means The moment when the moon descends below the horizon and disappears from view, the lunar counterpart to a sunset.

from A transparent English compound of "moon" and "set," built on the same logic that gives us "sunset." The "moon" half traces back through Old English mona to a deep Germanic root, a distant cousin of Latin mensis (month) and Greek mene (moon) — a nod to the ancient habit of measuring time by lunar cycles. The "set" half comes from Old English settan, "to cause to sit," the same word behind a setting sun and a hen set on her eggs: the celestial body, in this poetry, simply takes a seat below the edge of the world. "Moonset" itself is a relatively modern formation, modeled directly on its older solar sibling.