is an important "rule" in photography -- and one that works.
It states that the proper exposure for an object lit by the Sun (at noon, near sea level and on the equator, but almost anywhere on civilized Earth is about close enough ) will be f16 at a shutter speed equal to the reciprocal of the ISO speed.
Hence, with ISO 100 film or digital, the proper exposure for a sunlit photo would be 1/100th at f16 or equivalent; 1/200th at f11, 1/400th at f8, etc.
And what is Mr. Moon lit by?
That's right, the Sun! So a proper exposure for the moon is Sunny 16, but you can generally still hold enough detail in it at 1 stop over, so you can use Sunny 11 too.