Author Topic: Episode #688  (Read 7339 times)

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Offline CarbShark

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Re: Episode #688
« Reply #45 on: September 19, 2018, 03:35:21 PM »
Dude, the sun already has significantly
Here's another wrinkle: the barycenter of the Sun-Jupiter system is actually outside the radius of the sun.  Should we call Jupiter something other than a planet?

If the Moon keeps receding eventually it will be far enough away at its apogee that the gravitational attraction to Earth would be less than the pull of the Sun, or Venus, or even Mars or Jupiter. If that were to happen the Moon could be captured by the Sun, another planet or could even be expelled from the solar system.

The sun has already 'captured' the moon in the sense that it and the earth both orbit the sun.  The question is whether the moon can cease to orbit the earth, which is entirely a function of its orbital distance and its velocity relative to earth, and I'm pretty sure (though it's been a long time since I read about it in detail) that the earth and moon will become doubly-tidally-locked before the moon reaches escape velocity for its orbital distance from the earth.  At that point the moon will cease to recede, and we will be stuck together.

So, no, it's not possible, barring some sort of solar-system transforming collision with another star, for the moon to stop orbiting the Earth.

Capturing the Moon would entail having a stronger pull on it directly than the earth does. This could have the effect of preventing it's return from apogee or increasing its speed significantly on approach to perigee. Either could send it on a different trajectory. If this happened the most likely trajectory would be new orbit around the sun. Other trajectories could slingshot it out of the solar system; put it into orbit around venus, say, or even crash it into the sun or the earth.

To say it's "not possible" is false, but I agree it's not likely.

That’s not how it works.  The earth and the moon are gravitationally bound.  As long as that is true, the pull of the sun acts as though it is pulling on the barycenter of the earth-moon system, and any changes in the effects of the sun are felt by the system; they cannot be felt more by the moon than by the earth—the two will accelerate or decelerate as a pair.  The sun can’t just reach out and touch only the moon.

So you're saying that in a scenario when the moon is approaching apogee (moving away from Earth) and Earth is moving away from the sun and the moon is significantly closer to the sun than the earth (because the two have been receding for x number of years) that the sun would have not have a significantly increased gravitational attraction to the moon than the earth?

I think that's exactly how it works.

Quote


Incidentally, the sun’s pull on the moon already exceeds that of the earth by more than double.

Sure but we're talking about a scenario where those relative gravitational forces change.

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  But that’s irrelevant to it pulling the two apart because *all* that matters is the relative velocities ofbthe two bodies, and there’s no way for the sun to differentially accelerate the moon as long as it’s gravitationally bound to the earth. 
As the moon continues receeding then eventually the gravitational attraction to earth becomes weaker and weaker. There are times in their orbits that the Earth is moving away from the Sun and the Moon is moving toward the Sun and is closer to the sun than the Earth is.
 


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And the recession of of the moon due to tidal forces is just not adequate for it to achieve escape velocity before tidal locking.  Barring, as I said, a colission or something else from *outside* the earth-moon-sun system that can impart new velocity to the moon, it simply cannot escape the earth.

Again, I do not think this is likely. As I said before the Moon will probably not continue to recede indefinitely. But if it did, then yes, the Moon could stop orbiting the earth.

« Last Edit: September 19, 2018, 03:49:24 PM by CarbShark »
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Online Alex Simmons

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Re: Episode #688
« Reply #46 on: September 19, 2018, 05:01:22 PM »
If the Moon keeps receding eventually it will be far enough away at its apogee that the gravitational attraction to Earth would be less than the pull of the Sun, or Venus, or even Mars or Jupiter. If that were to happen the Moon could be captured by the Sun, another planet or could even be expelled from the solar system.
No.

Let's assume that the Sun won't vapourise the Earth-Moon system during it's red giant phase in about 5 billion years or so from now.

Moon and Earth are currently receding from each other however that won't continue forever. It only happens while there is a tidal energy "swap" going on. Eventually the Earth will also become tidally locked to the Moon and the Earth and Moon will no longer be receding from each other. Indeed the earth-Moon orbit will likely begin to decay again as the Earth-Moon orbital energy is slowly leaked to the Sun via tidal force swaps. We are talking something like 50 billion years into the future. Given another 10-20 billion years and the orbital decay may mean a collision or Roche limit reached.

But there is also the possibility that before then, during the Sun's red giant phase orbital drag due to Sun's atmosphere will cause the Moon's orbit to decay more quickly, possibly even to the point of reaching the Roche limit resulting in the destruction of the Moon.

The Moon is not going to leave Earth orbit.

As for other planets, the gradual loss of mass by the Sun will see the planet's orbit expand making planetary interactions weaker, not stronger.

About the only thing to fling something out of the Solar system would be a very close encounter with another star or hot jupiter.

Offline The Latinist

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Re: Episode #688
« Reply #47 on: September 19, 2018, 05:13:32 PM »
No, Ed, you’re just making shit up.  Unless the sun can impart velocity to the moon such that it exceeds escape velocity, it simply cannot pull the moon out of earth orbit.  And it cannot do that.  Not “is very unlikely to,” but “cannot.”  You’re talking about this like it’s some sort of chaotic system that can’t be predicted when it’s actually some of the best-understood and mechanistic physics in the universe.
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Online Alex Simmons

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Re: Episode #688
« Reply #48 on: September 19, 2018, 06:17:43 PM »
Beets.  Oxalic acid.  I guess I'm the only one here who prepares beets and the beet greens.
Many years ago when I was still racing I used beet juice in a self trial for exploring its potential ergogenic effects. Red pee and poo were certainly novel experiences during the experiment.

Dietary nitrates have been researched quite a bit since then and have been shown to be good for heart health and do have a positive influence on athletic aerobic capability.

Online Alex Simmons

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Re: Episode #688
« Reply #49 on: September 19, 2018, 06:57:14 PM »
No, Ed, you’re just making shit up.  Unless the sun can impart velocity to the moon such that it exceeds escape velocity, it simply cannot pull the moon out of earth orbit.  And it cannot do that.  Not “is very unlikely to,” but “cannot.”  You’re talking about this like it’s some sort of chaotic system that can’t be predicted when it’s actually some of the best-understood and mechanistic physics in the universe.
Yup

The escape velocity from Earth's gravitational well is √2 times the Moon's actual velocity.

This ratio applies to any object orbiting another. e.g. Earth's escape velocity from the Sun is √2 times the Earth's orbital velocity.

Hence it ain't leaving since as you say there is nothing to increase the Moon's velocity (relative to Earth). Only a very close encounter from a large extra solar visitor could do that.

Offline brilligtove

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Re: Episode #688
« Reply #50 on: September 19, 2018, 07:15:56 PM »
Well, I'm sure a direct hit from a rogue planet travelling through our solar system would do it.

Oh wait. That would be more of an 'essploderationistic' interaction, now wouldn't it. :)
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Offline CarbShark

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Re: Episode #688
« Reply #51 on: September 19, 2018, 07:18:01 PM »
No, Ed, you’re just making shit up.  Unless the sun can impart velocity to the moon such that it exceeds escape velocity, it simply cannot pull the moon out of earth orbit.  And it cannot do that.  Not “is very unlikely to,” but “cannot.”  You’re talking about this like it’s some sort of chaotic system that can’t be predicted when it’s actually some of the best-understood and mechanistic physics in the universe.
Yup

The escape velocity from Earth's gravitational well is √2 times the Moon's actual velocity.

This ratio applies to any object orbiting another. e.g. Earth's escape velocity from the Sun is √2 times the Earth's orbital velocity.

Hence it ain't leaving since as you say there is nothing to increase the Moon's velocity (relative to Earth). Only a very close encounter from a large extra solar visitor could do that.

Once again, this entire discussion is based on the proposition someone else suggested (bachfiend?) that the Moon would continue to recede from the Earth.

I said that probably would not happen (and I agree that without some outside force, it will not happen).

But, if that were to happen, if the Moon were to steadily recede from it's orbit gradually increasing its distance from the Earth, then yes, the Moon could be pulled away from the Earth. Actually, if the Moon (or any satellite) steadily increased its distance from the planet it orbits it would be inevitable that it would leave its orbit.

And if the Moon ever did so, then yes it would be classified as a planet if it orbited the Sun, or if it were ejected from the solar system.

It would be considered a satellite if it were captured by Venus.

It would not be considered an asteroid.



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Online arthwollipot

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Re: Episode #688
« Reply #52 on: September 19, 2018, 09:10:46 PM »
Add to that the  orbits are elliptical and in some cases can be influenced by other satellites and planets.

If by "in some cases" you mean "always", then yes.

Well, yes, but in some cases influenced enough to dynamically change the orbits.

No - in all cases, an object's orbit is influenced by the gravitational fields of other objects in space. That's how Neptune was discovered - because its precence was altering Uranus' orbit. Technically every object in space is feeling a gravitational influence from every other object in the universe but beyond a certain distance these influences are miniscule enough to be ignored for most practical purposes.
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Offline The Latinist

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Re: Episode #688
« Reply #53 on: September 19, 2018, 10:16:10 PM »
To be constitutionally incapable of admitting when one is wrong, as you have proved yourself over and over again, is incompatible with skepticism, CarbShark. It’s okay: everyone’s wrong sometimes, and this is one of your times.  Just admit that you didn’t know what you were talking about and move on. Perhaps even grow a little humbler because of the experience.
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Offline CarbShark

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Re: Episode #688
« Reply #54 on: September 19, 2018, 11:56:56 PM »
Go back and follow the conversation from the start. Someone else suggested that the moon would keep receding  from earth until, oh no, it became a planet

I said from the start that wouldn’t happen, but if it did continue to recede then yes it would stop being a satellite and start being a planet.

You have ignored that I was responding to a hypothetical and, probably because you hold a grudge over some past discussion, you’ve ignored my repeated disclaimers (how skeptical is that?)

I fully admit when I make mistakes and have whenever appropriate. In this case given the hypothetical of the moon continuing to proceed away from earth it’s a certainty it would eventually stop orbiting earth and get captured by the sun, or another planet or ejected from the solar system. Or collide with earth or something else.

But, as I’ve said repeatedly now, that’s not going to happen.


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Offline bachfiend

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Re: Episode #688
« Reply #55 on: September 20, 2018, 01:00:43 AM »
Go back and follow the conversation from the start. Someone else suggested that the moon would keep receding  from earth until, oh no, it became a planet

I said from the start that wouldn’t happen, but if it did continue to recede then yes it would stop being a satellite and start being a planet.

You have ignored that I was responding to a hypothetical and, probably because you hold a grudge over some past discussion, you’ve ignored my repeated disclaimers (how skeptical is that?)

I fully admit when I make mistakes and have whenever appropriate. In this case given the hypothetical of the moon continuing to proceed away from earth it’s a certainty it would eventually stop orbiting earth and get captured by the sun, or another planet or ejected from the solar system. Or collide with earth or something else.

But, as I’ve said repeatedly now, that’s not going to happen.


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The Moon will continue to recede from the Earth until the Earth becomes tidally locked to the Moon, which (as has been noted) will happen in around 60 billion years, considerably longer than the life expectancy of the solar system.

Basically, the Earth produces a bulge on the Moon on the side facing it.  As does the Moon produce a bulge on the Earth on the side facing it.  The Earth currently rotates once in around 24 hours and the Moon orbits the Earth once in around 28 days.  The Moon pulls on the bulge on the Earth slowing down the Earth’s rotation, so that the length of an Earth day increases.  The Earth pulls on the bulge in the Moon causing its orbital speed to increase.  So the Moon must continue to recede, until the bulge on the Earth stops rotating faster than the orbiting Moon and remains directly beneath the Moon all the time (ie the Earth is tidally locked).

The argument isn’t whether the Moon is a moon or a planet if it’s ejected.  It’s how the Moon could be a moon now, and then suddenly become a planet in a planetary pair when it has receded sufficiently.
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Offline CarbShark

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Re: Episode #688
« Reply #56 on: September 20, 2018, 02:15:29 AM »
Go back and follow the conversation from the start. Someone else suggested that the moon would keep receding  from earth until, oh no, it became a planet

I said from the start that wouldn’t happen, but if it did continue to recede then yes it would stop being a satellite and start being a planet.

You have ignored that I was responding to a hypothetical and, probably because you hold a grudge over some past discussion, you’ve ignored my repeated disclaimers (how skeptical is that?)

I fully admit when I make mistakes and have whenever appropriate. In this case given the hypothetical of the moon continuing to proceed away from earth it’s a certainty it would eventually stop orbiting earth and get captured by the sun, or another planet or ejected from the solar system. Or collide with earth or something else.

But, as I’ve said repeatedly now, that’s not going to happen.


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The Moon will continue to recede from the Earth until the Earth becomes tidally locked to the Moon, which (as has been noted) will happen in around 60 billion years, considerably longer than the life expectancy of the solar system.

Basically, the Earth produces a bulge on the Moon on the side facing it.  As does the Moon produce a bulge on the Earth on the side facing it.  The Earth currently rotates once in around 24 hours and the Moon orbits the Earth once in around 28 days.  The Moon pulls on the bulge on the Earth slowing down the Earth’s rotation, so that the length of an Earth day increases.  The Earth pulls on the bulge in the Moon causing its orbital speed to increase.  So the Moon must continue to recede, until the bulge on the Earth stops rotating faster than the orbiting Moon and remains directly beneath the Moon all the time (ie the Earth is tidally locked).
Nice summary of the wiki page.
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The argument isn’t whether the Moon is a moon or a planet if it’s ejected.  It’s how the Moon could be a moon now, and then suddenly become a planet in a planetary pair when it has receded sufficiently.

Here’s the thing, neither Metzger nor the IAU agree with you.

By both definitions, if the Moon were orbiting the Sun and not the Earth it would be a planet. But for as long as it’s orbiting Earth it’s a satellite.

Same for a good number of other satellites in the solar system.

But, by the IAU definition, they would not be considered planets if there were asteroids and dust in their orbits, sort of.

That would change them from planets to dwarf planets.

I think that makes less sense. Particularly since the size of the object is not part of the criteria as long as it’s big enough to be a sphere.




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Offline gebobs

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Re: Episode #688
« Reply #57 on: September 20, 2018, 09:21:05 AM »
How about make the criteria for a moon be if the barycenter lies outside a certain percentage of the distance between the centers of two objects. 

Why?

What's the scientific reason and purpose for introducing this arbitrary criteria?

Fark if I know. What's the scientific reason for the criteria being that the barycenter be within the radius of one of the bodies? Is that any less arbitrary?

Offline brilligtove

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Re: Episode #688
« Reply #58 on: September 20, 2018, 10:22:02 AM »
It is less arbitrary in that it is an intrinsic property of the bodies in the system, and not a tuned multiplier of that property.

I'd go with this:

| Gravitational
| Sphere
|
| Orbits
| Star
|
| Orbits Planet
| Barycentre in
| planet
| Orbits Planet
| Barycentre in
| space
|---------------|---------------|---------------|---------------|---------------
| Asteroid: | no| yes | no | no
| Moon:    | n/a | no | yes | no
| Planet:   | yes | yes | no* | yes*

*Ignore if the body orbits the sun only and has no satellites.
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Offline CarbShark

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Re: Episode #688
« Reply #59 on: September 20, 2018, 11:29:11 AM »
It is less arbitrary in that it is an intrinsic property of the bodies in the system, and not a tuned multiplier of that property.

I'd go with this:

| Gravitational
| Sphere
|
| Orbits
| Star
|
| Orbits Planet
| Barycentre in
| planet
| Orbits Planet
| Barycentre in
| space
|---------------|---------------|---------------|---------------|---------------
| Asteroid: | no| yes | no | no
| Moon:    | n/a | no | yes | no
| Planet:   | yes | yes | no* | yes*

*Ignore if the body orbits the sun only and has no satellites.

Again, what is the scientific purpose of these criteria? What makes it better than simply establishing that if they orbit each other it's a planet/satellite?

Are you dropping the definition of Dwarf Planet?

Jupiter is a planet, right? But the barycenter between Jupiter and the Sun is in space.

If that aspect changes the definition of whether an object is a satellite or a planet shouldn't that also change the definition of whether an object is a planet or something else?

Also, why using the term "moon" rather than satellite (or natural satellite)?

By this definition Ceres would be a planet?

By this definition, Pluto/Charon would be double planets?

By this definition Earth/Moon would become double planets when the moon recedes far enough that the barycenter is in space?

But here's the thing, all this discussion about what the definition of planet should be is exactly what Metzger wants, and is exactly why he's critical of the IAU.

The criteria should be handled via science and the scientific method and a consensus should evolve as these questions are studied and answered by scientists working in the field (astronomers) and publishing in peer review journals, etc.

What the IAU has done is issued a top down, authoritative criteria and definition which some argue (in this very thread) should end this discussion and only "malcontents" or outliers on the "fringe" dispute.

That's not scientific. That's not science.

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