A moonbow (also known as a moon rainbow or lunar rainbow) is a rainbow (concentric arcs of colors produced on a screen of water droplets) produced by moonlight rather than direct sunlight. Other than the difference in the light source, its formation is the same as for a solar rainbow: It is caused by the refraction and reflection of light in many water droplets, such as a rain shower or a waterfall, and is always positioned in the opposite part of the sky from the Moon relative to the observer.

A moon bow over the town of Kihei, seen from Kula, on Maui, Hawaii, in February 2016. Source: Wikicommons, Author: Arne-kaiser.

Moonbows are much fainter than solar rainbows, due to the smaller amount of light reflected from the surface of the Moon. Because the light is usually too faint to excite the cone color receptors in human eyes, it is difficult for the human eye to discern colors in a moonbow. As a result, a moonbow often appears to be white. However, the colors in a moonbow do appear in long exposure photographs. Because of this lesser light intensity, moonbows are much less common than solar rainbows and are dependent on the phase of the moon, being more likely with a brighter full moon.