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.

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

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"All truth passes through three stages. First, it is ridiculed, second it is violently opposed, and third, it is accepted as self-evident." Arthur Schopenhauer - renowned 19th Century German philosopher.

I suppose I should have added, within the context of the blog

The BA repeats the assertion in his blog (Breaking News– Pluto not a planet!) today:PS: I was finally able to post to the blog directley

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.

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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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.

Um . . . I'm confused. Why not? (And that's a way greater distance to move things.)

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Gillian

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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.

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?

Just the earth/moon in present conditions.

It's all in the OP

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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N6MAA10816
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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.

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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N6MAA10816
'Forensic anthropologist! That's why no gun'
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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

You can't delete a post after someone else has posted. No worries, anyway, it's my fault for not using the preview facility

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N6MAA10816
'Forensic anthropologist! That's why no gun'
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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...

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N6MAA10816
'Forensic anthropologist! That's why no gun'
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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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N6MAA10816
'Forensic anthropologist! That's why no gun'
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