1300: Galilean Moons

Explain xkcd: It's 'cause you're dumb.
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Galilean Moons
I'm SO glad I escaped. They almost had me caught in their weird ...thing.
Title text: I'm SO glad I escaped. They almost had me caught in their weird ...thing.


The 1:2:4 orbital resonance of Ganymede, Europa, and Io

Megan has somehow acquired a set of Galilean moons similar to the four primary moons of Jupiter. The positions of the moons in the successive panels are reminiscent of the observations made by Galileo Galilei in 1610, which proved for the first time that objects in the heavens could orbit something other than the Earth (today these observations can be reproduced on successive nights by anyone looking at Jupiter with binoculars). As each of Megan's moons passes close to Cueball, it says something different:

  • Io, the innermost and second smallest, says "Hi!".
  • Europa, the second-innermost and smallest, says "What's your name?".
  • Ganymede, the third moon from Jupiter and the largest in size, interrupts Europa by shouting "MOOOOOON!" (this might be a reference to an old joke: Q: Knock Knock. A: Who's there? Q: Interrupting Cow. A: Interrupting Co— Q: MOOOOOOO!).
  • Callisto, the farthest from Megan, expresses its annoyance at the antics of the other three moons.

Because the inner moons orbit Jupiter faster (due to Kepler's Third Law), they pass by Cueball more often: Io ten times, Europa five times, and Ganymede twice over the course of the comic. In fact, the outermost crater-scarred moon Callisto appears to have passed its closest approach to Cueball just before the first panel (perhaps before Megan and her retinue had walked up to Cueball) and does not approach Cueball again until the tenth panel. At that point, due to some apparent exertion on Callisto's part, it leaves Megan's orbit and begins to orbit Cueball instead. This process could be seen as analogous to the capture of moons from one planet to another, which can happen in less stable systems than our solar system if two planets were to pass close to each other but is mostly just whimsical. The humor derives from attributing human characteristics to the moon Callisto in attempting to escape from the other three moons.

The title text refers to the unusual orbital resonance among the three inner Galilean moons: Io has an orbital period of about 1.78 Earth days, Europa 3.55 days, and Ganymede 7.15 days, putting them into a 1:2:4 resonance. Callisto, with an orbital period of 16.69 days, is not part of the resonant system. This is illustrated in the animated picture at right, where you may notice that all conjunctions between Io and Europa take place at the "12 o'clock" position and all conjunctions between Europa and Ganymede take place at "6 o'clock" position. You may also notice at the animated picture that, unlike in the fifth and ninth panels of the comic, the three moons are never on the same side of Jupiter at the same time. It is thought that this resonance came about as the moons migrated outward due to tidal acceleration; because the inner moons migrated more quickly, first Io caught up with the 2:1 resonance with Europa and then the two of them evolved outward in lockstep until Europa caught up with the 2:1 resonance with Ganymede. If the Jupiter system were to continue its current evolutionary path for long enough (several billion years), Ganymede would eventually catch up to the 2:1 resonance with Callisto and Callisto would also be trapped in the resonance, becoming the fourth member of a 1:2:4:8 system. The title text expresses Callisto's relief at escaping such a fate, describing the relationship among the other three moons as "their weird ...thing." Callisto also escapes a common practice among certain groups of humans in which the members greet each other with meaningless phrases, usually an inside joke, whenever they meet, which could also be described as "their weird ...thing." The word "orbit" could finish Callisto's sentence, as it can also mean a sphere of influence or interest.

Later, in 2264: Satellite, Jill was orbited in a similar way by an abandoned satellite.


[Megan, who is being orbited by four small floating balls, approaches Cueball.]
Megan: Check it out!
Cueball: What?
Megan: I've got Galilean moons!
[Io is at the point in its orbit closest to Cueball.]
Io: Hi!
[Io, which completes a full orbit in each panel, is again near Cueball, as is Europa this time.]
Io: Hi!
Europa: What's your name?
[Io alone again.]
Io: Hi!
[Europa returns to its position near Cueball with Io, and Ganymede joins them.]
Io: Hi!
Europa: What's your—
Ganymede: MOOOOOON!
[Io alone again.]
Io: Hi!
[Europa and Io again.]
Io: Hi!
Europa: What's your name?
Callisto: [on the other side of Megan] Ugh.
[Io alone again.]
Io: Hi!
Callisto: So annoying.
[Europa, Ganymede, and Io again.]
Io: Hi!
Europa: What's y—
Ganymede: MOOOOOON.
[Io alone again. Callisto nudges toward Cueball.]
Io: Hi!
Callisto: ...almost... ...almoooost...
[Io and Europa again. Callisto enters an orbit around Cueball.]
Io: Hi!
Europa: What's your name?
Callisto: Yessss!
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"most likely Callisto, the outermost of the four" - seems that it's definitely Callisto, since its drawn with little craters - no? (talk) (please sign your comments with ~~~~)

Hmmm. The animation just added agrees with another animation I've seen, in that the three innermost moons never line up all on one side of Jupiter at the same time. So if "Hi" (Io) and "What's your name" (Europa) conjoin on the right side as we're looking, then "What's your name" and "MOOOON!" (Ganymede) should conjoin on the left side. Not that I'm being critical of course...

Just some guy (talk) 05:39, 6 December 2013 (UTC)

Some javascript application available on the net to see the 4 moons orbits around jupiter

HmmmHmmm (talk) 06:48, 6 December 2013 (UTC)

If the inner moons are tidally locked with Jupiter, can you ostensibly state that they're mooning the outer moons, whenever two such moons line up? lol 08:57, 6 December 2013 (UTC)

Even with the resonance, "MOOOOOON!" appears still not to have been able to escape with that effect alone until Cueball's own close approach to Megan brought his own gravity well close enough to hers to give rise to a viable transfer orbit. And appears to be now retrograde, relative to its last orbit. Or possibly on a free-return path, unless Cueball steps back before the return transfer happens or makes an appropriate sideways move to quash the orbital potential. 09:34, 6 December 2013 (UTC)

Hang on... "MOOOOOON!" isn't the "Ugh/So annoying/Almost/Yes!" one. Forgot to note the hint of shading. Still, the above applies to the disgusted/elated moon, clearly not liking either of the Valley Girls or the loudmouth Jock. 09:38, 6 December 2013 (UTC)

Is it just me or does "MOOOOOON!" have a subtle "MOON MOON" undertone? 12:26, 6 December 2013 (UTC)

Am I the only one thinking that the "MOOOOOON!" is a reference to the "SPAAAAACE!" module from portal 2? (talk) (please sign your comments with ~~~~)

No -- Sian (talk) (please sign your comments with ~~~~)

I propose a simpler explanation for Ganymede saying "MOOOOOON!". Europa has asked Ganymede the same question, "What's your name?", every time they go by each other for eons. Ganymede is yelling the answer before it is even completely asked, exasperated at having to repeat it for the umpteeen gazillionth time. --uhillard (talk) 17:05, 12 December 2013 (UTC)

Ganymede interrupts Europa, reminding me of the knock-knock joke, "interrupting cow," "interrupting cow wh..." "MOOOOOO". Ganymede is an interrupting moon. ~~charles

Maybe moons converse with Cueball, not between them itself? 13:14, 6 December 2013 (UTC)

Or with Megan, when opposite to her face? 13:39, 6 December 2013 (UTC)

I think the moons are clearly conversing with Cueball. Remember that Io executes a full orbit between every panel. --BlueMoonlet (talk) 17:27, 6 December 2013 (UTC)
Animation is incorrect

The current animation has the wrong speed of the outermost moon, which is currently orbiting at a 5:1 ratio to the innermost. They should all line up along a vertical line once every four rotations. In fact the current animation never lines up all three moons at the same time (at least, not on the same side of the planet). -Greg 16:06, 6 December 2013 (UTC)

The animation is correct. If you look closely at only Europa and Ganymede, you'll see that they are also in a 2:1 resonance, with conjunctions always taking place at the "6 o'clock" position. Io and Ganymede are in a 4:1 resonance, with conjunctions taking place at 12 o'clock, 4 o'clock, and 8 o'clock. The comic is incorrect in having all three moons on the same side of Jupiter at the same time. That never happens in the actual system, though I don't mind it in the name of artistic license. --BlueMoonlet (talk) 17:27, 6 December 2013 (UTC)
The innermost orbit completes 5 rotations for each 1 of the outermost. How is that a 4:1? 18:16, 6 December 2013 (UTC)
I think you need to count more carefully. Start when both moons are at "12 o'clock". In the time it takes for Ganymede to get back to that position, I see Io go around 4 times. --BlueMoonlet (talk) 18:19, 6 December 2013 (UTC)
You're probably counting from 1 to 5, instead of from 0 to 4. I.e. When they're lined up to start, you could call that conjunction #1, but they've done 0 orbits. Wwoods (talk) 19:42, 6 December 2013 (UTC)
If it was five to one the planets would line up every other orbit of Ganymede

Did someone change the animation? Because when I watch it, they all line up on the right side. 22:44, 6 December 2013 (UTC)

Not the animation I've been seeing. Starting at an arbitrary point:
Io@12, Europa@12, Ganymede@3 (I+E Conjunction)
Io@8, Europa@10, Ganymede@2 (I+G Opposition)
Io@12, Europa@6, Ganymede@12 (I+G Conjunction, E Opposed)
Io@4, Europa@2, Ganymede@10 (I+G Opposition)
Io@12,Europa@12, Ganymede@9 (I+E Conjunction)
Io@8, Europa@10, Ganymede@8 (I+G Conjunction)
Io@12, Europa@6, Ganymede@6 (E+G Conjunction, I Opposed)
Io@4, Europa@2, Ganymede@4 (I+G Conjunction)
...then repeat
That's one cycle of Europa and Ganymede, two cycles of Io and Europa (relative to each other, alone) four cycles of Io and Ganymede (likewise).
There are two 'in-line' conditions, when Ganymede is in conjunction with one of the other moons, the remaining one in opposition, with Ganemedes other Io conjunctions having Europa off at an angle and the other Io/Europa conjunctions having Ganymede off at a right-angle. If I've managed to note all that down correctly. (Note, this is nothing to do with the following plot regarding the XKCD motions, which I quite admire!) 08:09, 7 December 2013 (UTC)

Hard to tell exactly which moon was which, until I plotted their cyclic orbits.
-- DaveC426913 (talk) (please sign your comments with ~~~~)

"You may also notice at the animated picture that, unlike in the fifth and ninth panels of the comic, the three moons are never on the same side of Jupiter at the same time." The animated picture doesn't match this text. In the animated picture it looks like the three moons are on the same side of Jupiter at least twice for each cycle of Ganymede. -- 04:26, 7 December 2013 (UTC)

It means they are never lining up on the same side of Jupiter. 23:53, 7 December 2013 (UTC)
That picture, which is very helpful, confirms that the three moons are all in conjunction with each other in panels 5 and 9 of the comic. In the animation, Io and Europa have their conjunctions at 12 o'clock, so (if the comic were correct) this picture would imply that Ganymede should sometimes also be at 12 o'clock during an I+E conjunction. In fact, the animation shows that Ganymede is always at either 3 o'clock or 9 o'clock during an I+E conjunction. --BlueMoonlet (talk) 18:25, 12 December 2013 (UTC)

Also note that "Io" looks like "Lo" in many fonts, so Io saying Hi is a probably a little Lo-Hi (Low High) pun. And Io passes 10 times, a Io-10 pun. And Europa saying "what's your name" is maybe a pun on Europa sounding like "You Are" a bit? 14:42, 10 December 2013 (UTC) Martin.

Animation is correct

Randall's Moon Meetings are just a part of the joke, all four Galilean Moons never would meet in that way in reality. Randall does not present science publications here, but just comics, playing with real things. --Dgbrt (talk) 22:28, 12 December 2013 (UTC)

Is it possible that a person having moons is a reference to Ioun Stones? (talk) (please sign your comments with ~~~~)