long . lines and ripples

The Apparent Position of Callisto, Jupiter's Outermost Gallilean Moon

On Sunday night (August 22nd, 2021), this was the view from the Chicago South Side, looking southeast over the lake, of the four Galilean moons. They are easily visible through 10x30 binoculars. From a sketch I made around midnight:

Drawing of appearance of Gallilean moons of Jupiter, night of 2021-08-30
Colors black/white inverted

What attracted my attention about Callisto's position is that lies well outside the apparent plane of the other three moons. I usually see the visible Galilean lined up with one another, like we are looking at them edge-on from Earth, in the same orbital plane.

A quick illustration shows how the configuration in the sketch can occur. None of the four moons orbits in the exact same plane, although their inclinations are very close to one another. And the orbital plane of Jupiter is slightly tilted toward Earth. These factors mean that we see the orbits of the Gallilean moons at a slight angle to us, like we are looking at very narrow ellipse rather than single-dimension lines:

graphic of moon orbits
Orbital paths of Gallilean moons. Apparent tilt of moons' orbit relative to Earth is exaggerated for illustration. Actual tilt is between one and two degrees.

Callisto, the outermost moon, has a much larger orbit than the other three moons, meaning it travels up and down across the largest distance in both dimensions (see arrow in illustration), and giving it more latitude to reach an apparent position "above" or "below" the other moons, as we see it from Earth.