Moonraker (sail)

The highest sail is moonsail, lower sky sail and royal sail

A moonraker, also known as a moonsail, hope-in-heaven, or hopesail, is a sail flown immediately above the skysail (see sail-plan) on the very top of the royal mast on large, square rigged sailing ships of the clipper era. None of the four- and five-masted square rigged ships carried a moonsail.

This is a relatively unusual sail only used on ships built primarily for speed.