Two moons, circling

All the planets in the solar system orbit the Sun roughly in the same plane. That means that from the side, the orbits of the planets would all be very close to falling on the same line. Since we orbit the Sun on the Earth, we're on that line, too. As it happens, Jupiter's poles are almost perfectly perpendicular to the line. In other words, its equator lines up with the plane of the solar system... and its big moons also orbit the planet right above the equator. All this together means that when we look at Jupiter, the moons appear to orbit the giant planet on a line, too. They swing back and forth, moving "left to right" and "right to left" over time. In fact, if you watch Jupiter for long enough you'll certainly see a moon pass directly over the planet's face, and sometimes you can see ...