Elliptical Orbit

[JustStuit]

Why are the planets traveling in an elliptical orbit. According to GR the sun bends space time so the planets orbit. I'm assuming the pull of other planets would not cause it. Would it be the angle it orginally came at and began being pulled by the sun and then it just continues an imperfect orbit? Also, does this orbit slowly become circular, just taking many billions of years? I was just wondering about this.

[[Tycho?]]

How the matter was moving at the star of the solar system, other planets (yes they can have a large effect), collisions... etc. But yeah, orbits are elliptical because they did not being as perfect orbits.

 

I think you are correct in saying the orbits will gradually become more circular due to tidal interactions with the object it is orbiting (unless the orbits are kept out of shape by a planet or something).

 

An interesting example is Triton, the largest moon of Neptune. It has a very close retrograde orbit around Neptune, and is under very large tidal forces. Its orbit is the most circular of any body in the solar system, its eccentricity is a zero to 16 decimal places. Which is pretty damn non-eccentric!

[h4tt3n]

']

I think you are correct in saying the orbits will gradually become more circular due to tidal interactions with the object it is orbiting (unless the orbits are kept out of shape by a planet or something).

 

An interesting example is Triton' date=' the largest moon of Neptune. It has a very close retrograde orbit around Neptune, and is under very large tidal forces. Its orbit is the most circular of any body in the solar system, its eccentricity is a zero to 16 decimal places. Which is pretty damn non-eccentric![/quote']

 

I'm quite sure the bit about orbits becoming circular is not true. Elliptic orbits are just as stable as circular ones. An elliptic orbit may be changed due to the gravitational pull from other planets, but so may circular orbits too. "Tidal interactions" between the planets will only affect their rotation, not the orbit itself: Earths pull on the moon has over time slowed it down to always turn the same side towards Earth, but it hasn't changed its orbit.

 

If Triton's orbit around Neptune has become circular due to gavitational pull, then what about Mercury? The Trition/Neptune mass ratio is about 4770, while the Sun/Mercury mass ratio is almost six millions, and the gravitiational force per mass is much much larger. Then why is Mercurys orbit still eccentric? Probably because elliptic orbits are stable...

 

I'm sure it can be shown mathematically too.

 

best regards,

Michael

[[Tycho?]]

I'm quite sure the bit about orbits becoming circular is not true. Elliptic orbits are just as stable as circular ones. An elliptic orbit may be changed due to the gravitational pull from other planets' date=' but so may circular orbits too. "Tidal interactions" between the planets will only affect their rotation, not the orbit itself: Earths pull on the moon has over time slowed it down to always turn the same side towards Earth, but it hasn't changed its orbit.

[/quote']

 

Nope

 

"...some of the Earth's rotational momentum is gradually being transferred to the Moon's orbital momentum, resulting in the Moon slowly receding from Earth at the rate of approximately 38 mm per year."

 

http://en.wikipedia.org/wiki/Moon

If Triton's orbit around Neptune has become circular due to gavitational pull, then what about Mercury? The Trition/Neptune mass ratio is about 4770, while the Sun/Mercury mass ratio is almost six millions, and the gravitiational force per mass is much much larger. Then why is Mercurys orbit still eccentric? Probably because elliptic orbits are stable...

 

I'm sure it can be shown mathematically too.

 

best regards,

Michael

 

"Another unique feature of Triton's orbit, arising from tidal effects on such a large moon so close to its primary, is that it is nearly a perfect circle with an eccentricity of zero to sixteen decimal places."

 

http://en.wikipedia.org/wiki/Triton_%28moon%29

 

The eliptical orbit itself may be stable, but the orbit alone does not account for tidal interactions, which definately do affect the orbit of bodies. I'm not definately sure on this since I can't back it up mathmatically, but it seems at least half likely.

[h4tt3n]

Nice links, it seems you are right after all. I never heard about this phenomenon before. If Triton is really a captured body, it makes a perfect circular orbit even more unlikely than if it was a "native" body like our moon - in other words this "circularizing" effect is quite big, at least in some cases.

 

It does seem to happen strictly to heavy sattelites orbiting close to their hosts, though. I wouldn't expect Halley's comet or Pluto to ever get circular orbits. And I'm still wondering why this effect hasn't straightened out Mercurys orbit... Any guesses?

 

Best regards

Michael

[[Tycho?]]

Nice links' date=' it seems you are right after all. I never heard about this phenomenon before. If Triton is really a captured body, it makes a perfect circular orbit even more unlikely than if it was a "native" body like our moon - in other words this "circularizing" effect is quite big, at least in some cases.

 

It does seem to happen strictly to heavy sattelites orbiting close to their hosts, though. I wouldn't expect Halley's comet or Pluto to ever get circular orbits. And I'm still wondering why this effect hasn't straightened out Mercurys orbit... Any guesses?

 

Best regards

Michael[/quote']

 

I actually looked up Mercury's orbit, its pretty freaking strange. Its eccentricity goes from very large, down to almost zero and then back up again. From what I could tell its because its in resonance with other planets, but I really dont see how it works.

 

I think we need someone in here with some more mathmatical knowledge, since I can't prove any of my arguments mathmatically.

[s pepperchin]

It isn't possible to write an equation for the solar sytem that takes the other bodies into account, atleast from what I've heard. Its a classic three body problem. If you place three objects in the fields of the others you can only figure out what is happening to one by treating everything else as a single object. In this case you would use the center of mass to figure out the reaction of a single body. This sounds like something that could be done with each body, but this doesn't take into full account the forces between each of them. It gives a good approximation but it isn't exact.

[Klaynos]

my cursary understanding of why eliptical orbits are allowed is as follows:

 

There is nothing actually teathering a planet to a set radius. This means when you take into account the kinetic energy of the planet you have to allow for a chanable radius. After solving the equations for the kinetic and potential energies for the planets you find that for any allowed energy the planet can do one of four things, spiral towards the sun, have a perfectly circular orbit, have an orbit that is allowed to move between two differnt radiuses, or an orbit which spirals outwards.

 

The image of the energy against radius graph can be found:

 

http://img.sparknotes.com/figures/F/ff797cbf7d96146efc9d42c55fdd303f/circ.gif

 

A very useful link which after googling for a while I found:

 

http://www.sparknotes.com/physics/gravitation/orbits/section1.html

 

After a quick glance through of what they're saying it all looks pretty good...

 

This obviousely does not take into account the fact that some planets have been hit by large bodies in the past which has altered their orbits...

 

Spiralling orbits are normally due to a decrease in the total energy of a body...

[timo]

It isn't possible to write an equation for the solar sytem that takes the other bodies into account, atleast from what I've heard.

Depends on what "an equation for the solar system" is for you. Force on body i is the sum of the gravitational forces exerted by the other bodies j : [math] m_i \vec a_i = \vec F_i = \sum_{j\neq i} \vec F_{ij} [/math]. Writing down the movement equation obviously isn´t a problem at all. The problem is not that you can´t write down the movement equations, the problem is that no one knows how to solve the movement equation for more than two bodies analytically.

But for example doing a numerical simulation of such a system is pretty straightforward and easy (just in case anyone in here likes writing physics simulations).

[[Tycho?]]

But do orbits become less eccentric over time? Triton apparently does, but do all orbiting bodies? Is it only tidal interactions and interactions with other bodies that causes it?