Editing 1300: Galilean Moons

{{comic
| number    = 1300
| date      = December 6, 2013
| title     = Galilean Moons
| image     = galilean_moons.png
| titletext = I'm SO glad I escaped. They almost had me caught in their weird ... thing.
}}

==Explanation==
Meghan has somehow acquired a set of {{w|Galilean moons}} similar to the four primary moons of {{w|Jupiter}}.  The positions of the moons in the successive panels are reminiscent of the observations made by {{w|Galileo Galilei|Galileo}} 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 Meghan's moon passes close to Cueball, it says something different.  Io, the innermost and smallest, says "Hi!"  Europa, the second-innermost and second-smallest, says "What's your name?"  Ganymede, the third moon from Jupiter but the largest in size, interrupts Europa to exclaim "MOOOOOON!"  

Because the inner moons orbit Jupiter faster (due to {{w|Kepler's laws of planetary motion#Third law|Kepler's Third Law}}), they pass by Cueball more often:  Io ten times, Europa five times, and Ganymede twice.  In fact, the outermost crater-scarred moon Callisto appears to have passed its closest approach to Cueball just before the first panel (perhaps before Meghan 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 Meghan'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. 

[[File:Galilean moon Laplace resonance animation.gif|thumb|365px|The 1:2:4 {{w|orbital resonance}} of {{w|Ganymede (moon)|Ganymede}}, {{w|Europa (moon)|Europa}}, and {{w|Io (moon)|Io}}]] <!-- DO NOT set the width to anything lower than 365px, as this breaks the animation for some -->

The title text refers to the unusual {{w|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 movie 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 in the movie that, unlike in the fifth panel 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 {{w|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."

==Transcript==
:[Megan approaches Cueball, orbited by four small floating balls.]
:Megan: Check it out!
:Cueball: What?
:Megan: I've got Galilean moons!

:[The moons converse as Io and Europa orbits synchronize.]
:Io: Hi!
:Europa: What's your name?

:[They are separated before Io can answer. Io stands alone now.]
:Io: Hi!

:[Europa returns to her position near Io, and Ganymede joins them.]
:Io: Hi!
:Europa: What's your—
:Ganymede: '''''MOOOOOON!'''''

:[Io is once again alone.]
:Io: Hi!

:[Europa and Io are together again, this time absent Ganymede.]
:Io: Hi!
:Europa: What's your name?
:Callisto: ''Ugh''.

:[Once more, Europa leaves Io alone before she can answer.]
:Io: Hi!
:Callisto: ''So annoying''.

:[Europa and Ganymede re-join Io.]
:Io: Hi!
:Europa: What's y—
:Ganymede: '''''MOOOOOON'''''.

:[Io is on her own once again. Callisto nudges herself toward Cueball.]
:Io: Hi!
:Callisto: ...''almost''...
:Callisto: ...''almoooost''...

:[Io and Europa are reunited, Ganymede by himself on the opposite side of Megan. Callisto enters an orbit around Cueball.]
:Io: Hi!
:Europa: What's your name?
:Callisto: ''Yessss!''

{{clear}}
{{comic discussion}}
[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Megan]]
[[Category:Astronomy]]