Meaning: The sun goes down below the horizon, which is the line where the sky and the earth meet.
Choice of Words: "Sun" is the star at the center of our solar system. "Sinks" means to go down or below the surface of something. "Below" means at a lower level or position. "Horizon" is the line where the sky and the earth or sea seem to meet. "Rim" is the outer edge of something.