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Kepler's Laws of Planetary Motion
Scientific laws describing motion of planets around the Sun
Figure 1: Illustration of Kepler's three laws with two planetary orbits.
The orbits are ellipses, with focal points F_{1} and F_{2} for the first planet and F_{1} and F_{3} for the second planet. The Sun is placed in focal point F_{1}.
The two shaded sectors A_{1} and A_{2} have the same surface area and the time for planet 1 to cover segment A_{1} is equal to the time to cover segment A_{2}.
The total orbit times for planet 1 and planet 2 have a ratio ${\textstyle \left({\frac {a_{1}}{a_{2}}}\right)^{\frac {3}{2}}}$.
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.^{[2]}
The ratio of the square of an object's orbital period with the cube of the semi-major of its orbit is the same for all objects orbiting the same primary.
The elliptical orbits of planets were indicated by calculations of the orbit of Mars.^{[3]} 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 longer its orbit, and vice versa.
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.
The Sun is at the center of the orbit.
The speed of the planet in the 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 not at all obvious:
The planetary orbit is not a circle, but an ellipse.
The Sun is not at 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) (see Earth's orbit).
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.
Nomenclature
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".^{[4]}^{[5]} 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.^{[6]} 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.^{[7]} Small also claimed, against the history, that these were empirical laws, based on inductive reasoning.^{[5]}^{[8]}
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.^{[9]}
History
Johannes Kepler published his first two laws about planetary motion in 1609 ^{[10]}
, having found them by analyzing the astronomical observations of Tycho Brahe.^{[11]}^{[1]}^{[12]} Kepler's third law was published in 1619.^{[13]}^{[1]} 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.^{[14]} His first law reflected this discovery.
Kepler in 1621 and Godefroy Wendelin in 1643 noted that Kepler's third law applies to the four brightest moons of Jupiter.^{[Nb 1]} 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. 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]}
Formulary
The mathematical model of the kinematics of a planet subject to the laws allows a large range of further calculations.
Figure 2: Kepler's first law placing the Sun at the focus of an elliptical orbit
Figure 4: Heliocentric coordinate system (r,?) for ellipse. Also shown are: semi-major axis a, semi-minor axis b and semi-latus rectum p; center of ellipse and its two foci marked by large dots. For , and for , .
Mathematically, an ellipse can be represented by the formula:
$r={\frac {p}{1+\varepsilon \,\cos \theta }},$
where $p$ is the semi-latus rectum, ? is the eccentricity of the ellipse, r is the distance from the Sun to the planet, and ? is the angle to the planet's current position from its closest approach, as seen from the Sun. So (r, ?) are polar coordinates.
For an ellipse 0 < ? < 1 ; in the limiting case ? = 0, the orbit is a circle with the Sun at the centre (i.e. where there is zero eccentricity).
The special case of a circle is ? = 0, resulting in r = p = r_{min} = r_{max} = a = b and A = ?r^{2}.
Second law
A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.^{[2]}
The same (blue) area is swept out in a fixed time period. The green arrow is velocity. The purple arrow directed towards the Sun is the acceleration. The other two purple arrows are acceleration components parallel and perpendicular to the velocity.
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 $dt\,$ the planet sweeps out a small triangle having base line $r$ and height $r\,d\theta$ and area $dA={\frac {1}{2}}\cdot r\cdot r\,d\theta$, so the constant areal velocity is ${\frac {dA}{dt}}={\frac {r^{2}}{2}}{\frac {d\theta }{dt}}.$
The area enclosed by the elliptical orbit is $\pi ab.\,$ So the period $P$ 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^{[13]} 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.^{[18]}
So it was known as the harmonic law.^{[19]}
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:
$mr\omega ^{2}=G{\frac {mM}{r^{2}}}$
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, $r$, with the semi-major axis, $a$, of the elliptical relative motion of one mass relative to the other, as well as replacing the large mass $M$ with $M+m$. However, with planet masses being so much smaller than the Sun, this correction is often ignored. The full corresponding formula is:
where $M$ is the mass of the Sun, $m$ is the mass of the planet, $G$ is the gravitational constant, $T$ is the orbital period and $a$ is the elliptical semi-major axis, and ${\text{AU}}$ 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:
"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 from Harmonies of the World by Kepler (1619)
Log-log plot of the semi-major axis (in Astronomical Units) versus the orbital period (in terrestrial years) for the eight planets of the Solar System.
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.^{[21]} Moreover, he does not assign a cause to gravity.^{[22]}
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.
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.
Acceleration vector
From the heliocentric point of view consider the vector to the planet $\mathbf {r} =r{\hat {\mathbf {r} }}$ where $r$ is the distance to the planet and ${\hat {\mathbf {r} }}$ is a unit vector pointing towards the planet.
where ${\hat {\boldsymbol {\theta }}}$ is the unit vector whose direction is 90 degrees counterclockwise of ${\hat {\mathbf {r} }}$, and $\theta$ 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:
is a constant, and ${\hat {\mathbf {r} }}$ is the unit vector pointing from the Sun towards the planet, and $r\,$ is the distance between the planet and the Sun.
Since mean motion $n={\frac {2\pi }{T}}$ where $T$ is the period, according to Kepler's third law, $\alpha$ has the same value for all the planets. So the inverse square law for planetary accelerations applies throughout the entire Solar System.
where $m_{\text{planet}}$ is the mass of the planet and $\alpha$ 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, $m_{\text{Sun}}$. So
where $m_{j}$ is the mass of body j, $r_{ij}$ is the distance between body i and body j, ${\hat {\mathbf {r} }}_{ij}$ 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
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.
Position as a function of time
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,?) of a planet as a function of the time t since perihelion, is the following four steps:
The Cartesian velocity vector can be trivially calculated as $\mathbf {v} ={\frac {\sqrt {\mu a}}{r}}\left\langle -\sin {E},{\sqrt {1-\varepsilon ^{2}}}\cos {E}\right\rangle$.^{[23]}
The important special case of circular orbit, ? = 0, gives ? = 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.
Mean anomaly, M
Figure 5: Geometric construction for Kepler's calculation of ?. The Sun (located at the focus) is labeled S and the planet P. The auxiliary circle is an aid to calculation. Line xd is perpendicular to the base and through the planet P. The shaded sectors are arranged to have equal areas by positioning of point y.
The Keplerian problem assumes an elliptical orbit and the four points:
When the mean anomaly M is computed, the goal is to compute the true anomaly ?. The function ? = f(M) is, however, not elementary.^{[24]} Kepler's solution is to use
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 ? from the eccentric anomaly E. Here are the details.
^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.) In 1621, Johannes Kepler had 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, page 554.
^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 ... )
^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; ... )
^Robert Small, An account of the astronomical discoveries of Kepler (London, England: J Mawman, 1804), pp. 298-299.
^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) ... ).
^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 quam , tanto diutius moratur Planeta in certo aliquo arcui excentrici apud ?, quam in æquali arcu excentrici apud ?." ( ... , as is longer than , so much longer will a planet remain on a certain arc of the eccentric near ? than on an equal arc of the eccentric near ?.) That is, the farther a planet is from the Sun (at the point ?), 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.
In 1621, Kepler restated his second law for any planet: Johannes Kepler, 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.)
^ ^{a}^{b}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.
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 ISBN0-7624-1348-4
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. ISBN978-0-471-59601-1..
Murray and Dermott, Solar System Dynamics, Cambridge University Press 1999, ISBN0-521-57597-4