Are the Earth and moon a double planet?

[Jakenorrish]

Due to the mass ration of the Earth and Moon (as opposed to the ratio of Saturn and Titan for example) should we class ourselves as a double planet?

Arguements please!!

[Betelgeuse]

Well, most moons are quite a bit smaller than their planets. Our Moon, however, is relatively large - around 1/4 of the diameter of the Earth, making the Earth-Moon system a double planet. Tiny Pluto and its moon Charon are also a double planet; Charon is about half the size of Pluto.

[antoniseb]

Originally posted by Jakenorrish@Dec 8 2004, 03:33 PM
should we class ourselves as a double planet?

I've answered this sort of question previously, but it bears repeating. Terms like "planet", "moon", "double-planet", "brown dwarf", "hot Jupiter", etc are relatively arbitrary at the limits. We now have the capacity to deal with the complexity of simply referring to every object according to its name and properties. As such, you may refer to the Earth and Moon as a double planet, and the meaning of your statement will be clear. Similarly, you in a different statement, can call the moon a satellite of the Earth without fear that you have made your reference to the system as a double planet less credible.

For what it's worth, by tradition, the moon is not a planet.

[StarLab]

One could compromise and call it a "satellite planet"... :unsure:

[antoniseb]

Originally posted by StarLab@Dec 8 2004, 04:35 PM
One could compromise and call it a "satellite planet"

Yes you could, and I think the idea you were trying to express would be understood. I think the important thing to remember is that you can't really try to correct someone for calling it something other than a "satellite planet".

We may in the future have various compund terms that describe planets and moons according to how they seem to have been formed [capture, collision, co-formed, a combination, something else], or by what they are mainly made of, or whether they have atmosphere and/or liquid water, or a significant magnetic field or molton core, whether they are closer or further than planet-synchronous, whether they are tidally locked to face their planet or star etc.

My point is that a single term cannot work for all intended uses.

[Matthew]

The moons gravity is so strong that the centre of gravity of the system is pulled quite a distance from the centre of the earth, which is major reason in us being a double planet sytem.

[StarLab]

Yeah..I believe the center of mass for the two bodies is somewhat into the mantle... I wonder how that affects the magnetism of the poles?

[John L]

I have to disagree with calling the Earth-Moon system a double planet. To me, a double planet refers to two bodies where the gravitational center of their mutual orbits does not lie within either of the bodies. Technically, there aren't any objects in a stable orbit that orbit around the exact center of the main body. This would be a perfectly circular orbit, which is inherently unstable. The moon orbits around a point within the Earth's mantle, making it a satellite of the Earth - its moon. In the Pluto-Charon system, both bodies orbit around a point outside of Pluto. These, therefore, constitute a double planet. That is if you consider Pluto a planet. I say they are a binary KBO, but that's just me.

[Jakenorrish]

I'm glad I asked this!

My own view is that when we make more detailed planetary discoveries outside our solar system we will then have to class planetary systems differently than we do now. Scientists will inevitably come up with a simple way of classing gas giant systems, rocky systems etc.

For instance Mars' moons are tiny by comparison to Earth and our own satellite. Saturn's Titan is big (Bigger than Pluto for instance), but in relation to its parent planet is small indeed. I therefore think that the Earth Moon system will eventually fall in to the category of double planet or something along those lines. Not sure if that makes total sense, but hope you get my drift!

Jake B)

[Betelgeuse]

Originally posted by Jakenorrish@Dec 9 2004, 09:03 AM
For instance Mars' moons are tiny by comparison to Earth and our own satellite. Saturn's Titan is big (Bigger than Pluto for instance), but in relation to its parent planet is small indeed. I therefore think that the Earth Moon system will eventually fall in to the category of double planet or something along those lines. Not sure if that makes total sense, but hope you get my drift!

Absolutely! A double planet system revolves around gravity/mass and size (as far as I know) - this is therefore the most obvious reason for earth and moon to be called a double planet system.

[Donald]

Due to the distances and masses of the satellites/outer planets, from Mars to Neptune -- I don't class the double comet Pluto/Charon as a planet -- all of them, from the tiny satellites of Mars to the largest around the gas giants, actually go around their primary planets.

The Moon does not go around Earth. The gravitational attraction of our sun on the Moon is over twice as great -- 2.2 times -- the gravitational attraction of Earth on the Moon. Therefore the path of the Moon has to be elliptical around our sun, which in fact, it is. That is, the Moon goes around our sun in an elliptical path and Earth goes around our sun in an elliptical path and those two orbits cross 24 times a year.

This has been known at least since 1874, that being the date of publication of the earliest book in which I can find those orbits. Wasley Krogdahl's "The Astronomical Universe," published in 1952 has those orbits. The National Geographic map of the front and back of the Moon, found in probably every middle school in the country has those orbits pictured at the top in the center of the chart.

Also, there are solar satellites in Jupiter's orbit, located far enough from the planet so that the solar gravitational attraction rules.

As far as the double planet question, if you were above the plane of the Earth/Moon system it would *look* like a double planet as the center of gravity of the system is about 1600km below Earth's surface. Looked at that way you could say that the Moon goes around Earth but you would also have to say that Earth goes around the Moon.

I've gotten into this over and over online. I was the Astronomy SysOp on the Orange County Astronomers' BBS for eight years before the Internet took over such operations. Also, I taught it in my astronomy classes in the planetarium and in middle school, high school, and college and university classes.

[withaGee]

My vote on the issue: I agree with John L's definition of a double planet; thus Terra/Luna is planet-and-satellite.

Donald's post raises some questions: 1): It would seem that both bodies in any 2-body system (where the center of gravity is not the center of either body) go around each other. But what about planets with multiple satellites? Are Mars through Neptune going around each of their own satellites? It's easy for me to conceptualize an Earth with two Lunas flying around as if tossed by a juggler, but how much are those other planets perturbed (if at all) by their moons?

And 2): While the orbits of the Earth and the Moon are generally elliptical around the Sun, they are both rather bumpy ellipses (the Moon's more so). Do you know of an online link, Donald, that contains a graphic representation of this?

Well, that's all for now. My head is now spinning.

Thanks for all the interesting concepts, fellow Forumites.

-G

[John L]

Go back to your Kepler. ALL objects orbit in ellipses. There are NO circular orbits. A circular orbit is inherently unstable. And the Moon orbits the Earth while the Earth orbits the sun. I disagree with the way Donald has characterized the Moon's orbit.

[rudeyd]

Originally posted by antoniseb+Dec 8 2004, 04:20 PM--></div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (antoniseb &#064; Dec 8 2004, 04:20 PM)</td></tr><tr><td id='QUOTE'><!--QuoteBegin-Jakenorrish@Dec 8 2004, 03:33 PM
should we class ourselves as a double planet?

I&#39;ve answered this sort of question previously, but it bears repeating. Terms like "planet", "moon", "double-planet", "brown dwarf", "hot Jupiter", etc are relatively arbitrary at the limits. We now have the capacity to deal with the complexity of simply referring to every object according to its name and properties. As such, you may refer to the Earth and Moon as a double planet, and the meaning of your statement will be clear. Similarly, you in a different statement, can call the moon a satellite of the Earth without fear that you have made your reference to the system as a double planet less credible.

For what it&#39;s worth, by tradition, the moon is not a planet.[/b][/quote]
Technically, the exact definition of a moon has never been determined. Moons that are extremely small around planets like Saturn I consider space debri with a predictable orbit. How do you call a rock that&#39;s a couple miles wide around a planet the size of Jupiter a "moon"?? Just my opinion. B)

[Jakenorrish]

What a series of excellent views. The answer like I said a bit earlier will probably come in the future, when our technology enables us to view planets orbitting other stars in greater detail.

I believe that we will eventually fall in to a different class than &#39;planet / moon&#39;. when we have more evidence to compare ourselves with. Whether that is &#39;double planet&#39; I&#39;m not sure.....

[Essel]

I have heard of a rock that has been locked by earth and it can be technically treated as earth&#39;s second moon. Anyone has more details pls?

Is there any tertiary body called satellite of a moon found anywhere?

Looking for answers pls?

[aeolus]

Originally posted by Essel@Dec 10 2004, 12:19 PM
I have heard of a rock that has been locked by earth and it can be technically treated as earth&#39;s second moon.

Might you be talking about a quasi-satellite of Earth? Earth has two such quasi satellites, both asteroids classifed as Near Earth Objects, that have orbital periods of something like .99...something years, so they orbit the sun once every year- making it look like, from our perspective, they orbit us. Its easiest to use the orbit simulator on the NEO webiste NASA has:

http://neo.jpl.nasa.gov

,click on orbit diagrams, and look up Cruithne & 2002 AA29 . If you look at it from the perspective of Earth, they orbit us, but really, they&#39;re orbiting the sun.

This is the only thing I can suggest for your question about tertiary (and quaternary?) moons of our home planet. If this isn&#39;t what you were looking for, my apologies- these quasi-satellites are cool nonetheless&#33;

[Essel]

Thanks aeolus&#33;

[Nyrath]

The definitions are vague. I remember back in the 1970&#39;s Isaac Asimov was pushing a definition of "planet" as a body large enough that gravity forced it to be spherical. The definition never caught on.

[Matthew]

Originally posted by Nyrath@Dec 11 2004, 03:46 AM
The definitions are vague. I remember back in the 1970&#39;s Isaac Asimov was pushing a definition of "planet" as a body large enough that gravity forced it to be spherical. The definition never caught on.

Actually that what we define a planet, too a point. Thats why asteroids won&#39;t be called planets, because of their small size which stops them from being spherical.

[Jakenorrish]

Some of Saturn and Jupiter&#39;s moons are spherical, whilst others are irregular, so maybe they&#39;d need reclassing on the strength of that?

[Betelgeuse]

Not necessarily - you can have definitions of satellites which allow "moons" to be classified as being what they are:-

Some would define a satellite as being

a body which revolves around another body of preponderant mass and which has a motion primarily and permanently determined by the force of attraction of that other body.* A parent body and its satellite revolve about their common center of gravity.

or even plainly,

celestial body orbiting around a planet&#33;

This site explains the bassis of classification of commercial satellites.

Hope this helps you,
Regards
Rigel

[Jakenorrish]

Very useful as always Rigel&#33;

Dda iawn,

Jake

[John L]

Originally posted by matthew+Dec 11 2004, 04:41 AM--></div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (matthew @ Dec 11 2004, 04:41 AM)</td></tr><tr><td id='QUOTE'> <!--QuoteBegin-Nyrath@Dec 11 2004, 03:46 AM
The definitions are vague. I remember back in the 1970&#39;s Isaac Asimov was pushing a definition of "planet" as a body large enough that gravity forced it to be spherical. The definition never caught on.

Actually that what we define a planet, too a point. Thats why asteroids won&#39;t be called planets, because of their small size which stops them from being spherical. [/b][/quote]
The asteroids Ceres and Vesta are both spherical due to their own gravity. The reason they aren&#39;t classified as planets is because they share the same orbit with hundreds of thousands of other bodies. When they were originally discovered in the mid-1800&#39;s, the astronomy texts of the time listed them as planets. It wasn&#39;t until the number of bodies in the space between Mars and Jupiter grew large that their designation was changed to asteroids. At one point Jupiter was listed as the 11th planet and Saturn as the 12th&#33;&#33;&#33;

That is why I feel the definition of a planet should be any object that orbits a star, that is in a spherical shape due to its own gravity, that has a differentiated core, mantel, and crust (whether solid, liquid, or gaseous), and that doesn&#39;t share its orbit around the central star with similarly sized bodies. The first, that it orbits a star, would differentiate between planets and the rest of the debris in the galaxy. The second and third places a limit on the size and mass reducing everything smaller to minor planet/asteroid/comet status. The last keeps things like the asteroids and KBO&#39;s from becoming swarms of planets that make learning their names quite difficult.

I feel that the definition of a moon should at least contain the caveat that the center of gravity around which it orbits should lie within the body of the planet it orbits. If not, then the situation is a double planet/binary body system.

[Jakenorrish]

That&#39;s a good way of looking at it John L.

I&#39;m swaying towards that way of looking at it, but in the context of the objects in our solar system, if you discount Pluto / Charon as being kuiper belt objects, then the Earth is closer in ratio to its moon than any of the other planets by a long long way&#33;

[Martin Dawson]

Why, Oh Why, must EVERYTHING have a category, a planet is a planet, a moon is a moon, now we are having "double planet&#39;s", what is a "double planet"??? think about it, they are both separate bodies, just because there&#39;s a size "issue" or they share a orbit around a common sun (or star...) some people think they have to dream up a new title, this remind&#39;s me of all the rubbish that was spoken (written...) about Pluto these last few years....
...one day in the far of distant future when every thing is known, will everything have a category, sub-category, a title, a (new) name or what.....
The Universe (and nature) is simple, let&#39;s keep it that way.....

[Erimus]

If you discount Pluto, as planetary astronomers are increasingly doing, then the Earth and Moon are most definitely a double planet. No other satellite of a major planet in the Solar System even comes close to the mass ratio of the Earth and the Moon

[Jakenorrish]

Hi Martin,

I think the simple answer to your well made point is:- for the same reason we have binary stars. We can then distinguish between say Mars type planet moon systems and our own very easily. The moon has a huge effect on the earths rotation, our tides etc. I don&#39;t think the same can be said of Phobos on Mars&#33;&#33;

[Buttercup]

Thread necromancy.

And now folks are wondering if Pluto and Charon are a double planet. Saw a brief article wherein it's wondered if Pluto and Charon share the same atmosphere (I am aware of Pluto's atmospheric anomaly).

Interesting. :-)

Comments? What would be THE determining factor of either Earth/Moon or Pluto/Charon being a genuine double planet?

Can't wait for New Horizons to "get out there."

[Centaur]

For what it's worth, by tradition, the moon is not a planet.

The Greek root for our word planet is "asteres planetai - wandering star", i.e. a celestial object that appears to move relative to the so-called fixed stars. In ancient times, both the Sun and Moon were considered to be planets. Of course, the Earth as the observation platform was not viewed as a wandering star (planet).