What is a moonraker? Also called a moonsail, a moonraker is the small sail, sometimes set in light winds, above the skysail.
What is the skysail? Used in a favorable light wind, it’s the light sail above the royal.
What is the royal? Also used in a favorable light wind, it’s the small sail above the topgallant sail.
What is the topgallant sail? It’s the sail above the topsail. Sometimes divided into upper topgallant sail and lower topgallant sail (depending on the era of the ship).
What is the topsail? It’s the sail above the course. Sometimes divided into upper topsail and lower topsail (depending on the era of the ship).
What is the course? The sails that hang from the lower yards of a square-rigged ship, now usually restricted to the foresail (the principal sail set on the foremast and the lowest on that mast) and mainsail (the lowest and largest sail on the mainmast, pronounced mains’l).
So, what is a moonraker? It’s that tiny sail six or even eight sails up. Generally only used on tall ships built for speed. I enjoyed flipping through A Sea of Words: A Lexicon and Companion to the Complete Seafaring Tales of Patrick O’Brian (by Dean King with John B. Hattendorf and J. Worth Estes) to figure it out.