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The Moon

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Barbara-B
826914.  Sun Jun 26, 2011 12:49 pm Reply with quote

True Posital, but there are moons out there that fit the definition of a moon; revolve around a plant; where the planet is static relative to the moon(s) (IE it is the moon that moves not the planet). This is not the case with the Earth/Luna system where both bodies are in motion relative to each other. Our Moon is unique, and this is another facet of of its uniqueness :-)

I've been trying to find my source, which was one of Isaac Asimov's science books, but that's a bit like trying to find a higgs-bosen in a large hadron collider.

 
Posital
826919.  Sun Jun 26, 2011 1:13 pm Reply with quote

Possibly - the tragedy of the moon?

 
Leith
826931.  Sun Jun 26, 2011 2:12 pm Reply with quote

The International Astronomical Union (IAU) has this to say on satellite classification (there appears to be some disagreement within their press office):

IAU wrote:
Q: Jupiter and Saturn, for example, have large spherical satellites in orbit around them. Are these large spherical satellites now to be called dwarf planets?
A: No. All of the large satellites of Jupiter (for example, Europa) and Saturn (for example, Titan) orbit around a common centre of gravity (called the "barycentre") that is deep inside of their massive planet. Regardless of the large size and shapes of these orbiting bodies, the location of the barycentre inside the massive planet is what defines large orbiting bodies such as Europa, Titan, etc. to be satellites rather than planets. [Actually, there has been no official recognition that the location of the barycenter is involved with the definition of a satellite.]

Source: IAU: Pluto and the Developing Landscape of Our Solar System
[Edit: fixed broken link]

The barycentre of any planet/moon system will be a little way out from the centre of the planet, but that of the Earth/Moon system is about 3/4 of the way to the Earth's surface. It remains below the surface of the Earth, though, which makes the Moon a moon by at least some people's definitions. In contrast, Pluto and Charon orbit around a barycentre that lies outside either body, hence they are now classified as a duel planet system.
http://en.wikipedia.org/wiki/Moon#Relationship_to_Earth

I also learned that the IAU have amended their rules for naming the moons of Jupiter and Saturn, having previously run out of approved mythological entities:
http://www.iau.org/public/naming/#satellites


Last edited by Leith on Sun Jun 26, 2011 6:32 pm; edited 1 time in total

 
Barbara-B
826942.  Sun Jun 26, 2011 3:01 pm Reply with quote

thanks Leith! :-)

I presume my Asimov Earth/moon system factiod originated before the IAU ruling.

 
Posital
826948.  Sun Jun 26, 2011 3:33 pm Reply with quote

So - how much matter/mass (you can assume the earth is of uniform density) would the earth have to lose for it to become a binary planet system - ie the barycentre is just on the surface of the planet.

Answers on a postcard... can't even imagine how to start answering this - since a change in mass would effect the orbits.

 
suze
826993.  Sun Jun 26, 2011 5:46 pm Reply with quote

Leith wrote:
In contrast, Pluto and Charon orbit around a barycentre that lies outside either body, hence they are now classified as a duel planet system.


Or at least, they probably should be.

We have to remember that Pluto was relegated from the Planetary Premiership in 2006, and now has to settle for dwarf planet status. But as is noted in the IAU document to which you link notes, for the present there is no rigorous definition of the term "binary dwarf planet", and so Pluto and Charon can't be one.

So for now, the official position is that the moon Charon is a satellite of the dwarf planet Pluto - but it is not ruled out that this may be revisited in the future.

 
Leith
827002.  Sun Jun 26, 2011 6:31 pm Reply with quote

Oops. Thanks Suze - I missed that update.

I got the "dual planet" status from the IAU's original 2006 press release on Pluto, which reads:
IAU wrote:
Q: Why is Pluto-Charon a “double planet” and not a “planet with a satellite”?

A: Both Pluto and Charon each are large enough (massive enough) to be spherical. Both bodies independently satisfy the definition of “planet”. The reason they are called a “double planet” is that their common centre of gravity is a point that is located in free space outside the surface of Pluto. Because both conditions are met: each body is “planet-like” and each body orbits around a point in free space that is not inside one of them, the system qualifies to be called a “double planet.”

They've obviously changed their minds / corrected their press officers since.

 
dr.bob
827134.  Mon Jun 27, 2011 9:43 am Reply with quote

Posital wrote:
So - how much matter/mass (you can assume the earth is of uniform density) would the earth have to lose for it to become a binary planet system - ie the barycentre is just on the surface of the planet.


Depends when you're talking about. Since the Moon is slowly moving away from the Earth, so the barycentre of the two-body system is also slowly moving further out from the centre of the Earth.

 
Zebra57
827238.  Mon Jun 27, 2011 12:04 pm Reply with quote

Do all astronomers accept the downgrading of Pluto?

 
samivel
827242.  Mon Jun 27, 2011 12:07 pm Reply with quote

Some American ones seem less keen on the idea.

 
dr.bob
827428.  Tue Jun 28, 2011 6:35 am Reply with quote

The number of astronomers that actually study planets is quite a small minority of astronomers generally, so most of them don't really give a toss either way.

 
Posital
827558.  Tue Jun 28, 2011 2:16 pm Reply with quote

dr.bob wrote:
Posital wrote:
So - how much matter/mass (you can assume the earth is of uniform density) would the earth have to lose for it to become a binary planet system - ie the barycentre is just on the surface of the planet.


Depends when you're talking about. Since the Moon is slowly moving away from the Earth, so the barycentre of the two-body system is also slowly moving further out from the centre of the Earth.

Ok - if your answer is "wait" - then can you calculate how long?

I suspect you'll need to take into account the sun's gravity...

 
dr.bob
827661.  Wed Jun 29, 2011 5:38 am Reply with quote

Dead easy. The location of the barycentre of the system is given by:

Mass of Moon x Distance to Moon / Sum of masses of Earth and Moon

The furthest the moon gets from the Earth (its apogee) is around 405,410 km (this varies quite a bit, but let's go with this value). So plugging in some numbers gives us:

7.3477 × 10^22 kg x 405,410 km / 6.0471 x 10^24 kg

= 4,926km from the Earth's centre.

To get that outside the Earth, we need to increase the distance from the Earth's centre to the barycentre to 6,378 km (the Earth's equatorial radius). If the masses are not changing, that means increasing the distance to the Moon to 524,909km: an increase of 119,499km. That will mean that, at apogee, the barycentre will just be above the surface of the Earth.

Currently the Moon is moving away from the Earth at a rate of 3.8cm/year so, to increase the distance by 119,499km will take about 3.1 billion years.

Simples!

 
Posital
827838.  Wed Jun 29, 2011 5:05 pm Reply with quote

If you put it like that, then it's simples... but any variation in the rate of orbit change will be quite significant over 3.1 billion years. So perhaps you're oversimplifying...

I suspect that the moon's orbit is accelerating away* from the earth due to its interactions with the sun (as I alluded earlier) - or will it be decelerating because of the action of the earth's oceans (etc) as the earth slows?

Or something else entirely?

*cumbersome phrasing, I know.

 
iamannoying.com
843720.  Sun Sep 04, 2011 9:37 pm Reply with quote

Since any variation in the rate of orbit change will be quite significant over 3.1 billion years: should we retrieve it, as long as there's a net gain?

[img]http://lpcoverlover.com/wp-content/uploads/2008/05/moon-flag.png[/img]

 

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