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Thread: The Earth/moon double planet blogs

  1. #1
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    The Earth/moon double planet blogs

    A few days ago, the BA blog concerned the IAU planet proposal. One of the BA's objections was that the moon's increasing distance from the Earth would eventually move their barycenter outside of the Earth in about 40 million years, making the moon a planet under the proposed definition.

    Today, he corrected his math, and the figure is now over 3 billion years.

    However, that is still incorrect, and not because it might be longer than the age of the sun.

    The moon is moving away from the earth because of the tidal interaction between the earth and moon, which slows the earth's rotation, and transfers its angular momentum to the moon. At a certain distance, the rotation of the earth will match the revolution of the moon, and the process will stop--in fact, it would reverse itself, because of the solar tidal effect, and the moon would start to move closer. At that distance, the earth/moon barycenter will still be within the interior of the earth. In other words, the conservation of angular momentum insures that it will never happen.

  2. #2
    without some sort of unforseen external event................

  3. #3
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    I suppose I should have added, within the context of the blog

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    The BA repeats the assertion in his blog (Breaking News– Pluto not a planet!) today:
    as I pointed out in my other post, in a billion years or so the Moon will be far enough away that the Earth-Moon center-of-mass will be outside the Earth.
    PS: I was finally able to post to the blog directley

  5. #5
    I don't understand how that argument about the Moon breaking free is valid. In some distant time in the future, the sun will swallow up the planet Mercury and Mercury will cease to exist-- does that mean Mercury isn't a planet right now? One day I will die and my bones will dry up and become dust-- does that mean I'm not a human being? Maybe I'm missing the point, but it seems really nitpicky.

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    I suppose the point is that if you move something just a little bit, that shouldn't make it necessary to redefine it.
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    Quote Originally Posted by ToSeek View Post
    I suppose the point is that if you move something just a little bit, that shouldn't make it necessary to redefine it.
    That's a valid point, but we have to be careful how it is phrased. We are talking about moving the moon out from the earth by another third. If we moved Mars in towards the sun by a third, it would no longer be a planet under the latest guidelines.

    However, my main objection to this has been the mechanism--the tidal braking mechanism will not move the moon that far away from the earth.

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    Quote Originally Posted by hhEb09'1 View Post
    That's a valid point, but we have to be careful how it is phrased. We are talking about moving the moon out from the earth by another third. If we moved Mars in towards the sun by a third, it would no longer be a planet under the latest guidelines.
    Um . . . I'm confused. Why not? (And that's a way greater distance to move things.)
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    Quote Originally Posted by Gillianren View Post
    Um . . . I'm confused. Why not? (And that's a way greater distance to move things.)
    It's still a "third", relatively. And that is in the spirit of the current definition, since Venus is much closer (in an absolute sense) to us than some of the outer orbits which have been said to be cleared.

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    Quote Originally Posted by hhEb09'1 View Post
    -In other words, the conservation of angular momentum insures that it will never happen..
    Are you saying that that is true for all systems? Or just the earth/moon in present "initial" conditions.

    I mean, other then tensile strength, there is no limit to the angular momentum of an Earth-sized planet, so there should be no (theoretical) limit to how much could be transferred to an Earth-like-planet's Moon-like-moon --- or am I missing something?

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    Quote Originally Posted by pghnative View Post
    Or just the earth/moon in present "initial" conditions.
    Just the earth/moon in present conditions.

    It's all in the OP

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    Quote Originally Posted by hhEb09'1 View Post
    Quote Originally Posted by Gillianren View Post
    Um . . . I'm confused. Why not? (And that's a way greater distance to move things.)
    It's still a "third", relatively. And that is in the spirit of the current definition, since Venus is much closer (in an absolute sense) to us than some of the outer orbits which have been said to be cleared.
    I think the point is, isn't the rule about the barycenters relating to non-star masses orbiting around each other?

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    Quote Originally Posted by Roy Batty View Post
    I think the point is, isn't the rule about the barycenters relating to non-star masses orbiting around each other?
    It was a rule of the earlier proposals, which are no longer on the table.
    That's what the blogs (and the OP) are about.

    The rule I mentioned to Gillianren is one of the ones passed by the IAU.

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    Oh well, thanks for correcting me. So quickly. Now I can't delete my double post

    I am getting well pee'd off with this board not allowing automatic nested quotes, at least to 2 levels!

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    Interesting. I just tried to post twice to this thread, and see if I could delete both of them. I could. Maybe we could just walk up the thread and delete em all

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    Quote Originally Posted by hhEb09'1 View Post
    Interesting. I just tried to post twice to this thread, and see if I could delete both of them. I could. Maybe we could just walk up the thread and delete em all
    You can't delete a post after someone else has posted. No worries, anyway, it's my fault for not using the preview facility

  17. #17
    I took care of the non-nested double above. If anyone encounters similar trouble, feel free to drop a note to the mods.

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    Quote Originally Posted by Wolverine View Post
    I took care of the non-nested double above. If anyone encounters similar trouble, feel free to drop a note to the mods.
    Thanks. & thanks for freaking me out just as I was trying to prove my point & finding my post had disappeared! LOL
    Now, back to the original programming...

  19. #19
    Doh! Sorry about that.

    I'm with you on the nested quote issue though -- it would be very helpful if they were the default (two-deep, anyway), but I'm not sure what would be involved in changing that.

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    Quote Originally Posted by Wolverine View Post
    Doh! Sorry about that.

    I'm with you on the nested quote issue though -- it would be very helpful if they were the default (two-deep, anyway), but I'm not sure what would be involved in changing that.
    Actually, I can still see the delete option on my previous posts so maybe it's time related instead?! anyway, I've sidetracked this thread enough, sorry!

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    Quote Originally Posted by hhEb09'1 View Post
    That's a valid point, but we have to be careful how it is phrased. We are talking about moving the moon out from the earth by another third. If we moved Mars in towards the sun by a third, it would no longer be a planet under the latest guidelines.
    Yes, but if we move the Moon outward by slightly less than a third, then it's still a Moon. If we move it slightly more than a third, then it's suddenly a planet, even though the difference is infinitesimal.
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  22. #22
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    Quote Originally Posted by ToSeek View Post
    Yes, but if we move the Moon outward by slightly less than a third, then it's still a Moon. If we move it slightly more than a third, then it's suddenly a planet, even though the difference is infinitesimal.
    Now I don't understand the objection. Is that an argument for fuzzy definitions, without well-defined boundaries?

    If the boundaries are sharp, then there will always be that potential for a small change to induce a classification change. One of the points that Mike Brown tries to make in his defense of the current rule is that the current rule may be fuzzy, but the classification is clear--given our present situation. I'm not sure that he successfully defends that notion, but I grant that it is an interesting idea.

  23. #23
    Quote Originally Posted by hhEb09'1 View Post
    Now I don't understand the objection. Is that an argument for fuzzy definitions, without well-defined boundaries?

    If the boundaries are sharp, then there will always be that potential for a small change to induce a classification change. One of the points that Mike Brown tries to make in his defense of the current rule is that the current rule may be fuzzy, but the classification is clear--given our present situation. I'm not sure that he successfully defends that notion, but I grant that it is an interesting idea.
    Given our lack of empirical data to feed into these models, I think it would be a mistake to try and nail down a definite boundary on planetary definitions - a fuzzy boundary implicitly accepts that it will in future be modified, which is IMO a far more sensible way to take the definitions when we lack enough data to prove anything either way.

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    Exclamation

    Quote Originally Posted by Roy Batty View Post
    You can't delete a post after someone else has posted.
    I did, just a few days ago, in another thread.

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    Quote Originally Posted by jkmccrann View Post
    Given our lack of empirical data to feed into these models, I think it would be a mistake to try and nail down a definite boundary on planetary definitions - a fuzzy boundary implicitly accepts that it will in future be modified, which is IMO a far more sensible way to take the definitions when we lack enough data to prove anything either way.
    We will always lack enough data to prove anything, though. Classification is an aid, not a scientific "result."

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