Looks like we discovered two more moons around Jupiter

[nathan1]

The Earth, Moon, and the Sun all orbit the center of the milky way, so really the moon is in orbit of the milky way, not the sun. I mean, over galactic timescales the orbit of the Moon around the center of the galaxy is nearly indistinguishable from the Sun's orbit.

[monstah]

9 hours ago, kerbiloid said:

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.

As far as I see it, this post won the internet, and what follows is a white dwarf of a thread.

There's no spoon, y'all.

[YNM]

42 minutes ago, HebaruSan said:

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.

We do have the debate on stars - where does a brown dwarf and a red dwarf meets. But an object discovery solved it.

Problem is, dust are far more numerous than the stars - for all reasons, planets are dusts - and trying to set it up simply as "being this big" and "being that big" is not going to make sense. Stars do this because their mass correlates to their light - what we see. Planets and rocky bodies on the other hand have nothing to be inferred from observation - no spectral lines for all practical reasons - and so the only way to class them is by mass, or orbit. As their mass and density vary wildly, orbit seems to be the only thing to really classify them.

[Green Baron]

58 minutes ago, monstah said:

As far as I see it, this post won the internet, and what follows is a white dwarf of a thread.

There's no spoon, y'all.

... with an accretionary disk ? :-)

You're right, i got too upset for something unimportant. Greetings to @K^2, no offense meant !

[monstah]

16 minutes ago, Green Baron said:

... with an accretionary disk ? :-)

Oh, white dwarves are still pretty!

[monstah]

Let me expand on this a little.

There is no spoon, but we still find it useful to have different names for "spoony" and "forky" things, even if sometimes a friggin' spork comes along and you have no idea why someone conceived such a thing, but we digress because celestial bodies just are, regardless of who concepts. *ahem* Anyway, what I was trying to say it that @K^2's argument is pretty damn compelling. The math is sound. But ultimately they're just boundaries between useful concepts, and if moon is the spork I don't think it matters that much if we call it a spoon of a fork.

Except, one definition means we've been to another planet, and the other doesn't.

[Green Baron]

But what exactly is the argument ?

The isolated look on the force comparison has no meaning here. The sun's field is almost the same for earth as for the moon. What matters is the change of the forces in the combined gravitational field as the moon revolves around the earth and is perturbed by the sun. But only as much as the suns gravitation changes over the distances the moon has on its orbit. We call that tidal forces.

Take the sun away, it wouldn't change the moons orbit around earth (well, a little, by the perturbations that result from different distances to the sun the moon has on its orbital path).

Spoon ... fork .... hunger ! I'll be back ... :-)

[monstah]

3 hours ago, Green Baron said:

But what exactly is the argument ?

You got me

We can probably agree unlike Jupiter's new found moons, ours is here to stay a long time, and leave it at that?

[CSE]

21 hours ago, K^2 said:

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.

As I understand it, there's a spent Apollo stage that alternates between orbiting the Earth and co-orbiting the Sun alongside the Earth. How's that for an unintended side-experiment from the space race.

[Gene Kerantz]

Jupiter is OP.  Devs: pls nerf!

[K^2]

9 hours ago, insert_name said:

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

Earth and Moon are in the same place w.r.t Sun to within a perturbation. Orbital elements do not typically include perturbation effects. We can talk about instantaneous orbital elements, which is a bit less useful, and they are still going to be almost identical. Keep in mind that the distance between Earth and Moon never exceeds 1.3ls, whereas distance between either of these and the Sun is closer to 8.25lm. To within precision I could show in an image of the orbit posted to this thread, Earth and Moon are in the same exact place.

The instantaneous eccentricity might vary enough to be visible. I'll see if I can come up with an animation that shows it, if there is enough of an effect to be visible.

 

@Green Baron Absolutely no offense taken. 

Edited June 9, 2017 by K^2

[kerbiloid]

9 hours ago, Green Baron said:

... with an accretionary disk ? :-)

There is no "disk". Only particles. Particles have trajectories. Some of them (recurring ones) you call "orbits", Bunch of "orbits" lazy astronomers call "disk" 'cuz they don't care.

[Adstri]

When will it end?

[magnemoe]

On 9.6.2017 at 4:53 AM, kerbiloid said:

There is no "disk". Only particles. Particles have trajectories. Some of them (recurring ones) you call "orbits", Bunch of "orbits" lazy astronomers call "disk" 'cuz they don't care.

Careful, you might be tasked with naming all the particles in Saturn's ring. 
You get an MK1 pod to stay in and can refuel at titan orbit.  

[kerbiloid]

5 hours ago, magnemoe said:

Careful, you might be tasked with naming all the particles in Saturn's ring. 

With pleasure! Prepare your moleskine...

[Green Baron]

On 09/06/2017 at 3:53 AM, kerbiloid said:

There is no "disk". Only particles. Particles have trajectories. Some of them (recurring ones) you call "orbits", Bunch of "orbits" lazy astronomers call "disk" 'cuz they don't care.

Without wanting to go into explaining our senses of perception or discussing what "is" and what "is not", one of the underlying principles of natural science is to find structures and patterns and to look for the physical reasons that form those. So, imo, yeah there "is" a disk. And an explanation "why" there "is" a disk.

[kerbiloid]

Disk is just a non-material abstraction summarizing physical events created by particles. There are no physical events caused by disk.

[Green Baron]

So, there is no earth ?

Protoplanetary disk, accretion disk, rings around a planet, they exist. They are formed by physical effects and they do cause physical events. You and me for example. Or are you just kidding ? :-)

[kerbiloid]

13 minutes ago, Green Baron said:

So, there is no earth ?

"Earth" is a non-material abstraction to describe the cloud of physical particles, of which "it" "consists".
Its trajectory is just a superposition of their trajectories. Its gravity is just a superposition of their gravities.

We just use it to avoid excessive detalization.
Say, we don't always measure time in Planck units, sometimes we vulgarly say "about a week".
Even more, we just anyway need to round the result because any measurement has a limited accuracy. So, "day", "week" and so are just colloquial names for accuracy "classes",

18 minutes ago, Green Baron said:

Protoplanetary disk, accretion disk, rings around a planet, they exist.

They "exist" as ideas. Science doesn't deal with ideas, it deals with measurable events. Measurable events are caused not by "disk", "Earth", etc, but by physical particles of the "astronomical objects" interacting with physical particles of "measuring device".
Of course, "measuring device" is also just a nickname for another cloud of particles.

20 minutes ago, Green Baron said:

You and me

My (and probably your) body is just one more cloud of particles unpredictably moving in a strange manner.
(Though I'm not even sure if you are not just letters spontaneously appearing on "my" "monitor", btw).
What is "mind" and where it "is", still nobody of "us" knows exactly.

[Dman979]

Locked at OP request.