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Radius

^{3}/Period

^{2}? How did he derive this equation?

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- Thread starter PiRsq
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In summary, Kepler came about to conclude that his third law is Radius3/Period2. He derived this equation by combining the centrifugal force law with Kepler's third law.

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Radius

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Kepler spent most of his life trying to derive his relationship between the period of orbit and the radius of orbit along with the rest of his laws. He never was able to and was very confounded and frustrated with it. Once Newton came along, he was able to derive Kepler's laws, which was one of the biggest tests of Newton's Laws, giving him a lot of credibility.

Cheers

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excercise for the viewer. Can you do what they did?

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So does R^{3} have any relationship with the fact that the planets are orbiting in 3 dimension?

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I can't explain how Kepler did it first, but I can attempt to go through a derivation. Hopefully it will be clear: I don't have the skill with the new latex tools to do graphics yet.

Given:

[tex] \mu = G*M[/tex] where G is the gravitational constant and M is the mass of the attracting body.

p is the semilatus rectum, the distance from the attracting focus to the ellipse perpendicular to the direction to periapsis, and is equal to [tex]a(1-e^2)[/tex]

a is the semimajor axis,

b is the semiminor axis and is equal to [tex]\sqrt{ap}[/tex]

e is the eccentricity,

P is the period,

[tex]\vec{r}[/tex] is the position vector from the attracting body

[tex]\vec{v}[/tex] is the velocity vector of the satellite

[tex] \nu [/tex] is the true anomaly, the angle from the closest point (periapsis) to the position vector

[tex]\vec{h}[/tex] is the angular momentum, is constant for the orbit, and is equal to:[tex] \vec{r} \times \vec{v} [/tex] and [tex] \sqrt{\mu p} [/tex]

The first steps aren't going to make sense without a picture. They take the angular momentum vector, and geometrically rearrange the terms getting:

[tex] h = r^2 \dot{\nu} [/tex]

or [tex] h = \frac{r^2d\nu}{dt} [/tex] (1)

Looking at the differentially small area swept out by the r vector as the satellite moves through a differentially small distance, you get

[tex]dA = \frac{1}{2}r^2d\nu[/tex] (2)

plugging (1) into (2) and rearranging gives

[tex]dt = \frac{2}{h}dA [/tex]

Integrating that over a complete revolution,

[tex]2*\pi[/tex] radians of [tex]\nu[/tex]

gives

[tex]P = \frac{2\pi a b}{h} [/tex]

where pi*a*b is the area of the ellipse.

From the geometry of the ellipse,

[tex] b = \sqrt{a^2(1-e^2)} = \sqrt{ap} [/tex]

Combining that with the definitions of h, gives:

[tex] P = 2 \pi \sqrt{\frac{a^3}{\mu}}[/tex]

rearranging once more gives:

[tex]\frac{a^3}{P^2} = \frac{\mu}{2\pi} = constant [/tex]

If you want to look through where I got all that from with pictures:

Vallado.*Fundamentals of Astrodynamics and Applications* pages 24-30

...fingers crossed that the Latex worked...

EDIT: not too bad... three edits

Given:

[tex] \mu = G*M[/tex] where G is the gravitational constant and M is the mass of the attracting body.

p is the semilatus rectum, the distance from the attracting focus to the ellipse perpendicular to the direction to periapsis, and is equal to [tex]a(1-e^2)[/tex]

a is the semimajor axis,

b is the semiminor axis and is equal to [tex]\sqrt{ap}[/tex]

e is the eccentricity,

P is the period,

[tex]\vec{r}[/tex] is the position vector from the attracting body

[tex]\vec{v}[/tex] is the velocity vector of the satellite

[tex] \nu [/tex] is the true anomaly, the angle from the closest point (periapsis) to the position vector

[tex]\vec{h}[/tex] is the angular momentum, is constant for the orbit, and is equal to:[tex] \vec{r} \times \vec{v} [/tex] and [tex] \sqrt{\mu p} [/tex]

The first steps aren't going to make sense without a picture. They take the angular momentum vector, and geometrically rearrange the terms getting:

[tex] h = r^2 \dot{\nu} [/tex]

or [tex] h = \frac{r^2d\nu}{dt} [/tex] (1)

Looking at the differentially small area swept out by the r vector as the satellite moves through a differentially small distance, you get

[tex]dA = \frac{1}{2}r^2d\nu[/tex] (2)

plugging (1) into (2) and rearranging gives

[tex]dt = \frac{2}{h}dA [/tex]

Integrating that over a complete revolution,

[tex]2*\pi[/tex] radians of [tex]\nu[/tex]

gives

[tex]P = \frac{2\pi a b}{h} [/tex]

where pi*a*b is the area of the ellipse.

From the geometry of the ellipse,

[tex] b = \sqrt{a^2(1-e^2)} = \sqrt{ap} [/tex]

Combining that with the definitions of h, gives:

[tex] P = 2 \pi \sqrt{\frac{a^3}{\mu}}[/tex]

rearranging once more gives:

[tex]\frac{a^3}{P^2} = \frac{\mu}{2\pi} = constant [/tex]

If you want to look through where I got all that from with pictures:

Vallado.

...fingers crossed that the Latex worked...

EDIT: not too bad... three edits

Last edited:

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Originally posted by PiRsq

So does R^{3}have any relationship with the fact that the planets are orbiting in 3 dimension?

No. What we are talking about is a porportional realtionship between the Period of an orbit and its radius.

To show the math:

Force of gravity is

[tex]F_{g}= \frac {GMm}{R^2}[/tex]

The centripetal force needed to hold the planet in a circular path is

[tex]F_{c} = \frac{mv^2}{R}[/tex]

For a circular orbit, these two forces are equal, so:

[tex] \frac {GMm}{R^2} = \frac{mv^2}{R}[/tex]

[tex] \frac {GMm}{R} = v^2[/tex]

Taking the squareroot of each side gives you the orbital velocity of the Planet:

[tex]\sqrt{ \frac {GMm}{R}} = v[/tex]

In one orbit the planet will travel the circumference of a circle with a radius of

[tex]C = 2\pi R[/tex]

The time or period of this orbit is equal to distance/velocity or:

[tex]P = \frac{2\pi R}{\sqrt{ \frac {GMm}{R}}}[/tex]

Rearranged:

[tex]P= 2\pi \sqrt{ \frac{R^3}{GM}}[/tex]

Now let's say that you want to compare the periods of two different orbits at different values of

[tex]\frac{P_{1}}{P_{2}}

= \frac{ 2\pi \sqrt{ \frac{R_{1}^3}{GM}}}

{ 2\pi \sqrt{ \frac{R_{2}^3}{GM}}}[/tex]

[tex]2\pi[/tex] cancels out:

[tex]\frac{P_{1}}{P_{2}}= \frac{ \sqrt{ \frac{R_{1}^3}{GM}}}{ \sqrt{ \frac{R_{2}^3}{GM}}}[/tex]

The squareroots combine:

[tex]\frac{P_{1}}{P_{2}}=\sqrt{ \frac{ \frac{R_{1}^3}{GM}}{ \frac{R_{2}^3}{GM}}}[/tex]

The [tex]GM[/tex]s drop out:

[tex]\frac{P_{1}}{P_{2}}=\sqrt{ \frac{ R_{1}^3}{ R_{2}^3}}[/tex]

Square both sides:

[tex]\left(\frac{P_{1}}{P_{2}}\right)^2={ \frac{ R_{1}^3}{ R_{2}^3}[/tex]

or

[tex]\frac{P_{1}^2}{P_{2}^2}={ \frac{ R_{1}^3}{ R_{2}^3} [/tex]

Kepler's Third law.

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Wow those are beautiful! Just beautiful how they fit into place! Thx guys for helping me understand.

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[tex]\frac{P_{1}^2}{P_{2}^2}={ \frac{ R_{1}^3}{ R_{2}^3} [/tex]

by the the equations of the ellipse described in Cartesian Coordinates?

[tex]\frac{x^2}{a^2} + { \frac{y^2}{b^2} = 1} [/tex]

By understanding Keplers 3rd law deals with 2 ellipses, one inner ellipse and one outer ellipse. Perhaps we could combine 2 ellipse equations described in Cartesian Coordinate to derive Keplers 3rd law.

The only thing is that, the ellipse equations in cartestian coordinates deal with two foci. Although, I don't know if that will even make a difference. Is there a possibility to derive Kepler's Third law from the Ellipse equations that describe the ellipse in Cartesian Coordinates? I just want to know this answer.

Ive actually started trying to do such derivation. But ofcrouse the ellipse equations that describe the ellipse in Cartesian Coordinates will have to be modified for its fit to be derived into Keplers 3rd law. Just tell me if there is a possibility to do such derivation before I waste a lot of time trying. I've found that in order to acquire time correctly among the ellipse equations is by the understanding the velocity of the elliptical orbiting object. We would have to some how bring the velocity in through Angular Momentum, [tex]\ p=mvr [/tex]

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It should be possible to derive Kepler's third law from his first two, since those two plus centripetal force are sufficient to prove the inverse square central force law, and the third law follows from inverse square and centripetal force.

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Yes, but if I can use Angular Momentum - p=mvr somehow, I thus can find the Velocity. Then by Velocity I can find the time. The only thing is, I would have to possibly add something to the Ellipse Equations, or modify the equations.

EDIT MESSAGE: 2 cartesian ellipse's have 2 focus. Perhaps I can develop the idea of only using the focus of the inner cartesian ellipse. Other then that, I would have to ofcrouse apply Angular Momentum for the 2 Elliptical bodies orbiting the Focus. I could also study the ellipse eccentricity and by this understand the Gravitational pull that causes the 2 elliptical orbiting bodies to orbit the Focus of a given mass. Then by this, I could determine the Mass of the Orbiting bodies. Therefore I could maybe develop a Cartesian version of Keplers Third Law. I could get the time of revolution with the Velocity in Angular Momentum, p=mvr. Also I could somehow determine the Mass of the Focus which is causing the 2 bodies to orbit elliptical by the gravitation pull; which I stated earlier I could possibly find the Gravitational force of the Focus opening the door to determining the Mass of the Focus. I would need to also find a Mathematical way to get the distances of the outer elliptical bodie from the Focus at any given point of the Outer Ellipse.

EDIT MESSAGE: 2 cartesian ellipse's have 2 focus. Perhaps I can develop the idea of only using the focus of the inner cartesian ellipse. Other then that, I would have to ofcrouse apply Angular Momentum for the 2 Elliptical bodies orbiting the Focus. I could also study the ellipse eccentricity and by this understand the Gravitational pull that causes the 2 elliptical orbiting bodies to orbit the Focus of a given mass. Then by this, I could determine the Mass of the Orbiting bodies. Therefore I could maybe develop a Cartesian version of Keplers Third Law. I could get the time of revolution with the Velocity in Angular Momentum, p=mvr. Also I could somehow determine the Mass of the Focus which is causing the 2 bodies to orbit elliptical by the gravitation pull; which I stated earlier I could possibly find the Gravitational force of the Focus opening the door to determining the Mass of the Focus. I would need to also find a Mathematical way to get the distances of the outer elliptical bodie from the Focus at any given point of the Outer Ellipse.

Last edited:

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Kudos to anyone who can remember Kepplers [tex]Fourth [/tex] law.

Kepler's Third Law, also known as the Law of Harmonies, states that the square of a planet's orbital period is directly proportional to the cube of its semi-major axis. In other words, the further a planet is from its star, the longer it takes to complete one orbit.

By using Kepler's Third Law and the fact that the gravitational force between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between them, one can derive the relationship between a planet's radius and its orbital period. This relationship is known as the Radius3/Period2 formula.

The Radius3/Period2 formula represents the relationship between a planet's radius and its orbital period. It states that the cube of a planet's semi-major axis (which is proportional to its radius) is equal to the square of its orbital period multiplied by a constant, known as the gravitational constant.

Kepler's Third Law is important in the study of planetary motion because it allows scientists to calculate the orbital period and distance of a planet from its star using only the mass of the star and the semi-major axis of the planet's orbit. This information is crucial in understanding the dynamics of our solar system and other planetary systems.

Johannes Kepler derived his Third Law by analyzing the observational data of his predecessor, Tycho Brahe. He noticed a pattern in the orbital periods and distances of the planets and used this data to formulate his Third Law, which became one of the three laws of planetary motion.

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