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In astronomy, **Kepler's laws of planetary motion**, published by Johannes Kepler between 1609 and 1619, describe the orbits of planets around the Sun. The laws modified the heliocentric theory of Nicolaus Copernicus, replacing its circular orbits and epicycles with elliptical trajectories, and explaining how planetary velocities vary. The three laws state that:

- Comparison to Copernicus
- Nomenclature
- History
- Formulary
- First law
- Second law
- Third law
- Planetary acceleration
- Acceleration vector
- Inverse square law
- Newton's law of gravitation
- Position as a function of time
- Mean anomaly, M
- Eccentric anomaly, E
- True anomaly, θ
- Distance, r
- See also
- Notes
- References
- Bibliography
- External links

- The orbit of a planet is an ellipse with the Sun at one of the two foci.
- A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
- The square of a planet's orbital period is proportional to the cube of the length of the semi-major axis of its orbit.

The elliptical orbits of planets were indicated by calculations of the orbit of Mars. From this, Kepler inferred that other bodies in the Solar System, including those farther away from the Sun, also have elliptical orbits. The second law helps to establish that when a planet is closer to the Sun, it travels faster. The third law expresses that the farther a planet is from the Sun, the slower its orbital speed, and vice versa.

Isaac Newton showed in 1687 that relationships like Kepler's would apply in the Solar System as a consequence of his own laws of motion and law of universal gravitation.

Johannes Kepler's laws improved the model of Copernicus. If the eccentricities of the planetary orbits are taken as zero, then Kepler basically agreed with Copernicus:

- The planetary orbit is a circle with epicycles.
- The Sun is approximately at the center of the orbit.
- The speed of the planet in the main orbit is constant.

The eccentricities of the orbits of those planets known to Copernicus and Kepler are small, so the foregoing rules give fair approximations of planetary motion, but Kepler's laws fit the observations better than does the model proposed by Copernicus. Kepler's corrections are:

- The planetary orbit is
*not*a circle with epicycles, but an*ellipse*. - The Sun is
*not*near the center but at a*focal point*of the elliptical orbit. - Neither the linear speed nor the angular speed of the planet in the orbit is constant, but the
*area speed*(closely linked historically with the concept of angular momentum) is constant.

The eccentricity of the orbit of the Earth makes the time from the March equinox to the September equinox, around 186 days, unequal to the time from the September equinox to the March equinox, around 179 days. A diameter would cut the orbit into equal parts, but the plane through the Sun parallel to the equator of the Earth cuts the orbit into two parts with areas in a 186 to 179 ratio, so the eccentricity of the orbit of the Earth is approximately

which is close to the correct value (0.016710218). The accuracy of this calculation requires that the two dates chosen are along the elliptical orbit's minor axis and the midpoints of each half are along the major axis. As the two dates chosen here are equinoxes, this will be correct when perihelion, the date the Earth is closest to the Sun, falls on a solstice. The current perihelion, near January 4, is fairly close to the solstice of December 21 or 22.

It took nearly two centuries for current formulation of Kepler's work to take on its settled form. Voltaire's *Eléments de la philosophie de Newton* (*Elements of Newton's Philosophy*) of 1738 was the first publication to use the terminology of "laws".^{ [1] }^{ [2] } The * Biographical Encyclopedia of Astronomers * in its article on Kepler (p. 620) states that the terminology of scientific laws for these discoveries was current at least from the time of Joseph de Lalande.^{ [3] } It was the exposition of Robert Small, in *An account of the astronomical discoveries of Kepler* (1814) that made up the set of three laws, by adding in the third.^{ [4] } Small also claimed, against the history, that these were empirical laws, based on inductive reasoning.^{ [2] }^{ [5] }

Further, the current usage of "Kepler's Second Law" is something of a misnomer. Kepler had two versions, related in a qualitative sense: the "distance law" and the "area law". The "area law" is what became the Second Law in the set of three; but Kepler did himself not privilege it in that way.^{ [6] }

Kepler published his first two laws about planetary motion in 1609,^{ [7] } having found them by analyzing the astronomical observations of Tycho Brahe.^{ [8] }^{ [9] }^{ [10] } Kepler's third law was published in 1619.^{ [11] }^{ [9] } Kepler had believed in the Copernican model of the Solar System, which called for circular orbits, but he could not reconcile Brahe's highly precise observations with a circular fit to Mars' orbit – Mars coincidentally having the highest eccentricity of all planets except Mercury.^{ [12] } His first law reflected this discovery.

In 1621, Kepler noted that his third law applies to the four brightest moons of Jupiter.^{ [Nb 1] } Godefroy Wendelin also made this observation in 1643.^{ [Nb 2] } The second law, in the "area law" form, was contested by Nicolaus Mercator in a book from 1664, but by 1670 his * Philosophical Transactions * were in its favour.^{ [13] }^{ [14] } As the century proceeded it became more widely accepted.^{ [15] } The reception in Germany changed noticeably between 1688, the year in which Newton's * Principia * was published and was taken to be basically Copernican, and 1690, by which time work of Gottfried Leibniz on Kepler had been published.^{ [16] }

Newton was credited with understanding that the second law is not special to the inverse square law of gravitation, being a consequence just of the radial nature of that law; while the other laws do depend on the inverse square form of the attraction. Carl Runge and Wilhelm Lenz much later identified a symmetry principle in the phase space of planetary motion (the orthogonal group O(4) acting) which accounts for the first and third laws in the case of Newtonian gravitation, as conservation of angular momentum does via rotational symmetry for the second law.^{ [17] }

The mathematical model of the kinematics of a planet subject to the laws allows a large range of further calculations.

The orbit of every planet is an ellipse with the Sun at one of the two foci.

Mathematically, an ellipse can be represented by the formula:

where is the semi-latus rectum, *ε* is the eccentricity of the ellipse, *r* is the distance from the Sun to the planet, and *θ* is the angle to the planet's current position from its closest approach, as seen from the Sun. So (*r*, *θ*) are polar coordinates.

For an ellipse 0 < *ε* < 1 ; in the limiting case *ε* = 0, the orbit is a circle with the Sun at the centre (i.e. where there is zero eccentricity).

At *θ* = 0°, perihelion, the distance is minimum

At *θ* = 90° and at *θ* = 270° the distance is equal to .

At *θ* = 180°, aphelion, the distance is maximum (by definition, aphelion is – invariably – perihelion plus 180°)

The semi-major axis *a* is the arithmetic mean between *r*_{min} and *r*_{max}:

The semi-minor axis *b* is the geometric mean between *r*_{min} and *r*_{max}:

The semi-latus rectum *p* is the harmonic mean between *r*_{min} and *r*_{max}:

The eccentricity *ε* is the coefficient of variation between *r*_{min} and *r*_{max}:

The area of the ellipse is

The special case of a circle is *ε* = 0, resulting in *r* = *p* = *r*_{min} = *r*_{max} = *a* = *b* and *A* = *πr*^{2}.

A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.

^{ [18] }

The orbital radius and angular velocity of the planet in the elliptical orbit will vary. This is shown in the animation: the planet travels faster when closer to the Sun, then slower when farther from the Sun. Kepler's second law states that the blue sector has constant area.

In a small time the planet sweeps out a small triangle having base line and height and area , so the constant areal velocity is

The area enclosed by the elliptical orbit is So the period satisfies

and the mean motion of the planet around the Sun

satisfies

The ratio of the square of an object's orbital period with the cube of the semi-major axis of its orbit is the same for all objects orbiting the same primary.

This captures the relationship between the distance of planets from the Sun, and their orbital periods.

Kepler enunciated in 1619^{ [11] } this third law in a laborious attempt to determine what he viewed as the "music of the spheres" according to precise laws, and express it in terms of musical notation.^{ [19] } It was therefore known as the *harmonic law*.^{ [20] }

Using Newton's law of gravitation (published 1687), this relation can be found in the case of a circular orbit by setting the centripetal force equal to the gravitational force:

Then, expressing the angular velocity in terms of the orbital period and then rearranging, we find Kepler's Third Law:

A more detailed derivation can be done with general elliptical orbits, instead of circles, as well as orbiting the center of mass, instead of just the large mass. This results in replacing a circular radius, , with the semi-major axis, , of the elliptical relative motion of one mass relative to the other, as well as replacing the large mass with . However, with planet masses being so much smaller than the Sun, this correction is often ignored. The full corresponding formula is:

where is the mass of the Sun, is the mass of the planet, is the gravitational constant, is the orbital period and is the elliptical semi-major axis, and is the Astronomical Unit, the average distance from earth to the sun.

The following table shows the data used by Kepler to empirically derive his law:

Planet | Mean distance to sun (AU) | Period (days) | (10^{-6} AU^{3}/day^{2}) |
---|---|---|---|

Mercury | 0.389 | 87.77 | 7.64 |

Venus | 0.724 | 224.70 | 7.52 |

Earth | 1 | 365.25 | 7.50 |

Mars | 1.524 | 686.95 | 7.50 |

Jupiter | 5.20 | 4332.62 | 7.49 |

Saturn | 9.510 | 10759.2 | 7.43 |

Upon finding this pattern Kepler wrote:^{ [21] }

I first believed I was dreaming... But it is absolutely certain and exact that the ratio which exists between the period times of any two planets is precisely the ratio of the 3/2th power of the mean distance.

— translated fromHarmonies of the Worldby Kepler (1619)

For comparison, here are modern estimates:

Planet | Semi-major axis (AU) | Period (days) | (10^{-6} AU^{3}/day^{2}) |
---|---|---|---|

Mercury | 0.38710 | 87.9693 | 7.496 |

Venus | 0.72333 | 224.7008 | 7.496 |

Earth | 1 | 365.2564 | 7.496 |

Mars | 1.52366 | 686.9796 | 7.495 |

Jupiter | 5.20336 | 4332.8201 | 7.504 |

Saturn | 9.53707 | 10775.599 | 7.498 |

Uranus | 19.1913 | 30687.153 | 7.506 |

Neptune | 30.0690 | 60190.03 | 7.504 |

Isaac Newton computed in his * Philosophiæ Naturalis Principia Mathematica * the acceleration of a planet moving according to Kepler's first and second law.

- The
*direction*of the acceleration is towards the Sun. - The
*magnitude*of the acceleration is inversely proportional to the square of the planet's distance from the Sun (the*inverse square law*).

This implies that the Sun may be the physical cause of the acceleration of planets. However, Newton states in his *Principia* that he considers forces from a mathematical point of view, not a physical, thereby taking an instrumentalist view.^{ [22] } Moreover, he does not assign a cause to gravity.^{ [23] }

Newton defined the force acting on a planet to be the product of its mass and the acceleration (see Newton's laws of motion). So:

- Every planet is attracted towards the Sun.
- The force acting on a planet is directly proportional to the mass of the planet and is inversely proportional to the square of its distance from the Sun.

The Sun plays an unsymmetrical part, which is unjustified. So he assumed, in Newton's law of universal gravitation:

- All bodies in the Solar System attract one another.
- The force between two bodies is in direct proportion to the product of their masses and in inverse proportion to the square of the distance between them.

As the planets have small masses compared to that of the Sun, the orbits conform approximately to Kepler's laws. Newton's model improves upon Kepler's model, and fits actual observations more accurately. (See two-body problem.)

Below comes the detailed calculation of the acceleration of a planet moving according to Kepler's first and second laws.

From the heliocentric point of view consider the vector to the planet where is the distance to the planet and is a unit vector pointing towards the planet.

where is the unit vector whose direction is 90 degrees counterclockwise of , and is the polar angle, and where a dot on top of the variable signifies differentiation with respect to time.

Differentiate the position vector twice to obtain the velocity vector and the acceleration vector:

So

where the **radial acceleration** is

and the **transversal acceleration** is

Kepler's second law says that

is constant.

The transversal acceleration is zero:

So the acceleration of a planet obeying Kepler's second law is directed towards the Sun.

The radial acceleration is

Kepler's first law states that the orbit is described by the equation:

Differentiating with respect to time

or

Differentiating once more

The radial acceleration satisfies

Substituting the equation of the ellipse gives

The relation gives the simple final result

This means that the acceleration vector of any planet obeying Kepler's first and second law satisfies the **inverse square law**

where

is a constant, and is the unit vector pointing from the Sun towards the planet, and is the distance between the planet and the Sun.

Since mean motion where is the period, according to Kepler's third law, has the same value for all the planets. So the inverse square law for planetary accelerations applies throughout the entire Solar System.

The inverse square law is a differential equation. The solutions to this differential equation include the Keplerian motions, as shown, but they also include motions where the orbit is a hyperbola or parabola or a straight line. (See Kepler orbit.)

By Newton's second law, the gravitational force that acts on the planet is:

where is the mass of the planet and has the same value for all planets in the Solar System. According to Newton's third law, the Sun is attracted to the planet by a force of the same magnitude. Since the force is proportional to the mass of the planet, under the symmetric consideration, it should also be proportional to the mass of the Sun, . So

where is the gravitational constant.

The acceleration of solar system body number *i* is, according to Newton's laws:

where is the mass of body *j*, is the distance between body *i* and body *j*, is the unit vector from body *i* towards body *j*, and the vector summation is over all bodies in the Solar System, besides *i* itself.

In the special case where there are only two bodies in the Solar System, Earth and Sun, the acceleration becomes

which is the acceleration of the Kepler motion. So this Earth moves around the Sun according to Kepler's laws.

If the two bodies in the Solar System are Moon and Earth the acceleration of the Moon becomes

So in this approximation, the Moon moves around the Earth according to Kepler's laws.

In the three-body case the accelerations are

These accelerations are not those of Kepler orbits, and the three-body problem is complicated. But Keplerian approximation is the basis for perturbation calculations. (See Lunar theory.)

Kepler used his two first laws to compute the position of a planet as a function of time. His method involves the solution of a transcendental equation called Kepler's equation.

The procedure for calculating the heliocentric polar coordinates (*r*,*θ*) of a planet as a function of the time *t* since perihelion, is the following five steps:

- Compute the
**mean motion***n*= (2*π*radians)/*P*, where*P*is the period. - Compute the
**mean anomaly***M*=*nt*, where*t*is the time since perilhelion. - Compute the
**eccentric anomaly***E*by solving Kepler's equation:where is the eccentricity. - Compute the
**true anomaly***θ*by solving the equation: - Compute the
**heliocentric distance***r*:where is the semimajor axis.

The Cartesian velocity vector can then be calculated as , where is the standard gravitational parameter.^{ [24] }

The important special case of circular orbit, *ε* = 0, gives *θ* = *E* = *M*. Because the uniform circular motion was considered to be *normal*, a deviation from this motion was considered an **anomaly**.

The proof of this procedure is shown below.

The Keplerian problem assumes an elliptical orbit and the four points:

*s*the Sun (at one focus of ellipse);*z*the perihelion*c*the center of the ellipse*p*the planet

and

- distance between center and perihelion, the
**semimajor axis**, - the
**eccentricity**, - the
**semiminor axis**, - the distance between Sun and planet.
- the direction to the planet as seen from the Sun, the
**true anomaly**.

The problem is to compute the polar coordinates (*r*,*θ*) of the planet from the **time since perihelion**, *t*.

It is solved in steps. Kepler considered the circle with the major axis as a diameter, and

- the projection of the planet to the auxiliary circle
- the point on the circle such that the sector areas |
*zcy*| and |*zsx*| are equal, - the
**mean anomaly**.

The sector areas are related by

The circular sector area

The area swept since perihelion,

is by Kepler's second law proportional to time since perihelion. So the mean anomaly, *M*, is proportional to time since perihelion, *t*.

where *n* is the mean motion.

When the mean anomaly *M* is computed, the goal is to compute the true anomaly *θ*. The function *θ* = *f*(*M*) is, however, not elementary.^{ [25] } Kepler's solution is to use

- ,
*x*as seen from the centre, the**eccentric anomaly**

as an intermediate variable, and first compute *E* as a function of *M* by solving Kepler's equation below, and then compute the true anomaly *θ* from the eccentric anomaly *E*. Here are the details.

Division by *a*^{2}/2 gives ** Kepler's equation **

This equation gives *M* as a function of *E*. Determining *E* for a given *M* is the inverse problem. Iterative numerical algorithms are commonly used.

Having computed the eccentric anomaly *E*, the next step is to calculate the true anomaly *θ*.

But note: Cartesian position coordinates with reference to the center of ellipse are (*a* cos *E*, *b* sin *E*)

With reference to the Sun (with coordinates (*c*,0) = (*ae*,0) ), *r* = (*a* cos *E* – *ae*, *b* sin *E*)

True anomaly would be arctan(*r*_{y/r}*x*), magnitude of *r* would be √*r* · *r*.

Note from the figure that

so that

Dividing by and inserting from Kepler's first law

to get

The result is a usable relationship between the eccentric anomaly *E* and the true anomaly *θ*.

A computationally more convenient form follows by substituting into the trigonometric identity:

Get

Multiplying by 1 + *ε* gives the result

This is the third step in the connection between time and position in the orbit.

The fourth step is to compute the heliocentric distance *r* from the true anomaly *θ* by Kepler's first law:

Using the relation above between *θ* and *E* the final equation for the distance *r* is:

- Circular motion
- Free-fall time
- Gravity
- Kepler orbit
- Kepler problem
- Kepler's equation
- Laplace–Runge–Lenz vector
- Specific relative angular momentum, relatively easy derivation of Kepler's laws starting with conservation of angular momentum

- ↑ In 1621, Johannes Kepler noted that Jupiter's moons obey (approximately) his third law in his
*Epitome Astronomiae Copernicanae*[Epitome of Copernican Astronomy] (Linz ("Lentiis ad Danubium"), (Austria): Johann Planck, 1622), book 4, part 2, pages 554–555. From pp. 554–555:*" … plane ut est cum sex planet circa Solem, … prodit Marius in suo mundo Ioviali ista 3.5.8.13 (vel 14. Galilæo) … Periodica vero tempora prodit idem Marius … sunt maiora simplis, minora vero duplis."*(… just as it is clearly [true] among the six planets around the Sun, so also it is among the four [moons] of Jupiter, because around the body of Jupiter any [satellite] that can go farther from it, orbits slower, and even that [orbit's period] is not in the same proportion, but greater [than the distance from Jupiter]; that is, 3/2 (*sescupla*) of the proportion of each of the distances from Jupiter, which is clearly the very [proportion] as is used for the six planets above. In his [book]*The World of Jupiter*[*Mundus Jovialis*, 1614], [Simon Mayr or] "Marius" [1573–1624] presents these distances, from Jupiter, of the four [moons] of Jupiter: 3, 5, 8, 13 (or 14 [according to] Galileo) [Note: The distances of Jupiter's moons from Jupiter are expressed as multiples of Jupiter's diameter.] … Mayr presents their time periods: 1 day 18 1/2 hours, 3 days 13 1/3 hours, 7 days 2 hours, 16 days 18 hours: for all [of these data] the proportion is greater than double, thus greater than [the proportion] of the distances 3, 5, 8, 13 or 14, although less than [the proportion] of the squares, which double the proportions of the distances, namely 9, 25, 64, 169 or 196, just as [a power of] 3/2 is also greater than 1 but less than 2.) - ↑ Godefroy Wendelin wrote a letter to Giovanni Battista Riccioli about the relationship between the distances of the Jovian moons from Jupiter and the periods of their orbits, showing that the periods and distances conformed to Kepler's third law. See: Joanne Baptista Riccioli,
*Almagestum novum*... (Bologna (Bononia), (Italy): Victor Benati, 1651), volume 1, page 492 Scholia III. In the margin beside the relevant paragraph is printed:*Vendelini ingeniosa speculatio circa motus & intervalla satellitum Jovis*. (Wendelin's clever speculation about the movement and distances of Jupiter's satellites.) From p. 492:*"III. Non minus Kepleriana ingeniosa est Vendelini … & D. 7. 164/1000. pro penextimo, & D. 16. 756/1000. pro extimo."*(No less clever [than] Kepler's is the most keen astronomer Wendelin's investigation of the proportion of the periods and distances of Jupiter's satellites, which he had communicated to me with great generosity [in] a very long and very learned letter. So, just as in [the case of] the larger planets, the planets' mean distances from the Sun are respectively in the 3/2 ratio of their periods ; so the distances of these minor planets of Jupiter from Jupiter (which are 3, 5, 8, and 14) are respectively in the 3/2 ratio of [their] periods (which are 1.769 days for the innermost [Io], 3.554 days for the next to the innermost [Europa], 7.164 days for the next to the outermost [Ganymede], and 16.756 days for the outermost [Callisto]).)

In physics, the **cross section** is a measure of the probability that a specific process will take place when some kind of radiant excitation intersects a localized phenomenon. For example, the Rutherford cross-section is a measure of probability that an alpha-particle will be deflected by a given angle during a collision with an atomic nucleus. Cross section is typically denoted *σ* (sigma) and is expressed in units of area, more specific in barns. In a way, it can be thought of as the size of the object that the excitation must hit in order for the process to occur, but more exactly, it is a parameter of a stochastic process.

In physics, an **orbit** is the gravitationally curved trajectory of an object, such as the trajectory of a planet around a star or a natural satellite around a planet. Normally, orbit refers to a regularly repeating trajectory, although it may also refer to a non-repeating trajectory. To a close approximation, planets and satellites follow elliptic orbits, with the center of mass being orbited at a focal point of the ellipse, as described by Kepler's laws of planetary motion.

In mathematics, the **polar coordinate system** is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction. The reference point is called the *pole*, and the ray from the pole in the reference direction is the *polar axis*. The distance from the pole is called the *radial coordinate*, *radial distance* or simply *radius*, and the angle is called the *angular coordinate*, *polar angle*, or *azimuth*. Angles in polar notation are generally expressed in either degrees or radians.

In mathematics, a **spherical coordinate system** is a coordinate system for three-dimensional space where the position of a point is specified by three numbers: the *radial distance* of that point from a fixed origin, its *polar angle* measured from a fixed zenith direction, and the *azimuthal angle* of its orthogonal projection on a reference plane that passes through the origin and is orthogonal to the zenith, measured from a fixed reference direction on that plane. It can be seen as the three-dimensional version of the polar coordinate system.

**Tidal acceleration** is an effect of the tidal forces between an orbiting natural satellite and the primary planet that it orbits. The acceleration causes a gradual recession of a satellite in a prograde orbit away from the primary, and a corresponding slowdown of the primary's rotation. The process eventually leads to tidal locking, usually of the smaller body first, and later the larger body. The Earth–Moon system is the best-studied case.

**Kinematics** is a subfield of physics, developed in classical mechanics, that describes the motion of points, bodies (objects), and systems of bodies without considering the forces that cause them to move. Kinematics, as a field of study, is often referred to as the "geometry of motion" and is occasionally seen as a branch of mathematics. A kinematics problem begins by describing the geometry of the system and declaring the initial conditions of any known values of position, velocity and/or acceleration of points within the system. Then, using arguments from geometry, the position, velocity and acceleration of any unknown parts of the system can be determined. The study of how forces act on bodies falls within kinetics, not kinematics. For further details, see analytical dynamics.

**Orbital mechanics** or **astrodynamics** is the application of ballistics and celestial mechanics to the practical problems concerning the motion of rockets and other spacecraft. The motion of these objects is usually calculated from Newton's laws of motion and law of universal gravitation. Orbital mechanics is a core discipline within space-mission design and control.

**Linear elasticity** is a mathematical model of how solid objects deform and become internally stressed due to prescribed loading conditions. It is a simplification of the more general nonlinear theory of elasticity and a branch of continuum mechanics.

In classical mechanics, the **Laplace–Runge–Lenz (LRL) vector** is a vector used chiefly to describe the shape and orientation of the orbit of one astronomical body around another, such as a binary star or a planet revolving around a star. For two bodies interacting by Newtonian gravity, the LRL vector is a constant of motion, meaning that it is the same no matter where it is calculated on the orbit; equivalently, the LRL vector is said to be *conserved*. More generally, the LRL vector is conserved in all problems in which two bodies interact by a central force that varies as the inverse square of the distance between them; such problems are called Kepler problems.

In astrodynamics or celestial mechanics, an **elliptic orbit** or **elliptical orbit** is a Kepler orbit with an 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. In a wider sense, it is a Kepler's orbit with negative energy. This includes the radial elliptic orbit, with eccentricity equal to 1.

In celestial mechanics the **specific angular momentum** plays a pivotal role in the analysis of the two-body problem. One can show that it is a constant vector for a given orbit under ideal conditions. This essentially proves Kepler's second law.

In astrodynamics an **orbit equation** defines the path of orbiting body around central body relative to , without specifying position as a function of time. Under standard assumptions, a body moving under the influence of a force, directed to a central body, with a magnitude inversely proportional to the square of the distance, has an orbit that is a conic section with the central body located at one of the two foci, or *the* focus.

**Spacecraft flight dynamics** is the application of mechanical dynamics to model how the external forces acting on a space vehicle or spacecraft determine its flight path. These forces are primarily of three types: propulsive force provided by the vehicle's engines; gravitational force exerted by the Earth and other celestial bodies; and aerodynamic lift and drag.

In classical mechanics, the **Kepler problem** is a special case of the two-body problem, in which the two bodies interact by a central force *F* that varies in strength as the inverse square of the distance *r* between them. The force may be either attractive or repulsive. The problem is to find the position or speed of the two bodies over time given their masses, positions, and velocities. Using classical mechanics, the solution can be expressed as a Kepler orbit using six orbital elements.

In electrodynamics, the **Larmor formula** is used to calculate the total power radiated by a nonrelativistic point charge as it accelerates. It was first derived by J. J. Larmor in 1897, in the context of the wave theory of light.

**Spherical multipole moments** are the coefficients in a series expansion of a potential that varies inversely with the distance R to a source, *i.e.*, as 1/*R*. Examples of such potentials are the electric potential, the magnetic potential and the gravitational potential.

In celestial mechanics, a **Kepler orbit** is the motion of one body relative to another, as an ellipse, parabola, or hyperbola, which forms a two-dimensional orbital plane in three-dimensional space. A Kepler orbit can also form a straight line. It considers only the point-like gravitational attraction of two bodies, neglecting perturbations due to gravitational interactions with other objects, atmospheric drag, solar radiation pressure, a non-spherical central body, and so on. It is thus said to be a solution of a special case of the two-body 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 classical mechanics, the **Udwadia–Kalaba formulation** is a method for deriving the equations of motion of a constrained mechanical system. The method was first described by Firdaus E. Udwadia and Robert E. Kalaba in 1992. The approach is based on Gauss's principle of least constraint. The Udwadia–Kalaba method applies to both holonomic constraints and nonholonomic constraints, as long as they are linear with respect to the accelerations. The method generalizes to constraint forces that do not obey D'Alembert's principle.

In geometry, the **major axis** of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The **semi-major axis** is the longest semidiameter or one half of the major axis, and thus runs from the centre, through a focus, and to the perimeter. The **semi-minor axis** of an ellipse or hyperbola is a line segment that is at right angles with the semi-major axis and has one end at the center of the conic section. For the special case of a circle, the lengths of the semi-axes are both equal to the radius of the circle.

The **Binet equation**, derived by Jacques Philippe Marie Binet, provides the form of a central force given the shape of the orbital motion in plane polar coordinates. The equation can also be used to derive the shape of the orbit for a given force law, but this usually involves the solution to a second order nonlinear ordinary differential equation. A unique solution is impossible in the case of circular motion about the center of force.

- ↑ Voltaire,
*Eléments de la philosophie de Newton*[Elements of Newton's Philosophy] (London, England: 1738). See, for example:- From p. 162:
*"Par une des grandes loix de Kepler, toute Planete décrit des aires égales en temp égaux : par une autre loi non-moins sûre, chaque Planete fait sa révolution autour du Soleil en telle sort, que si, sa moyenne distance au Soleil est 10. prenez le cube de ce nombre, ce qui sera 1000., & le tems de la révolution de cette Planete autour du Soleil sera proportionné à la racine quarrée de ce nombre 1000."*(By one of the great laws of Kepler, each planet describes equal areas in equal times ; by another law no less certain, each planet makes its revolution around the sun in such a way that if its mean distance from the sun is 10, take the cube of that number, which will be 1000, and the time of the revolution of that planet around the sun will be proportional to the square root of that number 1000.) - From p. 205:
*"Il est donc prouvé par la loi de Kepler & par celle de Neuton, que chaque Planete gravite vers le Soleil, ... "*(It is thus proved by the law of Kepler and by that of Newton, that each planet revolves around the sun … )

- From p. 162:
- 1 2 Wilson, Curtis (May 1994). "Kepler's Laws, So-Called" (PDF).
*HAD News*(31): 1–2. Retrieved December 27, 2016. - ↑ De la Lande,
*Astronomie*, vol. 1 (Paris, France: Desaint & Saillant, 1764). See, for example:- From page 390:
*" … mais suivant la fameuse loi de Kepler, qui sera expliquée dans le Livre suivant (892), le rapport des temps périodiques est toujours plus grand que celui des distances, une planete cinq fois plus éloignée du soleil, emploie à faire sa révolution douze fois plus de temps ou environ; … "*( … but according to the famous law of Kepler, which will be explained in the following book [i.e., chapter] (paragraph 892), the ratio of the periods is always greater than that of the distances [so that, for example,] a planet five times farther from the sun, requires about twelve times or so more time to make its revolution [around the sun]; … ) - From page 429:
*"Les Quarrés des Temps périodiques sont comme les Cubes des Distances. 892. La plus fameuse loi du mouvement des planetes découverte par Kepler, est celle du repport qu'il y a entre les grandeurs de leurs orbites, & le temps qu'elles emploient à les parcourir; … "*(The squares of the periods are as the cubes of the distances. 892. The most famous law of the movement of the planets discovered by Kepler is that of the relation between the sizes of their orbits and the times that the [planets] require to traverse them; … ) - From page 430:
*"Les Aires sont proportionnelles au Temps. 895. Cette loi générale du mouvement des planetes devenue si importante dans l'Astronomie, sçavior, que les aires sont proportionnelles au temps, est encore une des découvertes de Kepler; … "*(Areas are proportional to times. 895. This general law of the movement of the planets [which has] become so important in astronomy, namely, that areas are proportional to times, is one of Kepler's discoveries; … ) - From page 435:
*"On a appellé cette loi des aires proportionnelles aux temps, Loi de Kepler, aussi bien que celle de l'article 892, du nome de ce célebre Inventeur; … "*(One called this law of areas proportional to times (the law of Kepler) as well as that of paragraph 892, by the name of that celebrated inventor; … )

- From page 390:
- ↑ Robert Small,
*An account of the astronomical discoveries of Kepler*(London, England: J Mawman, 1804), pp. 298–299. - ↑ Robert Small, An account of the astronomical discoveries of Kepler (London, England: J. Mawman, 1804).
- ↑ Bruce Stephenson (1994).
*Kepler's Physical Astronomy*. Princeton University Press. p. 170. ISBN 978-0-691-03652-6. - ↑ Astronomia nova Aitiologitis, seu Physica Coelestis tradita Commentariis de Motibus stellae Martis ex observationibus G.V. Tychnonis.Prague 1609; Engl. tr. W.H. Donahue, Cambridge 1992.
- ↑ In his
*Astronomia nova*, Kepler presented only a proof that Mars' orbit is elliptical. Evidence that the other known planets' orbits are elliptical was presented only in 1621.

See: Johannes Kepler,*Astronomia nova*… (1609), p. 285. After having rejected circular and oval orbits, Kepler concluded that Mars' orbit must be elliptical. From the top of page 285:*"Ergo ellipsis est Planetæ iter; … "*(Thus, an ellipse is the planet's [i.e., Mars'] path; … ) Later on the same page:*" … ut sequenti capite patescet: ubi simul etiam demonstrabitur, nullam Planetæ relinqui figuram Orbitæ, præterquam perfecte ellipticam; … "*( … as will be revealed in the next chapter: where it will also then be proved that any figure of the planet's orbit must be relinquished, except a perfect ellipse; … ) And then:*"Caput LIX. Demonstratio, quod orbita Martis, … , fiat perfecta ellipsis: … "*(Chapter 59. Proof that Mars' orbit, … , is a perfect ellipse: … ) The geometric proof that Mars' orbit is an ellipse appears as Protheorema XI on pages 289–290.

Kepler stated that every planet travels in elliptical orbits having the Sun at one focus in: Johannes Kepler,*Epitome Astronomiae Copernicanae*[Summary of Copernican Astronomy] (Linz ("Lentiis ad Danubium"), (Austria): Johann Planck, 1622), book 5, part 1, III. De Figura Orbitæ (III. On the figure [i.e., shape] of orbits), pages 658–665. From p. 658:*"Ellipsin fieri orbitam planetæ … "*(Of an ellipse is made a planet's orbit … ). From p. 659:*" … Sole (Foco altero huius ellipsis) … "*( … the Sun (the other focus of this ellipse) … ). - 1 2 Holton, Gerald James; Brush, Stephen G. (2001).
*Physics, the Human Adventure: From Copernicus to Einstein and Beyond*(3rd paperback ed.). Piscataway, NJ: Rutgers University Press. pp. 40–41. ISBN 978-0-8135-2908-0 . Retrieved December 27, 2009. - ↑ In his
*Astronomia nova*... (1609), Kepler did not present his second law in its modern form. He did that only in his*Epitome*of 1621. Furthermore, in 1609, he presented his second law in two different forms, which scholars call the "distance law" and the "area law".- His "distance law" is presented in:
*"Caput XXXII. Virtutem quam Planetam movet in circulum attenuari cum discessu a fonte."*(Chapter 32. The force that moves a planet circularly weakens with distance from the source.) See: Johannes Kepler,*Astronomia nova*… (1609), pp. 165–167. On page 167, Kepler states:*" … , quanto longior est αδ quam αε, tanto diutius moratur Planeta in certo aliquo arcui excentrici apud δ, quam in æquali arcu excentrici apud ε."*( … , as αδ is longer than αε, so much longer will a planet remain on a certain arc of the eccentric near δ than on an equal arc of the eccentric near ε.) That is, the farther a planet is from the Sun (at the point α), the slower it moves along its orbit, so a radius from the Sun to a planet passes through equal areas in equal times. However, as Kepler presented it, his argument is accurate only for circles, not ellipses. - His "area law" is presented in:
*"Caput LIX. Demonstratio, quod orbita Martis, … , fiat perfecta ellipsis: … "*(Chapter 59. Proof that Mars' orbit, … , is a perfect ellipse: … ), Protheorema XIV and XV, pp. 291–295. On the top p. 294, it reads:*"Arcum ellipseos, cujus moras metitur area AKN, debere terminari in LK, ut sit AM."*(The arc of the ellipse, of which the duration is delimited [i.e., measured] by the area AKM, should be terminated in LK, so that it [i.e., the arc] is AM.) In other words, the time that Mars requires to move along an arc AM of its elliptical orbit is measured by the area of the segment AMN of the ellipse (where N is the position of the Sun), which in turn is proportional to the section AKN of the circle that encircles the ellipse and that is tangent to it. Therefore, the area that is swept out by a radius from the Sun to Mars as Mars moves along an arc of its elliptical orbit is proportional to the time that Mars requires to move along that arc. Thus, a radius from the Sun to Mars sweeps out equal areas in equal times.

*Epitome Astronomiae Copernicanae*[Summary of Copernican Astronomy] (Linz ("Lentiis ad Danubium"), (Austria): Johann Planck, 1622), book 5, page 668. From page 668:*"Dictum quidem est in superioribus, divisa orbita in particulas minutissimas æquales: accrescete iis moras planetæ per eas, in proportione intervallorum inter eas & Solem."*(It has been said above that, if the orbit of the planet is divided into the smallest equal parts, the times of the planet in them increase in the ratio of the distances between them and the sun.) That is, a planet's speed along its orbit is inversely proportional to its distance from the Sun. (The remainder of the paragraph makes clear that Kepler was referring to what is now called angular velocity.) - His "distance law" is presented in:
- 1 2 Johannes Kepler,
*Harmonices Mundi*[The Harmony of the World] (Linz, (Austria): Johann Planck, 1619), book 5, chapter 3, p. 189. From the bottom of p. 189:*"Sed res est certissima exactissimaque quod*proportio qua est inter binorum quorumcunque Planetarum tempora periodica, sit præcise sesquialtera proportionis*mediarum distantiarum, … "*(But it is absolutely certain and exact that the*proportion between the periodic times of any two planets is precisely the sesquialternate proportion*[i.e., the ratio of 3:2] of their mean distances, … ")

An English translation of Kepler's*Harmonices Mundi*is available as: Johannes Kepler with E. J. Aiton, A. M. Duncan, and J. V. Field, trans.,*The Harmony of the World*(Philadelphia, Pennsylvania: American Philosophical Society, 1997); see especially p. 411. - ↑ National Earth Science Teachers Association (9 October 2008). "Data Table for Planets and Dwarf Planets".
*Windows to the Universe*. Retrieved 2 August 2018. - ↑ Mercator, Nicolaus (1664).
*Nicolai Mercatoris Hypothesis astronomica nova, et consensus eius cum observationibus*[*Nicolaus Mercator's new astronomical hypothesis, and its agreement with observations*] (in Latin). London, England: Leybourn. - ↑ Mercator, Nic. (25 March 1670). "Some considerations of Mr. Nic. Mercator, concerning the geometrick and direct method of signior Cassini for finding the apogees, excentricities, and anomalies of the planets; …".
*Philosophical Transactions of the Royal Society of London*(in Latin).**5**(57): 1168–1175. doi:10.1098/rstl.1670.0018. Mercator criticized Cassini's method of finding, from three observations, an orbit's line of apsides. Cassini had assumed (wrongly) that planets move uniformly along their elliptical orbits. From p. 1174:*"Sed cum id Observationibus nequaquam congruere animadverteret, mutavit sententiam, & lineam veri motus Planetæ æqualibus temporibus æquales areas Ellipticas verrere professus est: … "*(But when he noticed that it didn't agree at all with observations, he changed his thinking, and he declared that a line [from the Sun to a planet, denoting] a planet's true motion, sweeps out equal areas of an ellipse in equal periods of time: … [which is the "area" form of Kepler's second law]) - ↑ Wilbur Applebaum (2000).
*Encyclopedia of the Scientific Revolution: From Copernicus to Newton*. Routledge. p. 603. Bibcode:2000esrc.book.....A. ISBN 978-1-135-58255-5. - ↑ Roy Porter (1992).
*The Scientific Revolution in National Context*. Cambridge University Press. p. 102. ISBN 978-0-521-39699-8. - ↑ Victor Guillemin; Shlomo Sternberg (2006).
*Variations on a Theme by Kepler*. American Mathematical Soc. p. 5. ISBN 978-0-8218-4184-6. - ↑ Bryant, Jeff; Pavlyk, Oleksandr. "Kepler's Second Law",
*Wolfram Demonstrations Project*. Retrieved December 27, 2009. - ↑ Burtt, Edwin.
*The Metaphysical Foundations of Modern Physical Science*. p. 52. - ↑ Gerald James Holton, Stephen G. Brush (2001).
*Physics, the Human Adventure*. Rutgers University Press. p. 45. ISBN 978-0-8135-2908-0. - ↑ Caspar, Max (1993).
*Kepler*. New York: Dover. ISBN 9780486676050. - ↑ I. Newton,
*Principia*, p. 408 in the translation of I.B. Cohen and A. Whitman - ↑ I. Newton,
*Principia*, p. 943 in the translation of I.B. Cohen and A. Whitman - ↑ Schwarz, René. "Memorandum № 1: Keplerian Orbit Elements → Cartesian State Vectors" (PDF). Retrieved 4 May 2018.
- ↑ Müller, M (1995). "Equation of Time – Problem in Astronomy". Acta Physica Polonica A. Retrieved 23 February 2013.

- Kepler's life is summarized on pages 523–627 and Book Five of his
*magnum opus*,*Harmonice Mundi*(*harmonies of the world*), is reprinted on pages 635–732 of*On the Shoulders of Giants*: The Great Works of Physics and Astronomy (works by Copernicus, Kepler, Galileo, Newton, and Einstein). Stephen Hawking, ed. 2002 ISBN 0-7624-1348-4 - A derivation of Kepler's third law of planetary motion is a standard topic in engineering mechanics classes. See, for example, pages 161–164 of Meriam, J.L. (1971) [1966].
*Dynamics, 2nd ed*. New York: John Wiley. ISBN 978-0-471-59601-1.. - Murray and Dermott, Solar System Dynamics, Cambridge University Press 1999, ISBN 0-521-57597-4
- V.I. Arnold, Mathematical Methods of Classical Mechanics, Chapter 2. Springer 1989, ISBN 0-387-96890-3

Wikimedia Commons has media related to Kepler motions . |

- B.Surendranath Reddy; animation of Kepler's laws: applet
- "Derivation of Kepler's Laws" (from Newton's laws) at
*Physics Stack Exchange*. - Crowell, Benjamin, Light and Matter, an online book that gives a proof of the first law without the use of calculus (see section 15.7)
- David McNamara and Gianfranco Vidali,
*Kepler's Second Law – Java Interactive Tutorial*, https://web.archive.org/web/20060910225253/http://www.phy.syr.edu/courses/java/mc_html/kepler.html, an interactive Java applet that aids in the understanding of Kepler's Second Law. - Audio – Cain/Gay (2010) Astronomy Cast Johannes Kepler and His Laws of Planetary Motion
- University of Tennessee's Dept. Physics & Astronomy: Astronomy 161 page on Johannes Kepler: The Laws of Planetary Motion
- Equant compared to Kepler: interactive model
- Kepler's Third Law:interactive model
- Solar System Simulator (Interactive Applet)
- Kepler and His Laws, educational web pages by David P. Stern

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