View source for Elliptic orbit

[[File:Elliptic orbit.gif|thumb|250px|A small body in space orbits a large one (like a planet around the sun) along an elliptical path, with the large body being located at one of the [[ellipse]] foci.The height of the kinetic energy decreases as the orbiting body's speed decreases and distance increases according to Kepler's laws.]]
[[File:orbit5.gif|thumb|250px|Two bodies with similar mass orbiting around a common [[Barycentric coordinates (astronomy)|barycenter]] with elliptic orbits.]]
[[File:Gravity Wells Potential Plus Kinetic Energy - Circle-Ellipse-Parabola-Hyperbola.png|thumb|250px|An elliptical orbit is depicted in the top-right quadrant of this diagram, where the [[gravity well|gravitational potential well]] of the central mass shows potential energy, and the kinetic energy of the orbital speed is shown in red. The height of the kinetic energy decreases as the orbiting body's speed decreases and distance increases according to Kepler's laws.]]
{{Astrodynamics}}
In [[astrodynamics]] or [[celestial mechanics]], an '''elliptic orbit''' or '''elliptical orbit''' is a [[Kepler orbit]] with an [[orbital eccentricity|eccentricity]] of less than 1; this includes the special case of a [[circular orbit]], with eccentricity equal to 0. In a stricter sense, it is a Kepler orbit with the eccentricity greater than 0 and less than 1 (thus excluding the circular orbit). In a wider sense, it is a Kepler orbit with negative [[Specific orbital energy|energy]]. This includes the radial elliptic orbit, with eccentricity equal to 1.

In a [[gravitational two-body problem]] with negative energy, both bodies follow [[Similarity (geometry)|similar]] elliptic orbits with the same [[orbital period]] around their common [[barycenter]]. Also the relative position of one body with respect to the other follows an elliptic orbit.

Examples of elliptic orbits include: [[Hohmann transfer orbit]], [[Molniya orbit]], and [[tundra orbit]].

==Velocity==
Under standard assumptions the [[orbital speed]] (<math>v\,</math>) of a body traveling along an '''elliptic orbit''' can be computed from the [[vis-viva equation]] as:
:<math>v = \sqrt{\mu\left({2\over{r}} - {1\over{a}}\right)}</math>
where:
*<math>\mu\,</math> is the [[standard gravitational parameter]],
*<math>r\,</math> is the distance between the orbiting bodies.
*<math>a\,\!</math> is the length of the [[semi-major axis]].

The velocity equation for a [[hyperbolic trajectory]] has either + <math>{1\over{a}}</math>, or it is the same with the convention that in that case ''a'' is negative.

== Orbital period ==
Under standard assumptions the [[orbital period]] (<math>T\,\!</math>) of a body traveling along an elliptic orbit can be computed as:
:<math>T=2\pi\sqrt{a^3\over{\mu}}</math>
where:
*<math>\mu\,</math> is the [[standard gravitational parameter]],
*<math>a\,\!</math> is the length of the [[semi-major axis]].
Conclusions:
*The orbital period is equal to that for a [[circular orbit]] with the orbital radius equal to the semi-major axis (<math>a\,\!</math>),
*For a given semi-major axis the orbital period does not depend on the eccentricity (See also: [[Kepler's laws of planetary motion#Third law|Kepler's third law]]).

==Energy==
Under standard assumptions, [[specific orbital energy]] (<math>\epsilon\,</math>) of elliptic orbit is negative and the orbital energy conservation equation (the [[Vis-viva equation]]) for this orbit can take the form:
:<math>{v^2\over{2}}-{\mu\over{r}}=-{\mu\over{2a}}=\epsilon<0</math>
where:
*<math>v\,</math> is the [[orbital speed]] of the orbiting body,
*<math>r\,</math> is the distance of the orbiting body from the [[central body]],
*<math>a\,</math> is the length of the [[semi-major axis]],
*<math>\mu\,</math> is the [[standard gravitational parameter]].
Conclusions:
*For a given semi-major axis the specific orbital energy is independent of the eccentricity.

Using the [[virial theorem]] we find:
*the time-average of the specific potential energy is equal to −2ε
**the time-average of ''r''<sup>−1</sup> is ''a''<sup>−1</sup>
*the time-average of the specific kinetic energy is equal to ε

=== Energy in terms of semi major axis ===
It can be helpful to know the energy in terms of the semi major axis (and the involved masses). The total energy of the orbit is given by
:<math>E = - G \frac{M m}{2a}</math>,

where a is the semi major axis.

==== Derivation ====
Since gravity is a central force, the angular momentum is constant:
:<math>\dot{\mathbf{L}} = \mathbf{r} \times \mathbf{F} = \mathbf{r} \times F(r)\mathbf{\hat{r}} = 0</math>

At the closest and furthest approaches, the angular momentum is perpendicular to the distance from the mass orbited, therefore:
:<math>L = r p = r m v</math>.

The total energy of the orbit is given by
:<math>E = \frac{1}{2}m v^2 - G \frac{Mm}{r}</math>.

We may substitute for v and obtain
:<math>E = \frac{1}{2}\frac{L^2}{mr^2} - G \frac{Mm}{r}</math>.

This is true for r being the closest / furthest distance so we get two simultaneous equations which we solve for E:
:<math>E = - G \frac{Mm}{r_1 + r_2}</math>

Since <math display="inline">r_1 = a + a \epsilon</math> and <math>r_2 = a - a \epsilon</math>, where epsilon is the eccentricity of the orbit, we finally have the stated result.

==Flight path angle==
The flight path angle is the angle between the orbiting body's velocity vector (= the vector tangent to the instantaneous orbit) and the local horizontal. Under standard assumptions of the conservation of angular momentum the flight path angle <math>\phi</math> satisfies the equation:
:<math>h\, = r\, v\, \cos \phi</math>
where:
* <math>h\,</math> is the [[specific relative angular momentum]] of the orbit,
* <math>v\,</math> is the [[orbital speed]] of the orbiting body,
* <math>r\,</math> is the radial distance of the orbiting body from the [[central body]],
* <math>\phi \,</math> is the flight path angle

<math>\psi</math> is the angle between the orbital velocity vector and the semi-major axis. <math>\nu</math> is the local true anomaly. <math>\phi = \nu + \frac{\pi}{2} - \psi</math>, therefore,
:<math>\cos \phi = \sin(\psi - \nu) = \sin\psi\cos\nu - \cos\psi\sin\nu = \frac{1 + e\cos\nu}{\sqrt{1 + e^2 + 2e\cos\nu}}</math>

where <math>e</math> is the eccentricity.

The angular momentum is related to the vector cross product of position and velocity, which is proportional to the sine of the angle between these two vectors. Here <math>\phi</math> is defined as the angle which differs by 90 degrees from this, so the cosine appears in place of the sine.{{Expand section|date=June 2008}}

==Equation of motion==
{{Main article|orbit equation}}

== Orbital parameters ==
The state of an orbiting body at any given time is defined by the orbiting body's position and velocity with respect to the central body, which can be represented by the three-dimensional [[Cartesian coordinates]] (position of the orbiting body represented by x, y, and z) and the similar Cartesian components of the orbiting body's velocity. This set of six variables, together with time, are called the [[orbital state vectors]]. Given the masses of the two bodies they determine the full orbit. The two most general cases with these 6 degrees of freedom are the elliptic and the hyperbolic orbit. Special cases with fewer degrees of freedom are the circular and parabolic orbit.

Because at least six variables are absolutely required to completely represent an elliptic orbit with this set of parameters, then six variables are required to represent an orbit with any set of parameters. Another set of six parameters that are commonly used are the [[orbital elements]].

==Solar System==

In the [[Solar System]], [[planet]]s, [[asteroid]]s, most [[comet]]s and some pieces of [[space debris]] have approximately elliptical orbits around the Sun.  Strictly speaking, both bodies revolve around the same focus of the ellipse, the one closer to the more massive body, but when one body is significantly more massive, such as the sun in relation to the earth, the focus may be contained within the larger massing body, and thus the smaller is said to revolve around it. The following chart of the [[apsis|perihelion and aphelion]] of the [[planet]]s, [[dwarf planet]]s and [[Halley's Comet]] demonstrates the variation of the eccentricity of their elliptical orbits. For similar distances from the sun, wider bars denote greater eccentricity. Note the almost-zero eccentricity of Earth and Venus compared to the enormous eccentricity of Halley's Comet and Eris.
{{Distance from Sun using EasyTimeline}}

==Radial elliptic trajectory==
A [[radial trajectory]] can be a [[Line segment#As a degenerate ellipse|double line segment]], which is a [[Ellipse#Degenerate ellipse|degenerate ellipse]] with semi-minor axis = 0 and eccentricity = 1. Although the eccentricity is 1, this is not a parabolic orbit. Most properties and formulas of elliptic orbits apply. However, the orbit cannot be closed. It is an open orbit corresponding to the part of the degenerate ellipse from the moment the bodies touch each other and move away from each other until they touch each other again. In the case of point masses one full orbit is possible, starting and ending with a singularity. The velocities at the start and end are infinite in opposite directions and the potential energy is equal to minus infinity.

The radial elliptic trajectory is the solution of a two-body problem with at some instant zero speed, as in the case of [[Falling (physics)|dropping]] an object (neglecting air resistance).

{{see also|Free fall#Inverse-square law gravitational field}}

==History==
The [[Babylonian astronomy|Babylonians]] were the first to realize that the Sun's motion along the [[ecliptic]] was not uniform, though they were unaware of why this was; it is today known that this is due to the Earth moving in an elliptic orbit around the Sun, with the Earth moving faster when it is nearer to the Sun at [[perihelion]] and moving slower when it is farther away at [[aphelion]].<ref>{{citation|title=Babylon to Voyager and beyond: a history of planetary astronomy|author=David Leverington|publisher=[[Cambridge University Press]]|year=2003|isbn=0-521-80840-5|pages=6–7}}</ref>

In the 17th century, [[Johannes Kepler]] discovered that the orbits along which the planets travel around the Sun are ellipses with the Sun at one focus, and described this in his [[Kepler's laws of planetary motion|first law of planetary motion]]. Later, [[Isaac Newton]] explained this as a corollary of his [[Newton's law of universal gravitation|law of universal gravitation]].

== See also ==
* [[Apsis]]
* [[Characteristic energy]]
* [[Ellipse]]
* [[List of orbits]]
* [[Orbital eccentricity]]
* [[Orbit equation]]
* [[Parabolic trajectory]]

==References==
{{Reflist}}
*{{cite journal| last = D'Eliseo| first = MM| year = 2007| title = The first-order orbital equation| journal = American Journal of Physics| volume = 75| issue = 4| pages = 352–355| doi = 10.1119/1.2432126|bibcode = 2007AmJPh..75..352D }}
*{{cite journal| last = D'Eliseo| first = MM| year = 2009| title = The gravitational ellipse| journal = Journal of Mathematical Physics| volume = 50| pages = 022901–022901 |doi = 10.1063/1.3078419|bibcode = 2009JMP....50a2901M |arxiv = 0802.2435 | last2 = Mironov| first2 = Sergey V.}}
*{{cite book|last=Curtis|first=Howard|title=[[Orbital Mechanics for Engineering Students]]|year=2009|publisher=[[Butterworth-Heinemann]]|isbn=978-0123747785}}

==External links==
* [https://web.archive.org/web/20110719090338/https://wgpqqror.homepage.t-online.de/work.html JAVA applet animating the orbit of a satellite] in an elliptic Kepler orbit around the Earth with any value for semi-major axis and eccentricity.
* [http://www.perseus.gr/Astro-Lunar-Scenes-Apo-Perigee.htm Apogee - Perigee] Lunar photographic comparison
* [http://www.perseus.gr/Astro-Solar-Scenes-Aph-Perihelion.htm Aphelion - Perihelion] Solar photographic comparison
* http://www.castor2.ca

{{orbits|state=expanded}}

[[Category:Orbits]]