Keplerian orbit
 For further closely relevant mathematical developments see also Twobody problem, also Gravitational twobody problem, and Kepler problem.
In celestial mechanics, a Kepler orbit (or Keplerian orbit) describes the motion of an orbiting body as an ellipse, parabola, or hyperbola, which forms a twodimensional orbital plane in threedimensional space. (A Kepler orbit can also form a straight line.) It considers only the pointlike gravitational attraction of two bodies, neglecting perturbations due to gravitational interactions with other objects, atmospheric drag, solar radiation pressure, a nonspherical central body, and so on. It is thus said to be a solution of a special case of the twobody problem, known as the Kepler problem. As a theory in classical mechanics, it also does not take into account the effects of general relativity. Keplerian orbits can be parametrized into six orbital elements in various ways.
In most applications, there is a large central body, the center of mass of which is assumed to be the center of mass of the entire system. By decomposition, the orbits of two objects of similar mass can be described as Kepler orbits around their common center of mass, their barycenter.
Contents
Introduction
From ancient times until the 16th and 17th centuries, the motions of the planets were believed to follow perfectly circular geocentric paths as taught by the ancient Greek philosophers Aristotle and Ptolemy. Variations in the motions of the planets were explained by smaller circular paths overlaid on the larger path (see epicycle). As measurements of the planets became increasingly accurate, revisions to the theory were proposed. In 1543, Nicolaus Copernicus published a heliocentric model of the solar system, although he still believed that the planets traveled in perfectly circular paths centered on the sun.
Johannes Kepler
In 1601, Johannes Kepler acquired the extensive, meticulous observations of the planets made by Tycho Brahe. Kepler would spend the next five years trying to fit the observations of the planet Mars to various curves. In 1609, Kepler published the first two of his three laws of planetary motion. The first law states:
More generally, the path of an object undergoing Keplerian motion may also follow a parabola or a hyperbola, which, along with ellipses, belong to a group of curves known as conic sections. Mathematically, the distance between a central body and an orbiting body can be expressed as:
 $r(\backslash nu)\; =\; \backslash frac\{a(1e^2)\}\{1+e\backslash cos(\backslash nu)\}$
where:
 $r$ is the distance
 $a$ is the semimajor axis, which defines the size of the orbit
 $e$ is the eccentricity, which defines the shape of the orbit
 $\backslash nu$ is the true anomaly, which is the angle between the current position of the orbiting object and the location in the orbit at which it is closest to the central body (called the periapsis)
Alternately, the equation can be expressed as:
 $r(\backslash nu)\; =\; \backslash frac\{p\}\{1+e\backslash cos(\backslash nu)\}$
Where $p$ is called the semilatus rectum of the curve. This form of the equation is particularly useful when dealing with parabolic trajectories, for which the semimajor axis is infinite.
Despite developing these laws from observations, Kepler was never able to develop a theory to explain these motions.^{[1]}
Isaac Newton
Between 1665 to 1666, Isaac Newton developed several concepts related to motion, gravitation and differential calculus. However, these concepts were not published until 1687 in the Principia, in which he outlined his laws of motion and his law of universal gravitation. His second of his three laws of motion states:
The acceleration a of a body is parallel and directly proportional to the net force acting on the body, is in the direction of the net force, and is inversely proportional to the mass of the body:
 $\backslash mathbf\{F\}\; =\; m\backslash mathbf\{a\}\; =\; m\backslash frac\{d^2\backslash mathbf\{r\}\}\{dt^2\}$
Where:
 $\backslash mathbf\{F\}$ is the force vector
 $m$ is the mass of the body on which the force is acting
 $\backslash mathbf\{a\}$ is the acceleration vector, the second time derivative of the position vector $\backslash mathbf\{r\}$
Strictly speaking, this form of the equation only applies to an object of constant mass, which holds true based on the simplifying assumptions made below.
Newton's law of gravitation states:
Every point mass attracts every other point mass by a force pointing along the line intersecting both points. The force is proportional to the product of the two masses and inversely proportional to the square of the distance between the point masses:
 $F\; =\; G\; \backslash frac\{m\_1\; m\_2\}\{r^2\}$
where:
 $F$ is the magnitude of the gravitational force between the two point masses
 $G$ is the gravitational constant
 $m\_1$ is the mass of the first point mass
 $m\_2$ is the mass of the second point mass
 $r$ is the distance between the two point masses
From the laws of motion and the law of universal gravitation, Newton was able to derive Kepler's laws, demonstrating consistency between observation and theory. The laws of Kepler and Newton formed the basis of modern celestial mechanics until Albert Einstein introduced the concepts of special and general relativity in the early 20th century. For most applications, Keplerian motion approximates the motions of planets and satellites to relatively high degrees of accuracy and is used extensively in astronomy and astrodynamics.
Simplified two body problem
To solve for the motion of an object in a two body system, two simplifying assumptions can be made:
 1. The bodies are spherically symmetric and can be treated as point masses.
 2. There are no external or internal forces acting upon the bodies other than their mutual gravitation.
The shapes of large celestial bodies are close to spheres. By symmetry, the net gravitational force attracting a mass point towards a homogeneous sphere must be directed towards its centre. The shell theorem (also proven by Isaac Newton) states that the magnitude of this force is the same as if all mass was concentrated in the middle of the sphere, even if the density of the sphere varies with depth (as it does for most celestial bodies). From this immediately follows that the attraction between two homogeneous spheres is as if both had its mass concentrated to its center.
Smaller objects, like asteroids or spacecraft often have a shape strongly deviating from a sphere. But the gravitational forces produced by these irregularities are generally small compared to the gravity of the central body. The difference between an irregular shape and a perfect sphere also diminishes with distances, and most orbital distances are very large when compared with the diameter of a small orbiting body. Thus for some applications, shape irregularity can be neglected without significant impact on accuracy.
Planets rotate at varying rates and thus may take a slightly oblate shape because of the centrifugal force. With such an oblate shape, the gravitational attraction will deviate somewhat from that of a homogeneous sphere. This phenomenon is quite noticeable for artificial Earth satellites, especially those in low orbits. At larger distances the effect of this oblateness becomes negligible. Planetary motions in the Solar System can be computed with sufficient precision if they are treated as point masses.
Two point mass objects with masses $m\_1$ and $m\_2$ and position vectors $\backslash mathbf\{r\}\_1$ and $\backslash mathbf\{r\}\_2$ relative to some inertial reference frame experience gravitational forces:
 $m\_1\; \backslash ddot\{\backslash mathbf\{r\}\}\_1\; =\; \backslash frac\{G\; m\_1\; m\_2\}\{r^2\}\; \backslash mathbf\{\backslash hat\{r\}\}$
 $m\_2\; \backslash ddot\{\backslash mathbf\{r\}\}\_2\; =\; \backslash frac\{G\; m\_1\; m\_2\}\{r^2\}\; \backslash mathbf\{\backslash hat\{r\}\}$
where $\backslash mathbf\{r\}$ is the relative position vector of mass 1 with respect to mass 2, expressed as:
 $\backslash mathbf\{r\}\; =\; \backslash mathbf\{r\}\_1\; \; \backslash mathbf\{r\}\_2$
and $\backslash mathbf\{\backslash hat\{r\}\}$ is the unit vector in that direction and $r$ is the length of that vector.
Dividing by their respective masses and subtracting the second equation from the first yields the equation of motion for the acceleration of the first object with respect to the second:

$\backslash ddot\{\backslash mathbf\{r$
(})
where $\backslash mu$ is the gravitational parameter and is equal to
 $\backslash mu\; =\; G(m\_1\; +\; m\_2)$
In many applications, a third simplifying assumption can be made:
 3. When compared to the central body, the mass of the orbiting body is insignificant. Mathematically, m_{1} >> m_{2}, so μ = G (m_{1} + m_{2}) ≈ Gm_{1}.
This assumption is not necessary to solve the simplified two body problem, but it simplifies calculations, particularly with Earthorbiting satellites and planets orbiting the sun. Even Jupiter's mass is less than the sun's by a factor of 1047,^{[2]} which would constitute an error of 0.096% in the value of μ. Notable exceptions include the Earthmoon system (mass ratio of 81.3), the PlutoCharon system (mass ratio of 8.9) and binary star systems.
Under these assumptions the differential equation for the two body case can be completely solved mathematically and the resulting orbit which follows Kepler's laws of planetary motion is called a "Kepler orbit". The orbits of all planets are to high accuracy Kepler orbits around the Sun. The small deviations are due to the much weaker gravitational attractions between the planets, and in the case of Mercury, due to general relativity. The orbits of the artificial satellites around the Earth are, with a fair approximation, Kepler orbits with small perturbations due to the gravitational attraction of the sun, the moon and the oblateness of the Earth. In high accuracy applications for which the equation of motion must be integrated numerically with all gravitational and nongravitational forces (such as solar radiation pressure and atmospheric drag) being taken into account, the Kepler orbit concepts are of paramount importance and heavily used.
Orbital elements
It is worth mentioning that any Keplerian trajectory can be defined by six parameters. The motion of an object moving in threedimensional space is characterized by a position vector and a velocity vector. Each vector has three components, so the total number of values needed to define a trajectory through space is six. An orbit is generally defined by six elements (known as Keplerian elements) that can be computed from position and velocity, three of which have already been discussed. These elements are convenient in that of the six, five are unchanging for an unperturbed orbit (a stark contrast to two constantly changing vectors). The future location of an object within its orbit can be predicted and its new position and velocity can be easily obtained from the orbital elements.
Two define the size and shape of the trajectory:
 Semimajor axis ($a\backslash ,\backslash !$)
 Eccentricity ($e\backslash ,\backslash !$)
Three define the orientation of the orbital plane:
 Inclination ($i\backslash ,\backslash !$) defines the angle between the orbital plane and the reference plane.
 Longitude of the ascending node ($\backslash Omega\backslash ,\backslash !$) defines the angle between the reference direction and the upward crossing of the orbit on the reference plane (the ascending node).
 Argument of periapsis ($\backslash omega\backslash ,\backslash !$) defines the angle between the ascending node and the periapsis.
And finally:
 True anomaly ($\backslash nu$) defines the position of the orbiting body along the trajectory, measured from periapsis. Several alternate values can be used instead of true anomaly, the most common being $M$ the mean anomaly and $T$, the time since periapsis.
Because $i$, $\backslash Omega$ and $\backslash omega$ are simply angular measurements defining the orientation of the trajectory in the reference frame, they are not strictly necessary when discussing the motion of the object within the orbital plane. They have been mentioned here for completeness, but are not required for the proofs below.
Mathematical solution of the differential equation (1) above
For movement under any central force, i.e. a force parallel to r, the specific relative angular momentum $\backslash bold\{H\}\; =\; \backslash bold\{r\}\; \backslash times\; \{\backslash dot\{\backslash bold\{r\}\}\}$ stays constant:
$\backslash dot\; \{\backslash bold\{H\}\}\; =\; \backslash frac\{d\}\{dt\}\backslash left(\backslash bold\{r\}\; \backslash times\; \{\backslash dot\{\backslash bold\{r\}\}\}\backslash right)\; =\; \backslash dot\{\backslash bold\{r\}\}\; \backslash times\; \{\backslash dot\{\backslash bold\{r\}\}\}\; +\; \backslash bold\{r\}\; \backslash times\; \{\backslash ddot\{\backslash bold\{r\}\}\}\; =\backslash bold\{0\}\; +\; \backslash bold\{0\}\; =\; \backslash bold\{0\}$
Since the cross product of the position vector and its velocity stays constant, they must lie in the same plane, orthogonal to $\backslash bold\{H\}$. This implies the vector function is a plane curve.
Because the equation has symmetry around its origin, it is easier to solve in polar coordinates. However, it is important to note that equation (1) refers to linear acceleration $\backslash left\; (\backslash ddot\{\backslash bold\{r\}\}\; \backslash right\; )$, as opposed to angular $\backslash left\; (\backslash ddot\{\backslash theta\}\; \backslash right\; )$ or radial $\backslash left\; (\backslash ddot\{r\}\; \backslash right\; )$ acceleration. Therefore, one must be cautious when transforming the equation.
Introducing a cartesian coordinate system $(\backslash hat\{\backslash bold\{x\}\}\; \backslash \; ,\; \backslash \; \backslash hat\{\backslash bold\{y\}\})$ and polar unit vectors $(\backslash hat\{\backslash bold\{r\}\}\; \backslash \; ,\; \backslash \; \backslash hat\{\backslash boldsymbol\backslash theta\})$ in the plane orthogonal to $\backslash bold\{H\}$:
$\backslash hat\{\backslash bold\{r\}\}=\backslash cos(\backslash theta)\backslash hat\{\backslash bold\{x\}\}\; +\; \backslash sin(\backslash theta)\backslash hat\{\backslash bold\{y\}\}$
$\backslash hat\{\backslash boldsymbol\backslash theta\}=\backslash sin(\backslash theta)\backslash hat\{\backslash bold\{x\}\}\; +\; \backslash cos(\backslash theta)\backslash hat\{\backslash bold\{y\}\}$
We can now rewrite the vector function $\backslash bold\{r\}$ and its derivatives as:
$\backslash bold\{r\}\; =r\; (\; \backslash cos\backslash theta\; \backslash hat\{x\}\; +\; \backslash sin\; \backslash theta\; \backslash hat\{y\})\; =\; r\backslash hat\{\backslash mathbf\{r\}\}$
$\backslash dot\{\backslash bold\{r\}\}\; =\; \backslash dot\; r\; \backslash hat\; \{\backslash mathbf\; r\}\; +\; r\; \backslash dot\; \backslash theta\; \backslash hat\; \{\backslash boldsymbol\{\backslash theta\}\}$
$\backslash ddot\{\backslash bold\{r\}\}\; =\; (\backslash ddot\; r\; \; r\backslash dot\backslash theta^2)\backslash hat\{\backslash mathbf\{r\}\}\; +\; (r\; \backslash ddot\backslash theta\; +\; 2\; \backslash dot\; r\; \backslash dot\backslash theta)\; \backslash hat\{\backslash boldsymbol\backslash theta\}$
(see "Polar coordinates#Vector calculus"). Substituting these into (1), we find:
$(\backslash ddot\; r\; \; r\backslash dot\backslash theta^2)\backslash hat\{\backslash mathbf\{r\}\}\; +\; (r\; \backslash ddot\backslash theta\; +\; 2\; \backslash dot\; r\; \backslash dot\backslash theta)\; \backslash hat\{\backslash boldsymbol\backslash theta\}\; =\; \backslash left\; (\backslash frac\{\backslash mu\}\{r^2\}\backslash right\; )\backslash hat\{\backslash mathbf\{r\}\}\; +\; (0)\backslash hat\{\backslash boldsymbol\backslash theta\}$
This gives the nonordinary polar differential equation:
$\backslash ddot\{r\}\; \; r\; \{\backslash dot\{\backslash theta$ 

(}) 
 }}
In order to solve this equation, we must first eliminate all time derivatives. We find that:
$H\; =\; \backslash bold\{r\}\; \backslash times\; \{\backslash dot\{\backslash bold\{r\}\}\}\; =\; (r\backslash cos(\backslash theta),\; r\backslash sin(\backslash theta),\; 0)\; \backslash times\; (\backslash dot\{r\}\backslash cos(\backslash theta)r\backslash sin(\backslash theta)\backslash dot\{\backslash theta\},\; \backslash dot\{r\}\backslash sin(\backslash theta)+r\backslash cos(\backslash theta)\backslash dot\{\backslash theta\},\; 0)\; =\; (0,0,r^2\backslash dot\backslash theta)\; =\; r^2\backslash dot\backslash theta$


(}) 
Taking the time derivative of (3), we get
} 

(}) 
Equations (3) and (4) allow us to eliminate the time derivatives of $\backslash theta$. In order to eliminate the time derivatives of $r$, we must use the chain rule to find appropriate substitutions:


(}) 
} 

(}) 
Using these four substitutions, all time derivatives in (2) can be eliminated, yielding an ordinary differential equation for $r$ as function of $\backslash theta\backslash ,$.
$\backslash ddot\{r\}\; \; r\; \{\backslash dot\{\backslash theta\}\}^2\; =\; \; \backslash frac\; \{\backslash mu\}\; \{r^2\}$
$\backslash frac\; \{d^2r\}\; \{d\backslash theta^2\}\; \backslash cdot\; \{\backslash dot\; \{\backslash theta\}\}^2\; +\; \backslash frac\; \{dr\}\; \{d\backslash theta\}\; \backslash cdot\; \backslash ddot\; \{\backslash theta\}\; \; r\; \{\backslash dot\{\backslash theta\}\}^2\; =\; \; \backslash frac\; \{\backslash mu\}\; \{r^2\}$
$\backslash frac\; \{d^2r\}\; \{d\backslash theta^2\}\; \backslash cdot\; \backslash left\; (\backslash frac\{H\}\{r^2\}\; \backslash right\; )^2\; +\; \backslash frac\; \{dr\}\; \{d\backslash theta\}\; \backslash cdot\; \backslash left\; (\; \backslash frac\; \{2\; \backslash cdot\; H\; \backslash cdot\; \backslash dot\{r\}\}\; \{r^3\}\; \backslash right\; )\; \; r\backslash left\; (\backslash frac\{H\}\{r^2\}\; \backslash right\; )^2\; =\; \; \backslash frac\; \{\backslash mu\}\; \{r^2\}$


(}) 
The differential equation (7) can be solved analytically by the variable substitution


(}) 
Using the chain rule for differentiation one gets:


(}) 


(}) 
Using the expressions (10) and (9) for $\backslash frac\; \{d^2r\}\; \{d\backslash theta^2\}$ and $\backslash frac\; \{dr\}\; \{d\backslash theta\}$ one gets


(}) 
with the general solution


(}) 
where e and $\backslash theta\_0\backslash ,$ are constants of integration depending on the initial values for s and $\backslash frac\; \{ds\}\; \{d\backslash theta\}$.
Instead of using the constant of integration $\backslash theta\_0\backslash ,$ explicitly one introduces the convention that the unit vectors $\backslash hat\{x\}\; \backslash \; ,\; \backslash \; \backslash hat\{y\}$ defining the coordinate system in the orbital plane are selected such that $\backslash theta\_0\backslash ,$ takes the value zero and e is positive. This then means that $\backslash theta\backslash ,$ is zero at the point where $s$ is maximal and therefore $r=\; \backslash frac\; \{1\}\{s\}$ is minimal. Defining the parameter p as $\backslash frac\; \{H^2\}\{\backslash mu\}$ one has that
$r\; =\; \backslash frac\; \{1\}\{s\}\; =\; \backslash frac\; \{p\}\{1\; +\; e\; \backslash cdot\; \backslash cos\; \backslash theta\}$
Alternate derivation
Another way to solve this equation without the use of polar differential equations is as follows:
Define a unit vector $\backslash bold\{u\}$ such that $\backslash bold\{r\}\; =\; r\backslash bold\{u\}$ and $\backslash ddot\{\backslash bold\{r\}\}\; =\; \backslash frac\{\backslash mu\}\{r^2\}\backslash bold\{u\}$. It follows that
$\backslash bold\{H\}\; =\; \backslash bold\{r\}\; \backslash times\; \backslash dot\{\backslash bold\{r\}\}\; =\; r\backslash bold\{u\}\; \backslash times\; \backslash frac\{d\}\{dt\}(r\backslash bold\{u\})\; =\; r\backslash bold\{u\}\; \backslash times\; (r\backslash dot\{\backslash bold\{u\}\}+\backslash dot\{r\}\backslash bold\{u\})\; =\; r^2(\backslash bold\{u\}\; \backslash times\; \backslash dot\{\backslash bold\{u\}\})\; +\; r\backslash dot\{r\}(\backslash bold\{u\}\; \backslash times\; \backslash bold\{u\})\; =\; r^2\backslash bold\{u\}\; \backslash times\; \backslash dot\{\backslash bold\{u\}\}$
Now consider
$\backslash ddot\{\backslash bold\{r\}\}\; \backslash times\; \backslash bold\{H\}\; =\; \backslash frac\{\backslash mu\}\{r^2\}\backslash bold\{u\}\; \backslash times\; (r^2\backslash bold\{u\}\; \backslash times\; \backslash dot\{\backslash bold\{u\}\})\; =\; \backslash mu\backslash bold\{u\}\; \backslash times\; (\backslash bold\{u\}\; \backslash times\; \backslash dot\{\backslash bold\{u\}\})\; =\; \backslash mu[(\backslash bold\{u\}\backslash cdot\backslash dot\{\backslash bold\{u\}\})\backslash bold\{u\}(\backslash bold\{u\}\backslash cdot\backslash bold\{u\})\backslash dot\{\backslash bold\{u\}\}]$
(see Triple product#Vector triple product). Notice that
$\backslash bold\{u\}\backslash cdot\backslash bold\{u\}\; =\; \backslash bold\{u\}^2\; =\; 1$
$\backslash bold\{u\}\backslash cdot\backslash dot\{\backslash bold\{u\}\}\; =\; \backslash frac\{1\}\{2\}(\backslash bold\{u\}\backslash cdot\backslash dot\{\backslash bold\{u\}\}\; +\; \backslash dot\{\backslash bold\{u\}\}\backslash cdot\backslash bold\{u\})\; =\; \backslash frac\{1\}\{2\}\backslash frac\{d\}\{dt\}(\backslash bold\{u\}\backslash cdot\backslash bold\{u\})\; =\; 0$
Substituting these values into the previous equation, one gets:
$\backslash ddot\{\backslash bold\{r\}\}\backslash times\backslash bold\{H\}=\backslash mu\backslash dot\{\backslash bold\{u\}\}$
Integrating both sides:
$\backslash dot\{\backslash bold\{r\}\}\backslash times\backslash bold\{H\}=\backslash mu\backslash bold\{u\}\; +\; \backslash bold\{c\}$
Where c is a constant vector. Dotting this with r yields an interesting result:
$\backslash bold\{r\}\backslash cdot(\backslash dot\{\backslash bold\{r\}\}\backslash times\backslash bold\{H\})=\backslash bold\{r\}\backslash cdot(\backslash mu\backslash bold\{u\}\; +\; \backslash bold\{c\})\; =\; \backslash mu\backslash bold\{r\}\backslash cdot\backslash bold\{u\}\; +\; \backslash bold\{r\}\backslash cdot\backslash bold\{c\}\; =\; \backslash mu\; r(\backslash bold\{u\}\backslash cdot\backslash bold\{u\})+rc\backslash cos(\backslash theta)=r(\backslash mu\; +\; c\backslash cos(\backslash theta))$
Where $\backslash theta$ is the angle between $\backslash bar\{r\}$ and $\backslash bar\{c\}$. Solving for r:
$r\; =\; \backslash frac\{\backslash bold\{r\}\backslash cdot(\backslash dot\{\backslash bold\{r\}\}\backslash times\backslash bold\{H\})\}\{\backslash mu\; +\; c\backslash cos(\backslash theta)\}\; =\; \backslash frac\{(\backslash bold\{r\}\backslash times\backslash dot\{\backslash bold\{r\}\})\backslash cdot\backslash bold\{H\}\}\{\backslash mu\; +\; c\backslash cos(\backslash theta)\}\; =\; \backslash frac\{\backslash bold\{H\}^2\}\{\backslash mu\; +\; c\backslash cos(\backslash theta)\}$
Notice that $(r,\backslash theta)$ are effectively the polar coordinates of the vector function. Making the substitutions $p=\backslash frac\{\backslash bold\{H\}^2\}\{\backslash mu\}$ and $e=\backslash frac\{c\}\{\backslash mu\}$, we again arrive at the equation


(}) 
This is the equation in polar coordinates for a conic section with origin in a focal point. The argument $\backslash theta\backslash ,$ is called "true anomaly".
Properties of Trajectory Equation
For $e\backslash \; =\backslash \; 0\backslash ,$ this is a circle with radius p.
For $0\backslash \; <\; e\backslash \; <\backslash \; 1\backslash ,$ this is an ellipse with


(}) 
} 

(}) 
For $e\backslash \; =\backslash \; 1\backslash ,$ this is a parabola with focal length $\backslash frac\; \{p\}\{2\}$
For $e\backslash \; >\backslash \; 1\backslash ,$ this is a hyperbola with


(}) 
} 

(}) 
The following image illustrates an ellipse (red), a parabola (green) and a hyperbola (blue)
The point on the horizontal line going out to the right from the focal point is the point with $\backslash theta\; =\; 0\backslash ,$ for which the distance to the focus takes the minimal value $\backslash frac\; \{p\}\{1\; +\; e\}$, the pericentre. For the ellipse there is also an apocentre for which the distance to the focus takes the maximal value $\backslash frac\; \{p\}\{1\; \; e\}$. For the hyperbola the range for $\backslash theta\backslash ,$ is
 $\backslash left\; [\; \backslash cos^\{1\}\backslash left(\backslash frac\{1\}\{e\}\backslash right)\; <\; \backslash theta\; <\; \backslash cos^\{1\}\backslash left(\backslash frac\{1\}\{e\}\backslash right)\backslash right\; ]$
and for a parobola the range is
 $\backslash left\; [\; \backslash pi\; <\; \backslash theta\; <\; \backslash pi\; \backslash right\; ]$
Using the chain rule for differentiation (5), the equation (2) and the definition of p as $\backslash frac\; \{H^2\}\{\backslash mu\}$ one gets that the radial velocity component is
} 

(}) 
and that the tangential component (velocity component perpendicular to $V\_r$) is
} 

(}) 
The connection between the polar argument $\backslash theta\backslash ,$ and time t is slightly different for elliptic and hyperbolic orbits.
For an elliptic orbit one switches to the "eccentric anomaly" E for which


(}) 


(}) 
and consequently


(}) 


(}) 
and the angular momentum H is


(}) 
Integrating with respect to time t one gets


(}) 
under the assumption that time $t=0$ is selected such that the integration constant is zero.
As by definition of p one has


(}) 
this can be written
} 

(}) 
For a hyperbolic orbit one uses the hyperbolic functions for the parameterisation


(}) 


(}) 
for which one has


(}) 


(}) 
and the angular momentum H is


(}) 
Integrating with respect to time t one gets


(}) 
i.e.
} 

(}) 
To find what time t that corresponds to a certain true anomaly $\backslash theta\backslash ,$ one computes corresponding parameter E connected to time with relation (27) for an elliptic and with relation (34) for a hyperbolic orbit.
Note that the relations (27) and (34) define a mapping between the ranges
 $\backslash left\; [\; \backslash infin\; <\; t\; <\; \backslash infin\backslash right\; ]\; \backslash longleftrightarrow\; \backslash left\; [\backslash infin\; <\; E\; <\; \backslash infin\; \backslash right\; ]$
Some additional formulae
See also Equation of the center – Analytical expansions
For an elliptic orbit one gets from (20) and (21) that


(}) 
and therefore that


(}) 
From (36) then follows that
 $$
\tan^2 \frac{\theta}{2} = \frac{1\cos \theta}{1+\cos \theta}= \frac{1\frac{\cos Ee}{1e \cdot \cos E}}{1+\frac{\cos Ee}{1e \cdot \cos E}}= \frac{1e \cdot \cos E  \cos E+e}{1e \cdot \cos E + \cos Ee}= \frac{1+e}{1e} \ \cdot\ \frac{1\cos E}{1+\cos E}= \frac{1+e}{1e} \ \cdot\ \tan^2 \frac{E}{2}
From the geometrical construction defining the eccentric anomaly it is clear that the vectors $(\backslash \; \backslash cos\; E\backslash \; ,\backslash \; \backslash sin\; E\backslash \; )$ and $(\backslash \; \backslash cos\; \backslash theta\backslash \; ,\backslash \; \backslash sin\; \backslash theta\backslash \; )$ are on the same side of the xaxis. From this then follows that the vectors $\backslash left(\; \backslash cos\backslash frac\{E\}\{2\}\backslash \; ,\backslash \; \backslash sin\backslash frac\{E\}\{2\}\; \backslash right)$ and $\backslash left(\; \backslash cos\backslash frac\{\backslash theta\}\{2\}\backslash \; ,\backslash \; \backslash sin\backslash frac\{\backslash theta\}\{2\}\; \backslash right)$ are in the same quadrant. One therefore has that
} 

(}) 
and that


(}) 


(}) 
where "$\backslash operatorname\{arg\}(x\backslash \; ,\backslash \; y)$" is the polar argument of the vector $(\backslash \; x\backslash \; ,\backslash \; y\backslash \; )$ and n is selected such that $\backslash left\; E\; \backslash theta\backslash right\; <\; \backslash pi$
For the numerical computation of $\backslash operatorname\{arg\}(x\backslash \; ,\backslash \; y)$ the standard function ATAN2(y,x) (or in double precision DATAN2(y,x)) available in for example the programming language FORTRAN can be used.
Note that this is a mapping between the ranges
 $\backslash left\; [\; \backslash infin\; <\; \backslash theta\; <\; \backslash infin\backslash right\; ]\; \backslash longleftrightarrow\; \backslash left\; [\backslash infin\; <\; E\; <\; \backslash infin\; \backslash right\; ]$
For an hyperbolic orbit one gets from (28) and (29) that


(}) 
and therefore that


(}) 
As
 $$
\tan^2 \frac{\theta}{2} = \frac{1\cos\theta}{1+\cos \theta}= \frac{1\frac{e\cosh E}{e \cdot \cosh E1}}{1+\frac{e\cosh E}{e \cdot \cosh E1}}= \frac{e \cdot \cosh E  e +\cosh E}{e \cdot \cosh E + e \cosh E}= \frac{e+1}{e1}\ \cdot\ \frac{\cosh E1}{\cosh E+1}= \frac{e+1}{e1}\ \cdot\ \tanh^2 \frac{E}{2} and as $\backslash tan\; \backslash frac\{\backslash theta\}\{2\}$ and $\backslash tanh\; \backslash frac\{E\}\{2\}$ have the same sign it follows that
} 

(}) 
This relation is convenient for passing between "true anomaly" and the parameter E, the latter being connected to time through relation (34). Note that this is a mapping between the ranges
 $\backslash left\; [\; \backslash cos^\{1\}\backslash left(\backslash frac\{1\}\{e\}\backslash right)\; <\; \backslash theta\; <\; \backslash cos^\{1\}\backslash left(\backslash frac\{1\}\{e\}\backslash right)\backslash right\; ]\; \backslash longleftrightarrow\; \backslash left\; [\backslash infin\; <\; E\; <\; \backslash infin\; \backslash right\; ]$
and that $\backslash frac\{E\}\{2\}$ can be computed using the relation
 $\backslash tanh\; ^\{1\}x=\backslash frac\{1\}\{2\}\backslash ln\; \backslash left(\; \backslash frac\{1+x\}\{1x\}\; \backslash right)$
From relation (27) follows that the orbital period P for an elliptic orbit is
} 

(}) 
As the potential energy corresponding to the force field of relation (1) is
 $\backslash frac\; \{\backslash mu\}\; \{r\}$
it follows from (13), (14), (18) and (19) that the sum of the kinetic and the potential energy
 $\backslash frac\}$
(})
} 

(}) 
Determination of the Kepler orbit that corresponds to a given initial state
This is the "initial value problem" for the differential equation (1) which is a first order equation for the 6dimensional "state vector" $(\backslash \; \backslash bar\{r\}\backslash \; ,\backslash bar\{v\}\backslash \; )$ when written as
$\backslash dot\; \{\backslash bar\{v$ 

(}) 
$\backslash dot\; \{\backslash bar\{r$ 

(}) 
For any values for the initial "state vector" $(\backslash \; \backslash bar\{r\_0\}\backslash \; ,\backslash bar\{v\_0\}\backslash \; )$ the Kepler orbit corresponding to the solution of this initial value problem can be found with the following algorithm:
Define the orthogonal unit vectors $(\backslash hat\{r\}\backslash \; ,\backslash \; \backslash hat\{t\})$ through


(}) 


(}) 
with $r\; >\; 0$ and $V\_t\; >\; 0$
From (13), (18) and (19) follows that by setting


(}) 
one gets a Kepler orbit that for true anomaly $\backslash theta$ has the same r, $V\_r$ and $V\_t$ values as those defined by (50) and (51).
If this Kepler orbit then also has the same $(\backslash hat\{r\}\backslash \; ,\backslash \; \backslash hat\{t\})$ vectors for this true anomaly $\backslash theta$ as the ones defined by (50) and (51) the state vector $(\backslash bar\{r\}\backslash \; ,\backslash \; \backslash bar\{v\})$ of the Kepler orbit takes the desired values $(\backslash \; \backslash bar\{r\_0\}\backslash \; ,\backslash bar\{v\_0\}\backslash \; )$ for true anomaly $\backslash theta$.
The standard inertially fixed coordinate system $(\backslash hat\{x\}\backslash \; ,\backslash \; \backslash hat\{y\})$ in the orbital plane (with $\backslash hat\{x\}$ directed from the centre of the homogeneous sphere to the pericentre) defining the orientation of the conical section (ellipse, parabola or hyperbola) can then be determined with the relation


(}) 


(}) 
Note that the relations (53) and (54) has a singularity when $V\_r=0$ and
 $V\_t=V\_0=\backslash sqrt\{\backslash frac\{\backslash mu\}\{p\}\}=\backslash sqrt\{\backslash frac\{\backslash mu\}\{\backslash frac\}$
i.e.
} 

(}) 
which is the case that it is a circular orbit that is fitting the initial state $(\backslash \; \backslash bar\{r\_0\}\backslash \; ,\backslash bar\{v\_0\}\backslash \; )$
The osculating Kepler orbit
For any state vector $(\backslash bar\{r\}\; ,\; \backslash bar\{v\}\; )$ the Kepler orbit corresponding to this state can be computed with the algorithm defined above. First the parameters $p\; ,\; e\; ,\; \backslash theta$ are determined from $r\; ,\; V\_r\; ,\; V\_t$ and then the orthogonal unit vectors in the orbital plane $\backslash hat\{x\}\; ,\; \backslash hat\{y\}$ using the relations (56) and (57).
If now the equation of motion is
$\backslash ddot\; \{\backslash bar\{r$ 

(}) 
where
 $\backslash operatorname\{\backslash bar\{F\}\}(\backslash bar\{r\},\backslash dot\; \{\backslash bar\{r\}\},t)$
is a function other than
 $\backslash mu\; \backslash cdot\; \backslash frac\; \{\backslash hat\{r\}\}\; \{r^2\}$
the resulting parameters
$p\; ,\backslash ,\; e\; ,\backslash ,\; \backslash theta,\backslash ,\; \backslash hat\{x\}\; ,\backslash ,\; \backslash hat\{y\}$
defined by $\backslash bar\{r\},\backslash dot\; \{\backslash bar\{r\}\}$ will all vary with time as opposed to the case of a Kepler orbit for which only the parameter $\backslash theta$ will vary
The Kepler orbit computed in this way having the same "state vector" as the solution to the "equation of motion" (59) at time t is said to be "osculating" at this time.
This concept is for example useful in case
 $\backslash operatorname\{\backslash bar\{F\}\}(\backslash bar\{r\},\backslash dot\; \{\backslash bar\{r\}\},t)=\backslash mu\; \backslash cdot\; \backslash frac\; \{\backslash hat\{r\}\}\; \{r^2\}+\backslash operatorname\{\backslash bar\{f\}\}(\backslash bar\{r\},\backslash dot\; \{\backslash bar\{r\}\},t)$
where
 $\backslash operatorname\{\backslash bar\{f\}\}(\backslash bar\{r\},\backslash dot\; \{\backslash bar\{r\}\},t)$
is a small "perturbing force" due to for example a faint gravitational pull from other celestial bodies. The parameters of the osculating Kepler orbit will then only slowly change and the osculating Kepler orbit is a good approximation to the real orbit for a considerable time period before and after the time of osculation.
This concept can also be useful for a rocket during powered flight as it then tells which Kepler orbit the rocket would continue in in case the thrust is switchedoff.
For a "close to circular" orbit the concept "eccentricity vector" defined as $\backslash bar\{e\}=e\; \backslash cdot\; \backslash hat\{x\}$ is useful. From (53), (54) and (56) follows that
} 

(}) 
i.e. $\backslash bar\{e\}$ is a smooth differentiable function of the state vector $(\; \backslash bar\{r\}\; ,\backslash bar\{v\}\; )$ also if this state corresponds to a circular orbit.
See also
 Kepler's laws of planetary motion
 Elliptic orbit
 Hyperbolic trajectory
 Parabolic trajectory
 Radial trajectory
 Orbit modeling
Citations
References
 El'Yasberg "Theory of flight of artificial earth satellites", Israel program for Scientific Translations (1967)
External links
 JAVA applet animating the orbit of a satellite in an elliptic Kepler orbit around the Earth with any value for semimajor axis and eccentricity.