Eight planets

By this same logic, the Earth does not orbit the Sun. The deviation between the Earth's and Sun's paths about the center of the Milky Way is extremely small.

The Moon and Earth orbit each other around their common center of mass, which in turn orbits about the Sun, which in turn orbits about the Milky Way, which in turn ...

The Moon is classified as a moon and not a planet because the center of mass of the Earth-Moon system is inside the Earth. On the other hand, the center of mass of the Pluto-Charon system is outside the surface of Pluto and thus Charon cannot be deemed to be a moon of Pluto.

The intent of the IAU definition was to rule out Pluto as a planet not by virtue of Charon but by virtue of Neptune and the myriad Kuyper belt objects that cross Pluto's orbit. By similar logic, the 10,000 or so Aten and Apollo asteroids should rule out the Earth as a planet. The Earth has not yet cleared objects from the neighborhood of its orbit. (The Moon doesn't count, as the Earth has captured it.)

 

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I think you are correct on the above. I will just note that as the moon moves farther way from the Earth that some day the point about which they co-rotate will no longer be inside the Earth. Is the "moon" a planet then then?

Also, if memory serves me correctly (too lazy and tired just now to check) your comparsion to the sun neglects one very important difference (I think). Namely the force of gravity on the moon is mainly that from the sun's mass, whereas the the force of gravity on the Earth is 99.99+% from the sun, not the galactic center. Thus, even though it is true that both the sun and Earth are going arround the galactic center on essentially the same orbit, the object that the Earth is "orbiting" should be considered to be the one that is supplying the majority of the gravitational attaction acting on the Earth. Likewise the object the moon is "orbiting" is the one supplying the main gravitational force acting on the moon. (Again, by memory, that is the sun, not the Earth) So to be consistent the moon orbits the sun and so does the Earth.

 

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Now, (next day) I will prove my point again, with the numbers.

That was not my logic. My logic was: The object that qualifies as the thing a cosmic body is orbiting about is the object that is applying the greatest gravitational force on the body. When the object about which the body is orbiting is very dominate among the various gravitational forces acting on the body, then that the body's orbit is nearly an ellipse (including the circle case of course).

In my original post, I numerically showed that Earth's effect upon the moon's orbit is very small. So small (in comparison to that of the sun's effect upon the moon) that it is ridiculous to consider the moon as going around the Earth instead of around the sun. To make this point clear, I noted that if the moon's full orbit were displayed on your computer screen, with radius 1.000, the tiny deviations for an ellipse that are caused by the Earth’s gravity are "wiggles" of amplitude less than 0.003 and consequently you could not even notice the Earth's effect upon the orbit of the moon as it goes around the sun. I.e. on your computer screen, the Moon's orbit would appear to be a perfect ellipse with the sun at the focus as the "wiggles" would be too small for you to see them.

You misunderstood me, and so noted (correctly) that the sun and Earth are both on essentially the same orbit about the galactic center so by my logic (as you misunderstood it) the Earth does not orbit the sun.

I was not basing my point on the fact the orbits of Earth and moon around the sun are essentially the same, but upon the fact that the sun, not the Earth, supplies the dominate gravitational force acting on the moon producing this "essentially the same orbit" AS A RESULT. - I.e. My logic was actually: the sun's force overwhelmingly controls the orbit of the moon and makes it orbit about the sun instead of about the Earth, but as the main point of my post was that the Earth has not "cleared out its orbit", the Earth can not be a planet, if that is required of all planets. That seems to be the reason why Pluto is no longer a planet, if the reports in my local newspaper are reporting the IAU's "Planet rule" correctly.

Perhaps I did not make the logical foundation as clear as I could have because of my desire to know exactly how the IAU's planet rule was stated. If it states that only the object which is "gravitationally dominate" in any orbit is a planet, then Earth can remain a planet and Pluto can not be one (because part of the time, Pluto is closer to the sun than Neptune and Neptune is very dominate in that shared orbital space.)

You misunderstood my argument. - I.e. you though it was based on the RESULT that both moon and Earth have essentially same orbit. So you inferred (by your version of "my logic") that: "the Earth orbits the galactic center with the sun and does not orbit the sun, because both are in essentially the same orbit"

To numerically illustrate my actual logic, I now compare the Force applied to the moon by the sun, Fs, to the force, Fe, ~ applied to the moon by the Earth:

This is most easily done by noting that these forces are proportional to the centripetal acceleration of the moon they produce, which are in turn proportional to (omega)^2 times the distance to the moon, which in some very unusual system of units is:

Fs ~ (1/365)^2 times 1.5x10^8 = 1126
and
Fe ~ (1/28)^2 times 4x10^5 = 510*

So again we see that the object which is applying the greatest force on the moon, controlling its orbit etc., is the sun, not the Earth.

I do not know the number of days, d, (to stay in these strange units) required for the moon to make one trip around the galactic center or the distance, D, from the moon to the galactic center. Perhaps someone who does will compute d^2/D and compare it to Fs = 1126. My hunch is that in these units Fgc is small compared to unity. If Fgc < < Fs, the there is no reason, certainly not by my logic, to think that the moon is orbiting the galactic center instead of the sun, but in any case THE MOON IS NOT ORBITING THE EARTH as 99.99+ percent of the population is erroneously taught.
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*Actually it is much less as I here I have treated the Earth as stationary with the moon orbiting it. -I.e. neglected the fact sun is accelerating the Earth. the "tiny "wiggles" are a better measure of how little effect the Earth has on the moon's trajectory.

 

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I'm not sure if any of the rest of us knows about how long it takes for the Earth (number of days?) to make one trip around the galactic center, let alone the moon. If you do, let us know so that we can make a calendar note of it.

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Think about it this way.

• The Earth has captured the Moon gravitationally in the sense that the Moon's velocity with respect to the Earth is less than the escape velocity.
• Astronomers and NASA deem the Moon to be orbiting about the Earth.
  Who is more likely to be mistaken: professionals who have carried out multiple trips to the Moon, or BillyT?

The simplest way to look at the Sun-Earth-Moon system is that the Earth and Moon orbit each other and both orbit the Sun.

That is not the logic the IAU uses to determine whether some body is a satellite. The IAU has this to say about satellites:

Q: What is a “satellite” of a planet?
A: For a body that is large enough (massive enough) to satisfy the definition of “planet”, an object in orbit around the planet is called a “satellite” of the planet if the point that represents their common centre of gravity (called the “barycentre”) is located inside the surface of the planet.

Q: The Earth’s moon is spherical. Is the Moon now eligible to be called a “planet”?
A: No. The Moon is a satellite of the Earth. The reason the Moon is called a “satellite” instead of a “planet” is because the common centre of gravity between the Earth and Moon (called the “barycentre”) resides below the surface of the Earth.​

The IAU definition of "planet" in its entirety is

A planet is a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighbourhood around its orbit.​

As noted above, part of your problem is a misinterpretation of the meaning of "in orbit around the Sun". The IAU is clear on what it means to be in orbit about the Sun. On the other hand, the IAU is not clear on the meaning of "clearing the neighborhood around its orbit". Trying to read between the lines, my thoughts are that a planet accomplishes this in three ways:

• It can give a nearby object a gravitational boost to fling the nearby object far from the planet,
• It can capture a nearby object, making it a satellite of the planet, or
• It can smash into the nearby object, making it a part of the planet.

The presense of the Moon is not a problem with determining whether the Earth is a "planet". The presense of 10,000 near-Earth asteroids is a problem. These objects have not been captured by the Earth, flung away from the Earth's orbit, or collided with the Earth.

 

Don't look at it in real time, but instead: does Earth as a body has the capacity to do one of the three things you mentioned in the case of a close encounter with one and all of the 10000 asteroids.

Though I have to admit the IAU definition is less than perfect.

 

On (1):
We agree that the moon has less than the "escape velocity." Certainly, much less than escape velocity from the surface of the Earth, which is what this term usually refers to. The moon is, however, moving father from the Earth every year and will continue to do so for a long time. - In fact it will do so until the moon no longer "rises in the East" - Or alternatively so long as the moon makes tides on Earth, it will move farther away each year. It will "make tides" so long as its angular velocity is slower that the Earth's rotational rate (spin). Whether or not the moon will eventually escape I do not know, but I would guess "yes," but very far into the future and probably via some more complex interaction with Jupiter (of even a passing "Dark Visitor" as in my book) If it has not escaped by time the moon's "Earth year" (360 degree trip around Earth as seen from Earth's center) and Earth's day are equal, then it will need to wait for some "liberator" to come by. (I think you know all this and agree - In past you have been well informed.)

I do not like your term "captured," but will not get into deep discussion about whether or not the "Earth has captured the moon" - that is a complex question with several very uncertain answerS. About all that can be said with reasonable certainty is that if the Earth did capture the moon then some third body was passing by the Earth (This also true in the case of the Moon being pulled from the Earth, instead of "captured," as I believe most experts believe now, I think.)

However, your use of "capture" is not important as you explicitly stated what you mean by "captured" and I agree that the moon is gravitationally bound to the Earth (here I am sure we agree that "bound" means they orbit the sun with essentially the same duration year. For example we do not want to be stating that Pluto is bound to Neptune, even though one could argue that it has because of their 3 to 2 period resonance. - seem silly to use "bound" if the two objects can be on opposite sides of the sun!)

However, NONE of this and NONE of your point 1 matters wrt the Planet, or not, status of the Earth. To determine that question, we must look at the IAU's definition of a "planet."

On 2:
Can you offer any serious evidence for that statement about the belief of NASA experts? I will of course concede that many, like me, tend to casually think of it that way (we were all "brain washed" into doing so). I held that erroneous view seriously until about a year ago when Janus58 (who must be either a NASA or JPL etc professional) set me straight. I am quite confident that the vast majority of NASA professionals, if you ask for their "considered, serious opinion," will agree that the moon is orbiting the sun, and doing so in almost a perfect ellipse, with sun at the focus as of course, but moon's orbit about the sun does have very slight deviations from an ellipse caused mainly by Earth and Jupiter.

Again, the IAU definition, not what people many believe is what is important as the question is covered by definition, not opinion. One should apply the definition and then form you belief - not allow the belief you acquired before your were five, over-ride the definition! That is why I was asking what the new exact definition of a Planet is. Then, I will be able to decide if the Earth is, or is not, a planet. I think we (and most of the informed world) agree that the definition is not what may be printed in various dictionaries, but is what the IAU has recently stated. You said that the IAU's definition of a planet was the following:

We both agree that the Earth has satisfied conditions (a) and (b) and seem to also agree, me more strongly than you, that Earth has NOT irrefutably satisfied (c).

You then added some of your views to clarify the "(c) problem" of the Earth being a planet:

As the Earth CAN (and will) boost the moon far* from the Earth, the moon will first cease to be a "satellite" and then in all probably be in an orbit at approximately 1AU from the sun but with a year slightly different from that of the Earth. If this is true, then occasionally the moon and Earth will be on opposite sides of the sun! (Then certainly the Earth has not "clear out of this 1AU orbit" making the Earth not a planet, provided the current IAU's "planet definition" remains in effect.

This post is already too long, and it does not matter for our discussion of the Earth's status as a planet (or not), but I have a problem with your first Q&A on satellites: Where is the "surface" the barycentre must be inside to make object in orbit about a gas giant a "satellite" - does not the gas density just continuously decrease as one moves away from the mass center?

I will agree that the moon is now a "satellite of Earth" and hope you will agree that this is only "temporary." - I.e. at some future time the moon will clearly be a Planet, if the Earth is one, or both will not be planets as neither has cleared the other from the 1AU path about the sun. Also the barycentre will not inside the surface of either then, so neither is a "satellite" of the other, but they are still "gravitationally bound." - Even later, clearly neither will be a planet as at times they will be on opposite side of the sun yet both in essentially the 1AU orbit about the sun.

Perhaps you will agree with me that the new IAU definition is a mess with the status of various objects a function of the epic you are speaking of.
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*For example, Earth/moon barycentre will outside the surface of the Earth.

 

The same logic applies to man-made low-Earth and geocentric satellites and to the moons of Jupiter. These satellites also move in a near perfect ellipses about the Sun, with some very slight devations caused by the Earth and Jupiter. However, I am sure you would agree that our man-made satellites orbit the Earth and that Europa orbits about Jupiter.

I work on spacecraft guidance, navigation, and control at NASA. I have worked on planning trips to the Moon via L1. Does that qualify as a professional opinion? We use various levels of fidelity in planning a trip to the Moon. The coarsest is to ignore everything but the Earth and Moon. This alone yields good planning results. For more accuracy, we add in the Sun and other bodies as a "third body" gravitational attractors -- i.e., as perturbations on top of the Earth+Moon system. The Moon is orbiting around the Earth, and the two together orbit about the Sun.

That was direct from the IAU website.

The IAU does not consider the Earth's Moon to invalidate the Earth's status as a planet.

Don't shoot me, I'm just the messenger. That material is straight from the IAU website. (Rather, it was on their website; I got it from a cached copy. The website has been sanitized.)

Consider Pluto/Charon. The only difference beween the definitions "dwarf planet" and "planet" is the unclear statement about "clearing the neighboring of the orbit". Pluto and Charon are both considered "dwarf planets" because the barycenter of the Pluto/Charon system lies outside the surface of Pluto. Thus both the Earth and Moon would have to be considered planets (or co-planets) when the Moon moves far enough from the Earth that the barycenter is no longer inside the surface of the Earth.

Agreed. The IAU created a mess.

 

Yes it does and I would agree that any object bound to the Earth and close enough to the Earth for the Earth's gravitational field force on it to be greater than the sun's is "orbiting the Earth" and that both are rotating about their barycentre as it orbits the sun; however, beyond that separation, where the gravitational force of the sun on the object is greater than that of the Earth, I part company with you and prefer to say that the the two objects are orbiting the sun (and their barycehtre also is of course). Janus58, in the process of convincing me that the moon orbits the sun told me, as I recall, that when the object's orbit is far enough from the Earth so that all points in its orbit are ALWAYS curving towards the sun, is the same separation from Earth than makes the sun the dominate gravitational force.

Note the moon is never "curving" or "falling" towards the Earth, yet you want to claim it is orbiting the Earth! - that fact is what made me adopt the POV I now have instead of the one I was taught. The moon's trajectory is always concave towards the sun! (Even when it is closest to the sun!)

I really have a problem with the idea that object A is "orbiting object B" when some other object, S, is applying the majority of the gravitational force acting on object A, and the trajectory of object A is essentially an ellipse with object S at the focus, but if you do not that is OK. I see nothing wrong with accepting the "moon orbit the sun" result of this "rule" despite it sure looks like the moon orbits the Earth. It also sure looks like the Earth is flat and we have accepted that it is not. - Why all the resistance to accepting that the moon orbit the sun?

BTW, I worked about 15 year's in JHU/APL's space department, but must admit most of my time there was spent on biomedical things or energy systems. I (as you will probably agree) tend to think for myself and sometimes quite differently from others, so I was often called upon to at least sit in on "tiger teams" trying to find solutions to problems. I did however work on DISCOS and a little, on the earlier TRIAD that the US navy used before GPS for guiding ships etc. (both were invented at APL by two APL men while listening to the Doppler shift of sputnick (Earth's first artificial satellite).)

As we agree the IAU has made a mess of it, here is what the IAU would have said if I were King of the IAU:

(1) A satellite, S, of a celestial body, B, has their joint barycentre close enough to the mass center of B for less than 1 % of the mass of B to be more distant. That satellite, S, is also a "moon" of body B, if because of its self-gravity, it is essentially spherical, discounting any equatorial bulge effects due to its rotation AND body B is the more massive than S.

(2)A celestial object is also a moon of body B, if under it self-gravity it is essentially spherical, discounting any equatorial bulge effects due to its rotation AND body B is BOTH the more massive of the pair AND is producing the greatest gravitational force on that celestial body.

(3) A star, S, is a self-luminous mass that is self-luminous because of heat produced by nuclear fusion and /or gravitational collapse.

(4) A celestial body, P, which is neither a star, nor a moon, nor a satellite, is a planet of a star S iff a star S is normally producing the greatest gravitational force on P; however, P must at all times be essentially spherical, discounting any equatorial bulge effects due to its rotation. Objects that satisfy these "planet conditions" except for the final one about the shape are comets if their shape elongates significantly when near the star or asteroids if it does not AND it is not naturally essentially spherical, except for rotational distortions.*

(5)A set of celestial objects, none of which is self-luminous and all of which gravitational attract each other normally more strongly than any star does are celestial "clouds"

Please tell me why you think I should not be the "king of the IAU."

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*In order to comply with "full disclosure requirements" I admit that the inhabitants or Ceres, did provide a small financial consideration along with the text of this last condititon.

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(They are tired of just being the bigest asteroid and want to be a planet again.)

PS I will be gone for a couple of days (It is crazy at my age, but I am building a house all by my self overlooking a lake about an hours drive from Sao Paulo!)

 

And when I am stading on the salt flats of a dried up sea, I see the Earth is flat. So what. We need to go by the definitions, not our human impressions, BTW did you see the sunrise today?

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What you say about what we see is true but we also see all of the stars do the same, so by your argument's logic, the stars also orbit the Earth!

I did not think you were the Pope of Copernicus time re-incarnate , but perhaps I am wrong as you are advancing his same argument.

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SHAME ON YOU JAMES - I expected better from you!

How can you claim that the moon "orbits the Earth" (except by your human impression) if the moon never* ever is curved towards the Earth by the Earth's gravity! Certainly it is extremely wrong to state that fact is "irrelevant." Are you really that stuck in the "facts" you learned as a child and blind to the true?

No, because it is not a "similar argument." My argument is that the sun is the dominant gravitational force on the moon - that is not true about the force from the galactic center on the Earth or the moon. The Earth (and the moon ) orbit the object that is providing them with the dominate gravitational force on them. I.e they orbit the sun. If and when the black hole and other masses at the galactic center do provide more force on the Earth (and the moon) than the sun, then I will agree that the Earth is orbiting that center.
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*Never curved by the Earth's gravitational attraction on the moon alone. It is true that when the Earth is closer to the sun than the moon is, then the constant curving of the moon towards the sun makes it also cure towards ALL the planets that are closer to the sun, such as Venus, Mercury and at that time the Earth, but that is only an "accident" due to the fact that while curving towards the sun, because of the sun, the moon is curving towards these three planets also. (and toward many very distant stars that are hidden from view by the sunlight.) - again "so what?"

 

I agree if we are talking about language, but I was taking about physics and reality. Of course i say in common with maist everyone else the moon rises and the moon makes one revolution about the Earth every 28 days etc. But the fact is is the moon does not rise and is not going arround the Earth - it only appears to. yes if you select the "proper freferenc frame, then the trjaector of the moon is identical with that of a cab dirven by a drunk driver! but that does not make the true trajectory of the moon identiacal to that cab. (to the extent than any trjectory is to be considered "true" since all are somewhat arbitary as all are reference frame dependent)

As far as "fall" is concerned:
(1) while you were posting i added a foot note to my post to make it more accurate - be more precisely correct.
(2) "Fall" tends to be associated with the idea of movement in responce to gravitational forces. It is in the sense also that what i said is true - the moonn is allway mainly falling in responce to the sun's gravity, never in responce to that of the Earth, but admittedly when the Earth is on the same side of the moon as the sun, the the Earth's gravity make the radius of curvature toward the sun shorter than when the Earth is on the opposite side of the moon form the sun.
That is as the moon falls towards the sun it does so more rapidly when the Earth assists than when the Earth does not, but the moon alway falls towards the sun and only towards the earth (and Venus and Mercury etc. for all stars hidden by sun light) when these plantes and stars happen to be positioned such that the fall towrds the sun is als a fall towrds them.
Must break off now it is very late 3:30 Am for me and I will return to read your reply in about two days.

 

Saying that the Solar system is made of only eight planets makes us forget that there are many more interesting celestial bodies to study, that is Europa, Io, Ganymede, Titan, Enceladus, Triton, Pluto, Sedna, Xena...And it's not unlikely that they find other plutons because Brown and his team have only scanned around 5% of the deep sky.

www.titanexploration.com

 

Pluto is a planetoid.
http://sciforums.com/showthread.php?t=56367

 

Hey welcome, only want to point out that the pluton designation has been ditched, now Pluto and co go by the unimaginative name of "dwarf planets"