Last night was my first night watching the sky with a telescope, and I got to see the Galilean moons of Jupiter for the first time right away. Pretty awesome sight. Which led me to find out why they are aligned in a quasi-straight line.

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Turns out this alignment is pretty profound, and reveals a lot about the Solar system. Jupiter's moons are all on the same plane because of the origin of the solar system and conservation of momentum which flattens the cloud of dust orbiting the Sun into a disc the same way that a yo-yo spinning about you ends up farthest from you in a plane of rotation that includes you as the center of the rotation. Ultimately, I think it's gravity that pulls them toward the center and condenses the cloud into a plane. This is the same reason the planets are mostly in a plane. 

The fact that all 4 are on the same line on my first day of seeing them is no coincidence: it's always (mostly) true, due to the fact that the Earth is on the same plane of translation of the moons, which is probably related (through the common origin of Jupiter and its moons) also to the fact that the axial tilt of Jupiter is relatively small: only 3.13°. As a result this planet does not experience significant seasonal changes, in contrast to Earth and Mars for example.

The fact that they do align at all is not: they orbit in a pattern called Laplace resonance: for every four orbits that Io makes around Jupiter, Europa makes exactly two orbits and Ganymede makes exactly one. This is a stable pattern or attractor in the space of all orbits. Think of it this way: if the moons were slightly off each other, they would attract each other so the one that's a little faster than aligned gets slowed down, and the one that's a little slower gets sped up, and this repeats itself on each successive orbit until their orbits are multiples of each other so that every time they are close they are perfectly aligned. 2:1 orbital ratios are stable too because when two orbiting bodies are exactly opposite each other, this effect does not pull in either direction. 4:1 should not be as stable but the gravitational attraction between moons when a quarter of an orbit away is probably small enough that the primary effect is just the stabilizing attraction when the planets are close to each other. 

The other interesting thing about Jupiter is Jupiter's rotation is the fastest of all the Solar System's planets, completing a rotation on its axis in slightly less than ten hours; this creates an equatorial bulge easily seen through an Earth-based amateur telescope. Probably related to this through the speed of the original stellar cloud that gave birth to Jupiter and most of its moons, Jupiter's moons orbit really fast around Jupiter, too, with Io taking only 1.77 days to go around Jupiter. This makes for a very lively observation, as the positions of the moons change visibly and very significantly every night. Pretty amazing that we can see these moons move so fast so far away.