Rainbows do not appear over moons. Rainbows are caused by the refraction and reflection of sunlight in water droplets. The angle of the sun, the raindrops, and the observer's eye must be just right for a rainbow to form. The angle of the moon is not compatible to create a rainbow. Instead, the Moon can cause a moonbow, which is a rainbow created at night by moonlight, if all of the conditions for a rainbow are present.