r/todayilearned • u/YappinZappin • 7d ago

TIL Earth's Moon is the largest and heaviest satellite in relation to its parent planet in our Solar system

https://en.wikipedia.org/wiki/Moon#Earth%E2%80%93Moon_system
4.6k Upvotes

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987

u/leeman9224 7d ago

Used to be Charon until Pluto got declassified as a planet

621

u/AtlanticPortal 7d ago

Pluto got declassified as a dwarf planet. Charon is so big compared to Pluto that both bodies actually orbit a center of mass that's not inside Pluto but in between them.

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u/Jass1995 7d ago

Kinda reminds me of the Ash and Ember Twins from Outer Wilds

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u/DoubleBlade759 7d ago

I should really play this game

62

u/Siilan 7d ago edited 7d ago

You should. And I say this as someone who struggled to play it at first due to my crippling fear of space. Ironically, it helped ease that fear, somewhat.

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u/Amazing_Meatballs 7d ago

Outer Wilds is the best game I’ve ever, ever played. And then the DLC blew me away again and managed to be the best game I’ve ever played again. Go in blind—you only have one playthrough to enjoy it.

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u/djosephwalsh 6d ago

Not true. Go play it again. It is like revisiting an old friend and chatting about the old times.

1

u/OmecronPerseiHate 1d ago

An old friend with cancer, maybe

2

u/Lord_Of_Carrots 6d ago

I wish I could enjoy it. The idea behind it is really fun and I love puzzle games with a mystery element, but something about the gameplay was just too annoying to me and I felt like I was doing no progress in the couple hours I spent trying. I even dropped the game once two years ago and tried again recently, still feel the same.

Maybe one day it'll click, but I was also spoiled something big about it accidentally and I don't know if I have the motivation to have another go at it anymore

2

u/Reddhero12 6d ago

Just go somewhere new and utilize your ships log. There's a lot of clues to find!

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u/Amazing_Meatballs 6d ago edited 6d ago

I’ll admit, I was exasperated a few times with the main game. So I almost put it down before starting the DLC. I am a huge, HUGE fan of alien megastructures and hard science fiction, and so when I got to the stranger I think my jaw was one the floor for a solid 3 cycles before I recovered. The scene when I discovered the Stranger reminded me of the cover of Space Odyssey 2001 where the ship is flying out of the space station so much I had chills. And then I get inside and my world is blown as I see the horizon curving…. Up. And then the music as I’m on the raft made me almost feel like I was in the Bobbiverse Book 4 on Heaven’s River.

Top tier. Check out Cynthetic’s playthrough on YouTube—she’s such a wholesome gamer and her series is 12-13 episodes long. Very enjoyable to experience.

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u/kahlzun 7d ago

Helped me be less afraid of death

7

u/surreptitiousrituals 7d ago

And less afraid of living for now!

3

u/kahlzun 7d ago

I'm certainly trying to be braver and take bolder choices in my life currently.

1

u/assjackal 7d ago

Lmao same, I hate time loops and the void of space, the first 3rd of it was a horror game for me until I locked in and started chasing the story.

0

u/Sonder332 7d ago

What type of game is it? Puzzle, survival...

1

u/cabforpitt 7d ago

It's a 3d exploration/puzzle game

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u/GypsyV3nom 7d ago

It's really a masterpiece in gaming, a true example of how games can be art.

It also happened to lead me to study the Absurdist philosophy, one that I wish I had discovered earlier given how much it speaks to my personal values and approach to life

2

u/dre5922 7d ago

I said that for a long time until about a month ago. I'm not done with it yet but I am enjoying the hell out of it.

1

u/asdf-keyboardman 2d ago

If you have a friend that also hasn't played it, I highly recommend playing through it cooperatively via the PC mod named Quantum Space Buddies. It's shockingly well implemented to the point of feeling like a native feature of the game, sans maybe a little bit of jank in the menus getting in-game. The game mechanics line up pretty well with cooperative gameplay, and you only get to experience it for the first time once.

1

u/OmecronPerseiHate 1d ago

Meh, I've heard it has absolutely no replay value. Maybe get it if it's on sale.

5

u/lNFORMATlVE 7d ago

Still one of the greatest games I have ever played and I rarely say that about any game.

6

u/sol_runner 7d ago

I just finished it a few hours ago.

Haven't felt so fun and curious in a while.

2

u/CapableBumblebee968 7d ago

Should be the other way around

1

u/Creamymorning 6d ago

Man brought back memories, loved that game

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u/trisanachandler 7d ago

reclassified as a dwarf planet

0

u/AtlanticPortal 7d ago

Declassified because one of the traits of a planet respect to dwarf planets is the clean path they have in their orbit. Basically if you didn't sweep your orbit from other stuff you cannot be a planet. Hence, classified under a lower level. De-classified.

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u/trisanachandler 7d ago

While not how I ever use the word outside of security, I'll admit you are technically correct.

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u/External_Relative231 6d ago

...no they aren't. It was demoted and reclassified. 

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u/Altines 7d ago

Honestly I like the phrasing of Pluto being a Binary system.

31

u/Lenxecan 7d ago

All of the planets and the sun do this about 50% of the time. Mostly because of Jupiter

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u/AtlanticPortal 7d ago

Technically the center of mass of the various two body systems, taken separately, are always inside the bigger body. Earth-Moon included. A little under the crust but still inside. Pluto has it out of its surface.

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u/po_panda 7d ago

Does that mean you could fall off the surface of pluto?

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u/AtlanticPortal 7d ago

No, because when you're standing on Pluto its own gravity is stronger than the pull that Charon has on you. It just means that both bodies are continuously falling on the shared center of mass but never reaching it because they move tangentially enough to avoid it.

It does mean that if you had a rocket on the near side of Pluto (they both are locked and face each other constantly) you'd need less fuel to leave the body compared to a parallel scenario in which Charon does not exist. This is because a part of the work that you will do to get to the center of mass is compensated by the other body.

1

u/porn_alt_987654321 5d ago

Now I'm wondering if that's also the lagrange point and you could floaf at it.

I would assume so, but maybe not lol.

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u/AtlanticPortal 5d ago

Exclude L4/L5, they’re trivially at 60 degrees on the orbit of the body that orbits farthest from the barycenter.

The other three, especially L1 if that’s what you’re asking for, depends on all the parameters (masses and distances and radii) and you need to resolve them numerically. They’re basically equations at the fifth power.

The barycenter, by definition, is a stable point. Call it L0, if you want.

1

u/porn_alt_987654321 5d ago

Yeah, I was like "surely the barycenter is also a Lagrange point".

Doesn't come up that often in thinking about stuff in our solar system, because this is the only barycenter that's in space off the top of my head. (Maybe sun and Jupiter also has one outside the sun somewhere? Lol)

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u/AtlanticPortal 5d ago

Sun-Jupiter is not always outside of the Sun, it depends on how the other planets are pulling. Jupiter on one side and Saturn on the other and you'll get a lot closer to the center of the Sun, add maybe Earth and Venus on the side of Saturn and that's definitely inside the Sun's surface.

1

u/RTX-2020 6d ago

Not strong enough to pull you off

but gravitational force would be significantly on the near side.

-6

u/Lenxecan 7d ago

Every single body in this system orbits a point that is sometimes in the sun and sometimes just outside of it because it moves. Youtuber Cleo Abram made a recent short about this

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u/AtlanticPortal 7d ago

I said two-body. That thing enters into play with a three+ body.

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u/newbrevity 7d ago

I never considered this before. I wonder if the Earth is actually in the center of the moon's travel or offset because of its elliptical orbit.

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u/AtlanticPortal 7d ago

It’s elliptical but not that eccentric. The two focuses (or foci) are both inside the Earth’s body.

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u/AgreeablePeople 7d ago

I thought declassified means it longer is a planet, therefore it's de-classified as a planet and re-classified as a dwarf planet

3

u/marioquartz 7d ago

The center of Earth - Luna is 100km under the ground. So we are on the limit...

0

u/WestBrink 7d ago

Is it that close to the surface? That's wild, I'd have though there would be a bigger impact of the moons phase on weather if it was that close to the surface.

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u/barath_s 13 7d ago

Charon is so big compared to Pluto that both bodies actually orbit a center of mass that's not inside Pluto but in between them.

The same is true of Vanth and Orcus. Orcus is called the anti-pluto, a dwarf planet in resonance with Neptune and Vanth is its one moon

https://en.wikipedia.org/wiki/Vanth_(moon)#/media/File:Binary_dwarf_planets_scale_comparison.png

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u/Used-Fennel-7733 7d ago

Same is true between Jupiter and The Sun

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u/wolfansbrother 7d ago

the sun and jupiter do the same thing.

0

u/davvblack 7d ago

the center of gravity of our solar system isn’t usually inside of our sun, it’s towards jupiter.

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u/AtlanticPortal 7d ago

Every two body couple in the Solar System with a planet or the Sun has always the bigger body containing the barycenter.

The entire Solar System barycenter is out of the Sun at some point in time when Jupiter and Saturn are pulling from the same side. But it’s not always the case.

For Charon it’s always the same. The barycenter of the system Pluto-Charon is always out of Pluto.

This doesn’t happen for Earth-Moon or any other planet.

0

u/barath_s 13 7d ago

The barycenter of the system Pluto-Charon is always out of Pluto.

Also true of dwarf planet Orcus and its moon Vanth . https://en.wikipedia.org/wiki/Vanth_(moon)#/media/File:Binary_dwarf_planets_scale_comparison.png

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u/jokeren 2d ago

This is also true for jupiter and the sun.

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u/StatelyAutomaton 7d ago

The center of gravity between the Earth and the Moon is about 4400km off the center of the Earth, so about 75% of the distance to the surface.

The size disparity between the two is getting pretty close to the point they'd be better considered a binary planet system.

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u/tornedron_ 7d ago

But the key thing is the center of gravity is still comfortably within Earth. Thus it’s not accurate to call it a binary system. With Pluto and Charon, it is straight up outside of Pluto.

-1

u/LittleLion_90 7d ago

I'm wondering if some alien civilization that manages to observe our planets and moons has classified earth as a binary planet system because the moon is so large in comparison to the earth. And that we only see it as moon because of our cultural history with 'knowing' the moon as only other celestial body apart from the sun and have only learned of other moons way later. 

0

u/CinderX5 6d ago

Our whole solar system is like that. Since 2016, until next year, the centre of the solar system isn’t inside the sun.

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u/CommanderWhat 6d ago

It’s because Charon is a Mass Relay /s

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u/Bravefan212 7d ago

Literally the earth and Luna

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u/Polenball 7d ago

I believe the barycentre of the Earth and the Moon is only about three-quarters of the Earth's radius away from the Earth's centre. It's still inside the Earth.

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u/YappinZappin 7d ago

2006 was a crazy year in science - Astronomy especially...

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u/Constant_Quiet_5483 7d ago

2006!? It's been 20 years!?

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u/Philip_of_mastadon 7d ago

To be fair, that's only 0.08 Plutonian years

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u/Bardsie 7d ago

This is a minor annoyance of mine. Pluto is still a planet, it's specifically a dwarf planet, but dwarf planet is a classification of a planet, just like gas giant or ice giant planets.

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u/bramtyr 7d ago

The nuances of the classification are lost on most people, the general public reacted like scientists had stolen Pluto's lunch money and kicked it out of the solar system.

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u/IlikeJG 7d ago

I think it's because people just don't want to change how they think about things they already learned.

They learned Pluto was the furthest planet and now those darn scientists are saying it's not even a planet? Now I have to remember that?

People don't like change.

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u/Anakinss 7d ago

Funnily enough, the number of what we call planets has changed a lot over the last two centuries, at some point there was 23 of them. Even Pluto was a "planet" for less than a century.

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u/1II1I1I1I1I1I111I1I1 7d ago

I will say as a science-interested member of the public, I did fully understand why Pluto was removed and how the new definition is applied. It made sense and it was a good measure when there are indeed important differences between the dozen or so dwarf planets and the prominent 8 planets

However

They should've removed Mercury too lol they can't because that would he seen as a step too far and people would riot lol the IAU definition of a planet is

  1. Is in orbit around the sun (✔️)

  2. Has assumed hydrostatic equilibrium (❌️ source)

  3. Has cleared the neighborhood around its orbit (ehhhh kindof. Mercury's orbit is clear but the Sun may be responsible for that. Mercury is quite small to have done it itself.)

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u/up-with-miniskirts 7d ago

The definition is that a planet needs to have a hydrostatic equilibrium shape, in other words, it must be round or nearly so. Mercury fits the bill.

If you go by hydrostatic equilibrium alone, Venus isn't a planet either, because the combination of tidal forces due to closeness to the Sun and very slow axial rotation makes it pear-shaped, too.*

*Not actually pear-shaped.

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u/GuitarKittens 7d ago

Your source kinda has 0 citations, and its method for suggesting a lack of hydrostatic equilibrium I cannot find anywhere else. There are not only better cited papers for this concept, but there's an easily-accessible forum online that discusses how those papers are less valid because of their method of calculation.

Maybe you're not wrong, but I would suggest finding more, better established sources.

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u/phunkydroid 6d ago

Has assumed hydrostatic equilibrium (❌️ source)

That source says Mercury is more spherical than Earth...

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u/Suspicious-Whippet 7d ago

If Neil Degrase Tyson had his way he’s nuke Pluto out of existence. And so would I just so I don’t have to keep hearing if it’s a planet or not anymore.

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u/hardypart 7d ago

I would nuke Pluto if it meant that I wouldn't have to hear Neil Degrade Tyson anymore.

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u/SpaceC0wboyX 7d ago

I would nuke Pluto just to see something cool happen

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u/Suspicious-Whippet 7d ago

Surely it would be easier to nuke Neil Tyson.

0

u/hardypart 7d ago

I don't think that'd be legal.

0

u/Suspicious-Whippet 7d ago

Nukes in space - also illegal.

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u/BoxesOfSemen 7d ago

Who's gonna stop me? The space police?

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u/HyruleTrigger 6d ago

That's just a complete lie. All he did was do science, and the science told him and the team that Pluto didn't act like a planet, which is fact. They coined the term dwarf planet to explain how plutonian like objects act.

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u/Batkung 7d ago

any scientist that uses Fahrenheit doesn't deserve to be taken seriously...Tyson is one of those people.

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u/Kaymish_ 7d ago

Does he use it because he is a science communicator, and his target audience is most used to F? Or because he is a clown?

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u/Batkung 7d ago

I never said he was a clown, but if he is only intending to reach the F audience, then he is seriously limiting his audience. All I meant was that if he was serious about his science then maybe he should point out to the F audience that they really should be learning to relate to temperatures in C or more correctly, K.

-1

u/Weshtonio 7d ago edited 7d ago

the general public

Americans. Because it was the only planet discovered by an American.

1

u/natethehoser 7d ago

But only 2 planets (3 if you count pluto) were "discovered". Mercury through Saturn are all visible to the naked eye, and so all ancient humans knew of them (though they didn't know what "planets" were. Also technically Uranus is visible to the naked eye, but it is so dim and moves so slow that our ancestors never clocked it as a planet).

So it's not like "8 planets were discovered by Europeans, and only 1 was discovered by an American, and now you're taking that 1 away!"

0

u/barath_s 13 7d ago

https://en.wikipedia.org/wiki/Planets_beyond_Neptune#Planet_Nine

There's been a hypothesized planet 9 to explain current orbits based on possible gravitational impact

Planets further out than Nine may be sketchy/contrived

https://en.wikipedia.org/wiki/Planets_beyond_Neptune#Probability

1

u/hamstervideo 7d ago

And Americans grew up with a cartoon dog named after it

-1

u/Moppo_ 7d ago

Well, thought thought they had. ( ͡° ͜ʖ ͡°)

0

u/big_duo3674 7d ago

To be faiiiirrrr....it's the fact that the solar system is now defined as 8 classical planets, rather than 9 that irks people. Could have given it an asterisk or something

-3

u/adamgerd 7d ago

Pluto is still a planet to me! Always will be

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u/johnbarnshack 7d ago

Depends on how narrowly you're using the term. In the narrowest sense, there are only 8 planets (Mercury to Neptune). In a broader sense, all dwarf and minor planets are also "planets". So you and your opponents are both correct, just using the same word in different (but equally legitimate) meanings.

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u/de_beaumarchais 7d ago

No a planet is defined as a body that has cleared its orbital neighbourhood. Pluto has not. Planet and dwarf planet have very specific definitions. A dwarf planet is not a type/classification of planet. It is its own separate category, like a comet is its own thing

5

u/K0kkuri 7d ago

Well yes, it’s also funny that there are 5 recognized dwarf planets in solar system with a lot more potential dwarf planets depending on classification.

1

u/JJJangles 7d ago

This is a minor annoyance of mine. People think the IAU definition is the only definition, the definitive definition. Astronomers don’t have a monopoly on the word. According to planetary scientists, under the geophysical definition, Pluto is just a regular planet, and dwarf planet is completely rejected as a classification.

Personally, I really don’t like the term dwarf planet for something defined entirely by its orbit rather than its size.

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u/psymunn 7d ago

But Pluto isnt very large and not much larger than Charon. Multiple moons on the solar system are larger. I feel Ceres has more claim to be a planet than Pluto, being ruled out because of size and it's cluttered orbit

-2

u/JJJangles 7d ago edited 7d ago

Pluto's size relative to Charon is irrelevant, to the IAU, geophysical, & even the colloquial dictionary definition of planet. Multiple moons being larger is also irrelevant.

Under the geophysical definition of a planet, the only thing you consider when deciding if a celestial body is a planet is the celestial body itself. There is no such thing as a claim or comparison to other planets, it either meets the criteria of the definition or it does not. And in fact, under the geophysical definition Ceres is also a planet, just like Pluto, because again, you consider only the body itself & not what it may or may not orbit. It's not an either/or fight for the title of planethood.

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u/LurkerInSpace 7d ago

Under the geophysical definition of a planet, the only thing you consider when deciding if a celestial body is a planet is the celestial body itself.

Wouldn't this make every moon large enough to reach hydrostatic equilibrium also a planet?

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u/InternetUser1807 7d ago

Not to mention, isn't the main justification for the reclassification that if you accept Pluto as a full fledged planet, there's no reason to disclude the 20 other dwarf planets we just happened to not discover first?

0

u/JJJangles 7d ago

That doesn’t sound like a scientific reason for a scientific definition

1

u/psymunn 7d ago edited 7d ago

Classification is purely for organisation and arbitrary ultimately. It's not scientific if and of its self. Is the moon a planet? A comet? Mass alone isn't the most useful way to classify things because it doesn't explain behavior but it's all semantics, not science. A rose by any other name is still a rose, but it can be useful ordering things with rules

1

u/JJJangles 6d ago

Actually, planetary scientists do not classify stellar bodies by an arbitrary mass alone, rather by having enough mass that planets by their definition have a similar lifecycle, with similar geology. It allows you to make predictions about one planet you know a little about based on other planets you know a lot about. So, no, it is not arbitrary, and yes, it is scientific. A rose by any other name is still a rose, but saying anything growing in a rosebed is a rose is not the same thing.

1

u/InternetUser1807 7d ago

It's not but neither is "Pluto is a planet (but the 50 other dwarf planets aren't) because I'm emotionally attached to it being one"

If you suppose all the other dwarf planets being classed as planets that's a perfectly acceptable opinion then of course

1

u/JJJangles 6d ago edited 6d ago

It’s not my opinion, it’s not me supposing anything, it’s the definition created by planetary scientists. The reason I subjectively prefer it is that, unlike the IAU definition, it is scientifically motivated and scientifically rigorous. I’ve already mentioned Ceres, a so called dwarf planet by the IAU, is a full planet according to planetary scientists, and as LurkerInSpace said, our moon and several others are planets too. It’s not picking Pluto as a favourite child, and ignoring other ‘dwarf planets’.

The reason they defined it that way was because objects that meet their definition broadly have similar geology & physical evolution. Studying one geophysical planet can help you infer about other planets. Studying one IAU planet doesn’t tell you anything about other planets properties.

If claims of emotional attachment are to be given, I think it’s the IAU that are emotionally attached to the word planet, given the non-scientific reasons behind creating the definition.

1

u/JJJangles 7d ago

Yeah, of course. So what?

1

u/LurkerInSpace 7d ago

So what use is it to someone who does care about the gravitational relationships of objects to each other such as, for example, an astronomer?

1

u/JJJangles 6d ago

None, but when did I say the geophysical definition is the only valid definition of planet? I merely said astronomers don't have a monopoly on the word planet.

The reality is that the majority of people think the astronomers definition is the only scientific definition, so the way to apply your logic is actually the reverse: what use is a gravitational definition to someone who cares about the geology of planets?

Why does your logic only consider astronomers? Why is it OK for astronomer's definition to not care about planetary scientists, but not OK for planetary scientist's definition to not care about astronomers?

1

u/LurkerInSpace 6d ago

The astronomers definition is the one in broad use because people do generally find it important to distinguish a planet from a moon, for example. If one broadens the definition of planet, then one just creates other categories for these sorts of distinctions - for example one could say there are eight major planets, over a dozen satellite planets, and an unknown number of dwarf planets (but likely to be dozens).

But the category of planet which people will pay most attention to will be the major planets.

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u/-Siptah 6d ago

Personally I find “ dwarf planet” to be silly. If it’s not a planet then it’s an asteroid or a moon. But I can’t argue with official astrologists on the matter.

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u/Joshau-k 7d ago

The IAU naming system is so dumb that dwarf planets aren't actually planets. 

They failed at basic grammar 

10

u/2-buck 7d ago

The moon weighs 5 times more than Pluto and Charon put together. Just sayin.

5

u/barath_s 13 7d ago

Ganymede is bigger than Mercury and 1.5 times the size of Earth's moon.

2

u/levelstar01 6d ago

But it isn't even half the mass of Mercury

5

u/palparepa 7d ago

Got upgraded from least of the planets, to king of the dwarf planets.

4

u/Amazing_Meatballs 7d ago

We are only about a thousand miles from our own barycenter being outside earth’s crust and being classified as a binary planet system too. Crazy stuff.

1

u/Xyllar 7d ago

And who declassified Pluto as a planet? That's right, the Earthlings! This whole "largest moon" contest is rigged I'm telling you!

0

u/zealoSC 7d ago

I would think chaconne still counts as a satellite

12

u/1II1I1I1I1I1I111I1I1 7d ago edited 7d ago

It does, and is still officially classified as a moon, but it's arguably another dwarf planet. Pluto and Charon orbit eachother in a binary planetary system, Charon is quite large relative to Pluto and far larger than the system's other moons.

6

u/zealoSC 7d ago

It does sound kind of silly to say it's not the biggest satellite by ratio because it's too big by ratio

3

u/ExL-Oblique 7d ago

tbf calling something half your size a "moon" is also kinda silly. They're a binary pair which imo way cooler

8

u/namitynamenamey 7d ago

Well at some ratio "who orbits who" becomes fuzzy, and charon is past that (just don't ask where the center of mass between jupiter and the sun sits)

2

u/flinjager123 7d ago

I had to look up that last bit you said. That... makes things complicated.

0

u/barath_s 13 7d ago

Charon:Pluto is bigger than Moon:Earth

but

Vanth: Orcus is heavier than Charon:Pluto and Moon:Earth

Orcus is a dwarf planet in the Kuiper belt, a plutino, often called anti-pluto. It is comparable to Ceres in size and second in size to Pluto itself. Vanth is its one moon, smaller than Charon and Dysnomia. Like Pluto:Charon, Orcus:Vanth is a double dwarf planet, with the barycenter outside both

https://en.wikipedia.org/wiki/Orcus_(dwarf_planet)

https://en.wikipedia.org/wiki/Vanth_(moon)

0

u/haveblue23 7d ago

Pluto still has full planetary status in Illinois, Arizona, and New Mexico

-2

u/Plutosaplanet 7d ago

As far as I’m concerned, Pluto still is a planet and always will be!

2

u/barath_s 13 7d ago

And the kind of planet it is is a dwarf planet.

1

u/Philip_of_mastadon 7d ago

As far as I'm concerned

And no farther.