subiri

Subiri is a genuine build, and one you can take apart with no dictionary at all: sub under plus iri to go. To go under. The sun goes under the horizon, so la suno subiras - the sun sets.

That is the whole word. English hides the same picture behind an unrelated verb ("set") that a learner has to memorise; Esperanto shows its working. Compare leviĝi, to rise, for the other end of the day.

The noun is sunsubiro, sunset - three roots stacked (sun + sub + ir) plus the noun ending, and still perfectly readable. Sunleviĝo is sunrise on the same pattern. Prepositions used as prefixes like this are ordinary word-building, not a special construction; the compounding article has the general rule.

Subiri is not only for the sun. A ship, an empire or a reputation can subiri too, exactly as English things can go under.