[meteorite-list] Double Planets

From: lebofsky at lpl.arizona.edu <lebofsky_at_meteoritecentral.com>
Date: Fri, 18 Mar 2011 19:29:01 -0700 (MST)
Message-ID: <67fa10c41b288502cecd3ed15fb67cfc.squirrel_at_webmail.lpl.arizona.edu>

Hi Sterling:

I don't like to disagree with you (YES!), but I wonder if you have ever
been on a seesaw. If you move Charon away from Pluto, the center of mass
moves away from Pluto, not toward it, you have a longer lever arm.

You actually get it right when you talk about the Moon later on! Moving
the Moon 50% farther away puts the barycenter outside the Earth.

Larry

> Hi, Walter, List,
>
>> The dividing line between planet-moon and dual-planet seems to be
>> whether the center of gravity (barycenter) of the bodies is either in
>> space or beneath the surface of one of the bodies...
>
> You're not wrong, Walter.
>
> What would have made Pluto-Charon a true
> "double planet system" under those original IAU
> definitions (the ones that were shot down in favor
> of dwarfism) was the fact that the barycenter of
> the Pluto-Charon system was well OUTSIDE both
> bodies. This means that they truly orbit each other,
> dancing around a point in space between them.
>
> If Charon was much smaller or much further away
> from Pluto, the barycenter would move closer to
> the center of mass of Pluto until it was inside the
> planet. The barycenter of the Earth-Moon system
> lies inside the Earth, about 1710 km down in the
> mantle, wandering up and down a bit with the
> eccentricity and tilted, of course.
>
> All the figures for Earth-Moon and Pluto-Charon
> and the formulas for all that barycenter stuff are
> nicely accumulated in this:
> http://en.wikipedia.org/wiki/Barycenter#Barycenter_in_astronomy
>
> Interestingly, the path of the barycenter of the Sun
> and all planetary bodies taken together passes through
> the body of the Sun much of the time, even through the
> fusion core of the Sun, and yet part of the time that
> barycenter is outside the Sun. (There's a diagram.) It's
> a 179-year cycle. I wonder what that does to the surface
> and if it affects the sunspot cycle? Big arguments about that:
> http://www.google.com/webhp#hl=en&sugexp=ldymls&xhr=t&q=barycenter+jose&fp=a0e1d04ac32ef934
>
> Playing with the numbers... if the Moon were 40%
> heavier than it is, the barycenter would be just ABOVE
> the surface of the Earth, outside the planet, and we
> would meet the definition of "double planet."
>
> OR, if the Moon was the same weight but 335,000 miles
> away instead of 240,000, then too the barycenter of the
> system would be just ABOVE the surface of the Earth.
>
> Of course in a billion years or so, the Moon WILL be
> that far away, so relax... We'll get there.
>
> Be sure and scroll down to the animations of a number
> of double systems, including Earth-Moon and Pluto-Charon.
> It's like watching drunken mice waltz...
>
>
> Sterling K. Webb
> ------------------------------------------------------------------------------
> Dear Cap'n: I changed the subject line... I've reformed.
> ------------------------------------------------------------------------------
> ----- Original Message -----
> From: "Walter Branch" <waltbranch at bellsouth.net>
> To: <fallingfusion at wi.rr.com>; <meteorite-list at meteoritecentral.com>
> Sent: Friday, March 18, 2011 6:48 PM
> Subject: Re: [meteorite-list] The Term "Planetary"
>
>
> Hello Ryan,
>
> The Earth's moon is very large, relative to the planet it orbits. In
> the
> astronomical literature, the earth-moon system is sometimes refereed to
> as a
> dual planetary system. The dividing line between planet-moon and
> dual-planet seems to be whether the center of gravity (barycenter) of
> the
> bodies is either in space or beneath the surface of one of the bodies
> (don't
> quote me, though I may be wrong).
>
> I have seen Pluto-Charon sometimes referred to as a dual-planetary
> system
> (though now I guess it would be a dual dwarf planetary system.
>
> -Walter
>
> ----- Original Message -----
> From: <fallingfusion at wi.rr.com>
> To: <meteorite-list at meteoritecentral.com>
> Sent: Friday, March 18, 2011 2:44 PM
> Subject: [meteorite-list] The Term "Planetary"
>
>
> To the list,
>
> I was sitting here reading some emails, and just thought...
>
> Who in the world ever came up with the term "Planetary" in reference to
> meteorites.
>
> First of all, our Moon isn't a planet.. and secondly, to my knowledge,
> the
> only "Planetary" meteorites in current existence have an origin of Mars.
> Hence, "Martian" meteorites. Did I miss the big announcement of those
> from
> Venus and Mercury?
>
> Regards,
>
> Ryan
>
> Sent on the Sprint? Now Network from my BlackBerry?
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Received on Fri 18 Mar 2011 10:29:01 PM PDT