Looks like we discovered two more moons around Jupiter

[insert_name]

1 minute ago, Green Baron said:

Is that real or hypothetical ? If real, source pls. Because afaik only a few asteroids / kbos go withershins.

May be caught from outside or exchanged with neighbours (hypothetical) or forced by encounters with inner planets. These objects don't need much to change their path.

That diagram is from the article in the first post, pretty sure that he is referring to retrograde moons which are really common

[YNM]

@K^2 Defining moon by orbits... well.

There's one problem, anyway : orbits are for all reason the graph of gravitation, and so it might happen that the Earth and the Moon just happen to fall into one of the possible stable-yet-unusual conditions. I mean, the Moon were much closer to the Earth at one point (some estimates put initial revolution at 22 days (Earth rotation), with Earth's day being much faster than now), would that state makes the Moon classifiable to a moon or a co-orbiting body ? Also, up until now, clearly moon's gravitation affect Earth's characteristic, so I presume there should be some clearing up before one fully assume that gravitation and resulting path (orbit, or in case of single bodies, shape) determines the class of an object.

[K^2]

9 hours ago, Urses said:

All said above, would it not be a Definition for a true satelite?

A object that is able to change orbital Vector from prograde to retrograde to the main gravitational force (Sun) are satelites of the sub gravitational force (planet) otherwise those objekts are parts of a multiplanetal combines with the planet they are on same orbit....?

Urses

Very, VERY few natural satellites of planets ever change direction to retrograde with respect to Sol. This has to do with orbital speed around the Sun being very high, and that, in turn, is more of a statement on gravitational well of the Star than its pull on a specific body. All of the Saturn's moons are pulled by Saturn much stronger than by the Sun, yet only the few inner-most moons orbit fast enough to go retrograde with respect to Sun.

Definition based on strength of gravity, rather than orbital velocity, is a much better one. It's precise, and the only thing you need to ask is whether the orbit of the body around the Sun is convex or not. If it's convex, then the Sun always pulls stronger on it than the planet it co-orbits with. And again, this definition makes every moon a moon except for the Moon. It's a loss I'm willing to take.

Also, under definition I'm defending here, humans have been to another planet. Yes, it's a cheap populist selling point, but I think I've already made a solid argument from physics and categorization perspectives, so I don't feel bad about going for the populist vote now.

 

@YNM Earth's pull on the Moon is less than half of the Sun's. Putting aside whatever mess there was following the impact, or whatever event created the Moon, since formation, it's been influenced by the Sun a lot more than Earth. The fact that it moved out and is now influenced even less does nothing to weaken the point. Just the contrary. In general, however, yes. I suspect it to be entirely possible for an object that used to be the moon of a planet to migrate out and become a planet co-orbiting the primary. This is no different than capture event adding a moon to a planet, for example. Star systems evolve, and moon being a moon or not is definitely a question of whether it's orbiting a planet or the star. Our Moon is orbiting the Sun and merely co-orbits with Earth. That is a fact. Whether or not it's sufficient to qualify it is a planet is the core of the argument here. And while I don't particularly care in Earth-Moon case, we're doing ourselves a great disservice in terms of classifying exoplanets if we don't take care of such ambiguities now. And then there is no reason to create a special exception just for the Moon.

Edited June 8, 2017 by K^2

[Brotoro]

The Earth and Moon are gravitationally bound together. If the Sun were to suddenly vanish, the Earth and Moon would happily continue their relative dance as the fly out into deep space together. What the Sun is causing the pair to do (arcing through space in a relatively tight curve around the larger mass) is irrelevant to this relationship. And since the Moon is the junior partner in the system, it is classified as the satellite of the Earth.

[YNM]

@K^2 Fair enough indeed. Though it's interesting to think what other "orbits" may cause this - I mean, the Sun and the Moon is pretty much in a 1:12.(something) orbital ratios, what other ratios would cause a convex, repeating orbits around the central star ? Could this definition "erase" the possibility of inner bodies having a large natural satellite ? A good definition shouldn't cause too much a headache in some weird conditions I think...

38 minutes ago, Brotoro said:

The Earth and Moon are gravitationally bound together. If the Sun were to suddenly vanish, the Earth and Moon would happily continue their relative dance as the fly out into deep space together. What the Sun is causing the pair to do (arcing through space in a relatively tight curve around the larger mass) is irrelevant to this relationship. And since the Moon is the junior partner in the system, it is classified as the satellite of the Earth.

Well if the Earth were to vanish the Moon would orbit the Sun just as before again as well...

[Brotoro]

23 minutes ago, YNM said:

Well if the Earth were to vanish the Moon would orbit the Sun just as before again as well...

Yes... which proves that the Moon is a satellite of the Sun. And depending on the context, we classify satellites of the Sun as Planets (if they meet all of the IAU criteria), Dwarf Planets, minor planets (asteroids), comets, etc., etc., and moons (if they are natural objects gravitationally bound to a planet and are larger than some as yet undefined lower limit...below which they could be called moonlets). 

Edited June 8, 2017 by Brotoro

[kerbiloid]

Difference between solar influence on the Earth and Moon is  much weaker than the Earth influence on Moon, though solar influence on each other is greater than the Earth influence on Moon. So, the Earth and Moon are presumed married.

(Can't edit from pad) Not on each other, but on any of the two.

 

[Green Baron]

Morning *yawn*. Lol, funny how one can get excited about definitions.

If you define an orbit by the force that's being exerted the moon is in orbit around the sun. If you define it by Kepler's laws then it is in orbit around the earth. It is just a point of view. I do not accept the first idea because it leads to a wavy orbit and ignores the presence of hill spheres and barycenter, which are actually quite nice because with their help we can put satellites in places. Which actually works. Trojans stay where they are (or are attracted). Push them towards the planet and they enter an orbit around the planet, push them in the other direction and they fly around the sun. This is for ease of talk, just to know that if someone talks of an orbit in respect to a body he means that body.

Again:  even in astronaut around the moon would in a "sun orbit" according to that definition. @K^2, your definition leads to misunderstanding and unclear use of the word orbit. I doubt it will make into every day use. I am still waiting for the moons orbital parameters in a sun orbit.

Peace, love & understanding :-)

 

[Streetwind]

3 hours ago, Brotoro said:

Yes... which proves that the Moon is a satellite of the Sun.

Erm... no. By that definition, there are no subsatellites at all, and every single object in the solar system is always a satellite of the Sun. Remove Pluto, and Charon continues to orbit the Sun. Remove Jupiter, and all its captured bodies continue to orbit the Sun. You can do this for literally any object in any star system.

That makes this approach useless as a definition IMHO. If you define everything as the same thing, you don't have a definition in the first place, you just have a bunch of stuff with no differences.

Edited June 8, 2017 by Streetwind

[Brotoro]

Why is this a problem? The Moon DOES go around the Sun. The moons of Jupiter DO go around the Sun. Apollo 8 was going around the Earth along with the Moon back in 1968. Charon goes around the Sun.

BUT...these objects also go around other objects as they go around the Sun. So, in addition to going around the Sun, our Moon is gravitationally bound to the Earth and goes around the Earth (and would continue to do so if you remove the effects of the Sun). This is why our Moon is classified as a moon of a planet, even though it also is going around the Sun.

Our Moon is also going around the center of mass of our Galaxy...but that doesn't affect the fact that it is a satellite of the Earth.

MY point is not what happens to the Moon if you remove the Earth (YNM brought that up). My point is that the Moon would be going around the Earth whether the Sun was there or not. (But I certainly acknowlege the obvious fact that both the Earth and Moon are gravitationally bound to the Sun...it just doesn't matter to my point.)

Edited June 8, 2017 by Brotoro

[Green Baron]

The conclusion comes because you only look at the force the sun has on the moon. The same a similar force is exerted on the earth, so you can ignore it for the moons orbital considerations and replace it with the difference of the force on the earth and the moon. We go into tidal forces then, an then the suns influence is much less (not neglectable). This is clearly reflected in reality because the moon is in a bound rotation around earth, not sun.

1 minute ago, Brotoro said:

Why is this a problem? The Moon DOES go around the Sun. The moons of Jupiter DO go around the Sun. Apollo 8 was going around the Earth along with the Moon back in 1968. Charon goes around the Sun.

BUT...these objects also go around other objects as they go around the Sun. So, in addition to going around the Sun, our Moon is gravitationally bound to the Earth and goes around the Earth (and would continue to do so if you remove the effects of the Sun). This is why our Moon is classified as a moon of a planet, even though it also is going around the Sun.

Our Moon is also going around the center of mass of our Galaxy...but that doesn't affect the fact that it is a satellite of the Earth.

No, they wobble around the sun, no "round" there :-) But "round" in respect to the planet, not to the sun.

[kerbiloid]

There no 'orbits', only trajectories. They call 'orbit' a recurring trajectory. So, as the only purpose of this term is simplification, it's usually applied to the simplest possible shape, usually patched cones. Occam, desu.

[Brotoro]

9 minutes ago, Green Baron said:

The conclusion comes because you only look at the force the sun has on the moon. The same a similar force is exerted on the earth, so you can ignore it for the moons orbital considerations and replace it with the difference of the force on the earth and the moon. We go into tidal forces then, an then the suns influence is much less (not neglectable). This is clearly reflected in reality because the moon is in a bound rotation around earth, not sun.

No, they wobble around the sun, no "round" there :-) But "round" in respect to the planet, not to the sun.

I don't understand your point. Yes, the Earth and Moon and Jupiter and Charon and any other object that is in a planet/moon system follow paths with different degrees of wobble in their paths as they go around the Sun. Why would you think I claimed otherwise?

Edited June 8, 2017 by Brotoro

[Green Baron]

My point is, the conclusion "the moon orbits around the sun and not earth" is based on too little data. It only takes into account the force the sun exerts on the moon, not the fact that the same force is exerted on the earth as well. Better, on the barycenter of the system earth/moon.

The moon goes it goes "up and down" around the earth on it's path around the sun. And that is because it is in earth's orbit. Earth and moon and some other stuff together are in sun's orbit.

Would you, for example, accept that a tidal wave is in orbit around the body that causes it ?

I define "orbit" of a A around B as: A's path is at all time inside the hill sphere of B. That does not exclude that other external or internal bodies can have an influence (aka perturbations), but this influence does not put A inside another body's hill sphere. The orbit of A around B can approximately be described by Kepler's laws of motion.

You definition looks like: A is in orbit around B if B exerts the greatest gravitational force on it.

Whether B exerts that same force on another body that co-orbits with B is ignored. Kepler's laws are not applicable. The path of the moon around the sun can only very coarsely be described by them. A moon of Jupiter for example does weird things.

In my defintion the moon is in orbit around earth. The sun acts as a perturber. The tidal forces between moon and earth are dominated by those two, though the Sun and Jupiter must be taken into account for naval and navigational considerations (tidal heights and streams).

Are we together ?

:-)

[YNM]

1 hour ago, Streetwind said:

Erm... no. By that definition, there are no subsatellites at all, and every single object in the solar system is always a satellite of the Sun. Remove Pluto, and Charon continues to orbit the Sun. Remove Jupiter, and all its captured bodies continue to orbit the Sun. You can do this for literally any object in any star system.

That makes this approach useless as a definition IMHO.

"... continues to orbit the *insert secondary parent object here* *without *that* much deviation from before* "

I have to agree that orbits, and shape, pretty much clears it up. What differs a comet from an asteroid ? What differs a minor planet from planets ? What differs a moon from a moonlet ? Done it for you. Also, it easily accomodates that bodies do change classification, over time.

Edited June 8, 2017 by YNM

[Brotoro]

54 minutes ago, Green Baron said:

My point is, the conclusion "the moon orbits around the sun and not earth" is based on too little data. It only takes into account the force the sun exerts on the moon, not the fact that the same force is exerted on the earth as well. Better, on the barycenter of the system earth/moon.

The moon goes it goes "up and down" around the earth on it's path around the sun. And that is because it is in earth's orbit. Earth and moon and some other stuff together are in sun's orbit.

Would you, for example, accept that a tidal wave is in orbit around the body that causes it ?

I define "orbit" of a A around B as: A's path is at all time inside the hill sphere of B. That does not exclude that other external or internal bodies can have an influence (aka perturbations), but this influence does not put A inside another body's hill sphere. The orbit of A around B can approximately be described by Kepler's laws of motion.

You definition looks like: A is in orbit around B if B exerts the greatest gravitational force on it.

Whether B exerts that same force on another body that co-orbits with B is ignored. Kepler's laws are not applicable. The path of the moon around the sun can only very coarsely be described by them. A moon of Jupiter for example does weird things.

In my defintion the moon is in orbit around earth. The sun acts as a perturber. The tidal forces between moon and earth are dominated by those two, though the Sun and Jupiter must be taken into account for naval and navigational considerations (tidal heights and streams).

Are we together ?

:-)

My definition says nothing about what object exerts the greatest force on the Moon. Perhaps you are thinking of somebody else's definition. And I certainly never said that the Moon orbits around the Sun and NOT around the Earth (it moves around both).

My definition has to do with what objects are gravitationally bound together. If you have a group of two or more objects that are gradationally bound together as that group moves around the Sun, and if one of those objects is a planet*...then the smaller objects in that bound collection get classified as moons (or moonlets, if smaller than some as yet undefined limit). So our Moon is a moon of Earth. It doesn't matter to my definition how strong the Sun pulls on the objects in the bound collection compared to how strong they pull on each other (as long as the Sun doesn't pull strong enough to disrupt the system). The fact that both the Earth and Moon move in wobbly paths around the Sun during the course of a year does not affect my definition.

*...or dwarf planet, or minor planet... 

Edited June 8, 2017 by Brotoro

[Green Baron]

29 minutes ago, Brotoro said:

My definition says nothing about what object exerts the greatest force on the Moon. Perhaps you are thinking of somebody else's definition. And I certainly never said that the Moon orbits around the Sun and NOT around the Earth (it moves around both).

My definition has to do with what objects are gravitationally bound together. If you have a group of two or more objects that are gradationally bound together as that group moves around the Sun, and if one of those objects is a planet*...then the smaller objects in that bound collection get classified as moons (or moonlets, if smaller than some as yet undefined limit). So our Moon is a moon of Earth. It doesn't matter to my definition how strong the Sun pulls on the objects in the bound collection compared to how strong they pull on each other (as long as the Sun doesn't pull strong enough to disrupt the system). The fact that both the Earth and Moon move in wobbly paths around the Sun during the course of a year does not affect my definition.

*...or dwarf planet, or minor planet... 

Ah, oh, yeah. You are right, i argued with the wrong person. Should read more thoroughly. Sorry, i apologize for my eagerness :-)

And basically i agree with you concerning orbits and moons, i think. Yet we are missing a hard definition of "orbit" and "moon" but i think that when saying "a moon orbits a planet" most people think something similar to what you wrote.

The Sun will, as far as i know, not pull strong enough to tear the moon out. The moon's orbit rises because of impulse preservation due to the breaking effect of tidal forces, they exert mechanical friction, break the earths rotation and in response the moon gets away. But it will most probably not leave earth's orbit in the lifetime of the solar system.

 

[K^2]

3 hours ago, Green Baron said:

I am still waiting for the moons orbital parameters in a sun orbit.

I told them to you the first time you asked. They are exactly the same as Earth's. Moon's trajectory around the Sun is nearly as perfect an ellipse as Earth's.

[Green Baron]

I only saw the gravitational force for the sun and the earth on the moon, not the orbital elements. Maybe google failed me, i have to reconfigure my network access to reach google. Anyway, the moon performs a different motion around the sun as the earth, so the elements cannot be "exactly the same". "Nearly as perfect as earths ellipse" is too sloppy for my understanding and only valid if you see earth and moon "almost" as a single mass or from very far away. Which is ok, but not sufficient e.g. for orbit calculations inside the earth/moon system.

Maybe i misunderstand your point of view, sorry if i do ...

[KAL 9000]

The Moon orbits the Earth-Moon barycenter, as does Earth. The geometric point that is the Earth-Moon barycenter orbits the Solar System barycenter (mainly the Sun-Jupiter barycenter ). However, the Earth-Moon barycenter is inside Earth. I'm going to define "moon" as "an object that orbits the barycenter of it, a planet/dwarf planet/asteroid/comet, and any additional moons, so long as the barycenter is inside the planet/dwarf planet/asteroid/comet itself. And it has to be natural, and not a ring particle." 

Technically, the Solar System barycenter is outside the Sun, but that's a story debate for another day. 

Pluto and Charon are not a dwarf planet and a moon. They're two dwarf planets orbiting their barycenter (which is outside Pluto). What exactly Nix, Hydra, Styx, and Kerberos are is for another day.

Edited June 8, 2017 by KAL 9000

[Green Baron]

Agreed :-)

(Edit: though i would take the hillsphere as the main criteria. Nuances ....)

... just tried to argue against the idea the moon is in orbit of the sun. That'll be, for my understanding, too broadly a use of "orbit". 

We might end up with moon orbiting the Great Attractor ...

:-)

Edited June 8, 2017 by Green Baron

[Racescort666]

@Green Baron Not to further confuse the discussion but Wired did a pretty good article on exactly this about 5 years ago: https://www.wired.com/2012/12/does-the-moon-orbit-the-sun-or-the-earth/

I don't have a horse in this race because I don't really care whether our moon is considered a moon [insert "that's no moon" joke here]. There seems to be a lot of debate on how things get classified, taxonomy if you will, and much of the argument seems to come from trying to maintain our previously established taxonomy but including new data. The outrage over Pluto being reclassified as a dwarf planet is a good example. For years we were taught that Pluto was a planet but with improving technology, we realized that there were many similar bodies on the outer edge of the solar system and we faced a problem: have lots of things we call planets or figure out a better way of determining what a planet is. The rest is history.

I'm sure there would be similar backlash if the definition of a moon were redefined. To that end, I have heard a compelling argument that the terrestrial planets shouldn't be planets and that only gas planets should be called planets. After that whole argument, I took the stance where I don't care what we call things. After all, we literally named our planet "dirt". Good job humans.

[insert_name]

4 hours ago, K^2 said:

I told them to you the first time you asked. They are exactly the same as Earth's. Moon's trajectory around the Sun is nearly as perfect an ellipse as Earth's.

Um no, because that would say that they are in the same position

[Green Baron]

 

The article which i just skimmed makes imo the same mistake as pointed out above: it takes the force the sun has on the moon but ignores that nearly the same force is exerted on the earth as well.

Orbital elements are the semi major axis, eccentricity, argument of pe, longitude of an and inclination. If you give me these for the moon around the sun in a sun centered refernce system (choose the plane of reference) then i am content. I bet nobody can because they do not exist. But they exist for the earth, or more precisely for the system earth/moon as they are clearly in an orbit around the sun which can be described by the aforementioned elements.

Also the definition that puts the moon in a sun orbit is so generalized that, had i more time, i would start to calculate the g forces the galaxy (the Local Group, the Great Attractor ...) has on the moon. Do we have the moon then in a galactic orbit or even bigger ? I mean, yeah, in the same philosophical sense like i live on earth this is maybe the case, but does that make us smarter ? The orbital parameters help us to calculate orbits(!), define the in OP mentioned moons of Jupiter (topic :-)) and let us play ksp.

 

So what do you guys think about the application of Kepler's laws of motion, Newton's or Galilei's gravity, Hill or Roche sphere and Lagrange points in respect to orbiting bodies ? Too narrow-minded ?

:-)

Edited June 8, 2017 by Green Baron

[HebaruSan]

3 hours ago, KAL 9000 said:

Technically, the Solar System barycenter is outside the Sun, but that's a story debate for another day. 

I find it interesting that we're not having this debate about stars. They seem to be classified based on their intrinsic characteristics (mass, composition, luminosity, temperature, etc.) rather than how they orbit. You can have binary-and-up configurations, but that doesn't change whether we call them "dwarfs," "main sequence," etc. As long as it glows from fusion, it's a "star."

Maybe we need a similar system for smaller bodies. The difference between a "planet" and a "moon" could be whether it has an atmosphere above a certain density (for example; I'm not saying that's the perfect definition). Something intrinsic like this could even make sense of phrases like "rogue planet," which most definitely cannot have cleared any orbit.

Edited June 8, 2017 by HebaruSan