# The Naked Scientists Forum

### Author Topic: Why do planets have elliptical orbits?  (Read 13094 times)

#### MolonLabe

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##### Re: Why do planets have elliptical orbits?
« Reply #25 on: 09/10/2015 09:15:41 »
I know Pete and also know that he is knowledgeable in physics. Why would you even require him to provide such a derivation? If you think you know more then why not just post it?

I was merely commenting that he claimed to have explained why the orbits are ellipses, when in fact all he said was effectively "because Kepler said so". The OP might or might not be satisfied with that, and the tone of your post suggests that you think it is sufficient. If that is the expected level of explanation here, then I'm sorry to waste everybody's time, but I personally don't think it's satisfactory without the mathematical derivation (which I assumed would automatically be given somewhere).

As I mentioned in a post above which nobody appears to have read, the orbital motion of a planet is determined by the law of conservation of angular momentum. This is all nicely explained in the link you give above, or

http://farside.ph.utexas.edu/teaching/301/lectures/node155.htmlhere [nofollow]. (c.f. formula 582)

Because the force on the planet is always directed to one point, the centre of the sun, the angular momentum of the planet is constant. This alone determines that the orbit is a conic section, and despite PmbPhy's claim that I am ignorant of the laws of gravitation, I do know that the actual size of this force is not important in determining that. He also took me to task for suggesting that a planet could have a straight line trajectory. Whether this qualifies as an orbit or not, the fact is that in the hypothetical instance of a planet being introduced with zero angular momentum, the trajectory would be a straight line into the sun. I mentioned this for the sake of completeness to show that theoretically all conic sections are possible for orbital motion, that's all.
« Last Edit: 09/10/2015 09:19:24 by MolonLabe »

#### Thebox

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##### Re: Why do planets have elliptical orbits?
« Reply #26 on: 09/10/2015 09:24:29 »
Conic sections were considered by Thomas Harriot who corresponded with Kepler via one of Keplers associates. In the Harriot papers there is evidence that Kepler passed on work he had done on the orbits since Harriot showed the parabolic nature of the orbits of comets. Harriot also observed the moon and sunspots through a telescope and produce illustrations of both. I didn't read this on wikipedia. I researched it personally. I have a microfilm of the Harriot papers which I transferred to CD and passed to other researchers. I know Pete and also know that he is knowledgeable in physics. Why would you even require him to provide such a derivation? If you think you know more then why not just post it? We're here to help each other. It isn't a competition. You don't get a prize.

The truth be told a conic section does not exist of space, shapes of space are virtual, a body following a virtual shape is not really orbiting a shape value, the shape is just abstract in space, so the true answer would be, planets dont have elliptic orbits or any other shapes, they orbit according to gravitation path.

#### jeffreyH

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• The graviton sucks
##### Re: Why do planets have elliptical orbits?
« Reply #27 on: 10/10/2015 02:04:22 »
I know Pete and also know that he is knowledgeable in physics. Why would you even require him to provide such a derivation? If you think you know more then why not just post it?

I was merely commenting that he claimed to have explained why the orbits are ellipses, when in fact all he said was effectively "because Kepler said so". The OP might or might not be satisfied with that, and the tone of your post suggests that you think it is sufficient. If that is the expected level of explanation here, then I'm sorry to waste everybody's time, but I personally don't think it's satisfactory without the mathematical derivation (which I assumed would automatically be given somewhere).

As I mentioned in a post above which nobody appears to have read, the orbital motion of a planet is determined by the law of conservation of angular momentum. This is all nicely explained in the link you give above, or

http://farside.ph.utexas.edu/teaching/301/lectures/node155.htmlhere. (c.f. formula 582)

Because the force on the planet is always directed to one point, the centre of the sun, the angular momentum of the planet is constant. This alone determines that the orbit is a conic section, and despite PmbPhy's claim that I am ignorant of the laws of gravitation, I do know that the actual size of this force is not important in determining that. He also took me to task for suggesting that a planet could have a straight line trajectory. Whether this qualifies as an orbit or not, the fact is that in the hypothetical instance of a planet being introduced with zero angular momentum, the trajectory would be a straight line into the sun. I mentioned this for the sake of completeness to show that theoretically all conic sections are possible for orbital motion, that's all.

You have actually added some useful information, which is good. In words it often makes more sense to non mathematicians. A lot of members here are not conversant with the mathematics. An equation may be of limited use. The better approach is usually to post a link. Then people can review at leisure. I didn't mean to sound confrontational BTW.

#### trevorjohnson32

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##### Re: Why do planets have elliptical orbits?
« Reply #28 on: 04/12/2015 18:03:55 »
the gravity is stronger the closer the orbiting object is to the sun, this causes the sun to whip it around harder and throw it far away making the sides of the orbit longer then the turns. For near circular orbits this effect is smaller. The stronger gravity accelerates the planet when its closer to the star, then it balances when the faster moving planet pushes out to where gravity is weaker at which time it slows down and is drawn in closer again repeating the process. The balance of these two motions causes the elliptical orbit shape.

#### The Naked Scientists Forum

##### Re: Why do planets have elliptical orbits?
« Reply #28 on: 04/12/2015 18:03:55 »