The water nymph Rusalka sings to the moon of how much she yearns for the one she loves.
A much darker take on the Little Mermaid that much predates the Walt Disney version and is also based on a fairy tale, Rusalka is a water nymph or sprite who falls in love with a human prince. He, of course, has no idea she loves him. She sings wistfully about her hopeless love to the moon and then goes to the evil witch--who agrees to give her legs in exchange for her voice. The result, unsurprisingly, is tragic.
Although Dvorak is known today primarily for symphonic and chamber music, he also wrote 10 operas. Rusalka was his penultimate opera, written in the last few years of his life.