(n.) a satellite of the planet Jupiter
We should have a special category on this blog for words you’ll likely never, ever have any cause to use. Case in point, the word circumjovialist is a specific term for any satellite of the planet Jupiter.
Etymologically, the ‘circum–’ part here is fairly straightforward; it’s the same root as found in words like circumference, circumambulate (to walk all the way around something), circumambient (‘all-surrounding’), and circumbendibus (a roundabout way of doing something). This prefix comes from the Latin word circum, which literally means ‘around’ or ‘in a circle’, and is a fairly close relative of the word circus (in the sense that circus performances take place in a ring).
The ‘–jovial’ part here is a little more confusing, and something more of a story to tell.
As an adjective in its own right, jovial, meaning mirthful, has been in use in English since the early 1600s. Before then, it was used to describe anything inhabiting, resembling, or else occurring under the influence of the planet Jupiter; the two meanings are connected because mirthfulness was once associated with the influence of Jupiter in astrology.
Etymologically, jovial comes from Jove, an old poetic name for Jupiter. (The name Jupiter itself, incidentally, is a corruption of the Latin Jovus pater, ‘father Jove’, as Jupiter was considered the chief male deity in Roman mythology.)
Put these two elements together, then, and you have circumjovial—a word, with roots in Roman mythology, for anything that orbits the largest planet in our solar system.