text{p}}}}={\frac {\,a\,(1+e)\,}{\,a\,(1-e)\,}}={\frac {1+e}{1-e}}}$

Para la Tierra, la excentricidad orbital e ≈ 0.01671, la apoapsis es el afelio y la periapsis es el perihelio en relación con el sol.

For Earth's annual orbit path, the ratio of longest radius (r_a) / shortest radius (r_p) is ${\displaystyle {\frac {\,r_{\text{a}}\,}{r_{\text{p}}}}={\frac {\,1+e\,}{1-e}}{\text{ ≈ 1.03399 .}}}$

Examples

Gravity Simulator plot of the changing orbital eccentricity of Mercury, Venus, Earth, and Mars over the next 50000 years. The arrows indicate the different scales used, as the eccentricities of Mercury and Mars are much greater than those of Venus and Earth. The 0 point on this plot is the year 2007.

Eccentricities of Solar System bodies

Object	eccentricity
Triton	0.00002
Venus	0.0068
Neptune	0.0086
Earth	0.0167
Titan	0.0288
Uranus	0.0472
Jupiter	0.0484
Saturn	0.0541
Moon	0.0549
1 Ceres	0.0758
4 Vesta	0.0887
Mars	0.0934
10 Hygiea	0.1146
Makemake	0.1559
Haumea	0.1887
Mercury	0.2056
2 Pallas	0.2313
Pluto	0.2488
3 Juno	0.2555
324 Bamberga	0.3400
Eris	0.4407
Nereid	0.7507
Sedna	0.8549
Halley's Comet	0.9671
Comet Hale-Bopp	0.9951
Comet Ikeya-Seki	0.9999
C/1980 E1	1.057
ʻOumuamua	1.20[a]
C/2019 Q4 (Borisov)	3.5[b]

The eccentricity of the Earth's orbit is currently about 0.0167; the Earth's orbit is nearly circular. Venus and Neptune have even lower eccentricities. Over hundreds of 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).^[1]

The table lists the values for all planets and dwarf planets, and selected asteroids, comets, and moons. Mercury has the greatest orbital eccentricity of any planet in the Solar System (e = 0.2056). Such eccentricity is sufficient for Mercury to receive twice as much solar irradiation at perihelion compared to aphelion. Before its demotion from planet status in 2006, Pluto was considered to be the planet with the most eccentric orbit (e = 0.248). Other Trans-Neptunian objects have significant eccentricity, notably the dwarf planet Eris (0.44). Even further out, Sedna, has an extremely high eccentricity of 0.855 due to its estimated aphelion of 937 AU and perihelion of about 76 AU.

Most of the Solar System's asteroids have orbital eccentricities between 0 and 0.35 with an average value of 0.17.^[2] Their comparatively high eccentricities are probably due to the influence of Jupiter and to past collisions.

The Moon's value is 0.0549, the most eccentric of the large moons of the Solar System. The four Galilean moons have eccentricity < 0.01. Neptune's largest moon Triton has an eccentricity of 1.6×10⁻⁵ (0.000016),^[3] the smallest eccentricity of any known moon in the Solar System;^{[citation needed]} its orbit is as close to a perfect circle as can be currently^[when?] measured. However, smaller moons, particularly irregular moons, can have significant eccentricity, such as Neptune's third largest moon Nereid (0.75).

Comets have very different values of eccentricity. Periodic comets have eccentricities mostly between 0.2 and 0.7,^[4] but some of them have highly eccentric elliptical orbits with eccentricities just below 1, for example, Halley's Comet has a value of 0.967. Non-periodic comets follow near-parabolic orbits and thus have eccentricities even closer to 1. Examples include Comet Hale–Bopp with a value of 0.995^[5] and comet C/2006 P1 (McNaught) with a value of 1.000019.^[6] As Hale–Bopp's value is less than 1, its orbit is elliptical and it will return.^[5] Comet McNaught has a hyperbolic orbit while within the influence of the planets,^[6] but is still bound to the Sun with an orbital period of about 10⁵ years.^[7] As of a 2010 Epoch, Comet C/1980 E1 has the largest eccentricity of any known hyperbolic comet with an eccentricity of 1.057,^[8] and will leave the Solar System eventually.

ʻOumuamua is the first interstellar object found passing through the Solar System. Its orbital eccentricity of 1.20 indicates that ʻOumuamua has never been gravitationally bound to our sun. It was discovered 0.2 AU (30,000,000 km; 19,000,000 mi) from Earth and is roughly 200 meters in diameter. It has an interstellar speed (velocity at infinity) of 26.33 km/s (58,900 mph).

Mean eccentricity

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,^[9] but from 1800 to 2050 has a mean eccentricity of 0.00859.^[10]

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—while the opposite occurs in the southern hemisphere. As a result, in the northern hemisphere, fall and winter are slightly shorter than spring and summer—but in global terms this is balanced with them being longer below the equator. In 2006, the northern hemisphere summer was 4.66 days longer than winter, and spring was 2.9 days longer than fall due to the Milankovitch cycles.^[11][12]

Apsidal precession also slowly changes the place in the Earth's orbit where the solstices and equinoxes occur. Note that this is a slow change in the orbit of the Earth, not the axis of rotation, which is referred to as axial precession (see Precession § Astronomy). Over the next 10,000 years, the northern hemisphere winters will become gradually longer and summers will become shorter. However, any cooling effect in one hemisphere is balanced by warming in the other, and any overall change will be counteracted by the fact that the eccentricity of Earth's orbit will be almost halved.^[13] This will reduce the mean orbital radius and raise temperatures in both hemispheres closer to the mid-interglacial peak.

Exoplanets

Of the many exoplanets discovered, most have a higher orbital eccentricity than planets in our planetary system. Exoplanets found with low orbital eccentricity (near-circular orbits) are very close to their star and are tidally locked to the star. All eight planets in the Solar System have near-circular orbits. The exoplanets discovered show that the solar system, with its unusually low eccentricity, is rare and unique.^[14] One theory attributes this low eccentricity to the high number of planets in the Solar System; another suggests it arose because of its unique asteroid belts. A few other multiplanetary systems have been found, but none resemble the Solar System. The Solar System has unique planetesimal systems, which led the planets to have near-circular orbits. Solar planetesimal systems include the asteroid belt, Hilda family, Kuiper belt, Hills cloud, and the Oort cloud. The exoplanet systems discovered have either no planetesimal systems or one very large one. Low eccentricity is needed for habitability, especially advanced life.^[15] High multiplicity planet systems are much more likely to have habitable exoplanets.^[16][17] The grand tack hypothesis of the Solar System also helps understand its near-circular orbits and other unique features.^{[18][19][20][21][22][23][24][25]}

See also

• Equation of time

Footnotes

1. ʻOumuamua was never bound to the Sun, so its orbit is hyperbolic: e ≈ 1.20 > 1 .
2. C/2019 Q4 (Borisov) was never bound to the Sun, so its orbit is hyperbolic: e ≈ 3.5 >> 1 .

References

1. A. Berger & M.F. Loutre (1991). "Graph of the eccentricity of the Earth's orbit". Illinois State Museum (Insolation values for the climate of the last 10 million years). Archived from the original on 6 January 2018.
2. Asteroids Archived 4 March 2007 at the Wayback Machine
3. David R. Williams (22 January 2008). "Neptunian Satellite Fact Sheet". NASA.
4. Lewis, John (2 December 2012). Physics and Chemistry of the Solar System. Academic Press. ISBN 9780323145848.
5. "JPL Small-Body Database Browser: C/1995 O1 (Hale-Bopp)" (2007-10-22 last obs). Retrieved 5 December 2008.
6. "JPL Small-Body Database Browser: C/2006 P1 (McNaught)" (2007-07-11 last obs). Retrieved 17 December 2009.
7. "Comet C/2006 P1 (McNaught) – facts and figures". Perth Observatory in Australia. 22 January 2007. Archived from the original on 18 February 2011.
8. "JPL Small-Body Database Browser: C/1980 E1 (Bowell)" (1986-12-02 last obs). Retrieved 22 March 2010.
9. Williams, David R. (29 November 2007). "Neptune Fact Sheet". NASA.
10. "Keplerian elements for 1800 A.D. to 2050 A.D." JPL Solar System Dynamics. Retrieved 17 December 2009.
11. Data from United States Naval Observatory
12. Berger A.; Loutre M.F.; Mélice J.L. (2006). "Equatorial insolation: from precession harmonics to eccentricity frequencies" (PDF). Clim. Past Discuss. 2 (4): 519–533. doi:10.5194/cpd-2-519-2006.
13. Arizona U., Long Term Climate
14. exoplanets.org, ORBITAL ECCENTRICITES, by G.Marcy, P.Butler, D.Fischer, S.Vogt, 20 Sept 2003
15. Ward, Peter; Brownlee, Donald (2000). Rare Earth: Why Complex Life is Uncommon in the Universe. Springer. pp. 122–123. ISBN 0-387-98701-0.
16. Limbach, MA; Turner, EL (2015). "Exoplanet orbital eccentricity: multiplicity relation and the Solar System". Proc Natl Acad Sci U S A. 112 (1): 20–4. arXiv:1404.2552. Bibcode:2015PNAS..112...20L. doi:10.1073/pnas.1406545111. PMC 4291657. PMID 25512527.
17. Steward Observatory, University of Arizona, Tucson, Planetesimals in Debris Disks, by Andrew N. Youdin and George H. Rieke, 2015
18. Zubritsky, Elizabeth. "Jupiter's Youthful Travels Redefined Solar System". NASA. Retrieved 4 November 2015.
19. Sanders, Ray. "How Did Jupiter Shape Our Solar System?". Universe Today. Retrieved 4 November 2015.
20. Choi, Charles Q. "Jupiter's 'Smashing' Migration May Explain Our Oddball Solar System". Space.com. Retrieved 4 November 2015.
21. Davidsson, Dr. Björn J. R. "Mysteries of the asteroid belt". The History of the Solar System. Retrieved 7 November 2015.
22. Raymond, Sean. "The Grand Tack". PlanetPlanet. Retrieved 7 November 2015.
23. O'Brien, David P.; Walsh, Kevin J.; Morbidelli, Alessandro; Raymond, Sean N.; Mandell, Avi M. (2014). "Water delivery and giant impacts in the 'Grand Tack' scenario". Icarus. 239: 74–84. arXiv:1407.3290. Bibcode:2014Icar..239...74O. doi:10.1016/j.icarus.2014.05.009.
24. Loeb, Abraham; Batista, Rafael; Sloan, David (August 2016). "Relative Likelihood for Life as a Function of Cosmic Time". Journal of Cosmology and Astroparticle Physics. 2016 (8): 040. arXiv:1606.08448. Bibcode:2016JCAP...08..040L. doi:10.1088/1475-7516/2016/08/040.
25. "Is Earthly Life Premature from a Cosmic Perspective?". Harvard-Smithsonian Center for Astrophysics. 1 August 2016.

Further reading

• Prussing, John E.; Conway, Bruce A. (1993). Orbital Mechanics. New York: Oxford University Press. ISBN 0-19-507834-9.

External links

• World of Physics: Eccentricity
• The NOAA page on Climate Forcing Data includes (calculated) data from Berger (1978), Berger and Loutre (1991)^{[permanent dead link]}. Laskar et al. (2004) on Earth orbital variations, Includes eccentricity over the last 50 million years and for the coming 20 million years.
• The orbital simulations by Varadi, Ghil and Runnegar (2003) provides series for Earth orbital eccentricity and orbital inclination.
• Kepler's Second law's simulation