A vocabulary of landscape — a landscape of words
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moonbow
A moonbow is a rainbow produced by moonlight. It works on exactly the same physics as a solar rainbow, with light refracting and reflecting inside falling water drops, and it appears opposite the moon in the sky. It is far dimmer, because moonlight is far dimmer, so to the eye it usually appears white or faintly gray, and the color only shows in a long exposure.
Etymology
The English name is a plain compound. The older term is lunar rainbow, and Aristotle recorded the phenomenon and noted correctly that it was rare and that he had seen it only twice in his life. The requirements are demanding: a bright moon near full, low in the sky, a dark sky, and rain or spray in the opposite direction all at once.
Notes
The most reliable places to see one are permanent waterfalls, where the spray removes the need for rain. Cumberland Falls in Kentucky and Victoria Falls both produce them regularly on the nights around a full moon, and both publish schedules. Human color vision falls back on rods in dim light, which do not distinguish color, which is why the bow looks white even though the color is there.