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