The Moon has its own rainbows, but your eyes usually refuse to see them 🌙

A moonbow is the lunar counterpart of a rainbow: sunlight reflected off the Moon, refracted and reflected inside raindrops, then bent toward your eye at roughly the same angle as a solar rainbow. The physics is identical — only the light source is different. ^[1][2]

The catch is brightness. Moonlight is sunlight scattered off the lunar surface, so it arrives far dimmer than direct sunlight. That faintness means the colours in a moonbow are usually too weak to trigger the colour-sensitive cone cells in your retina, leaving a pale, whitish arc instead of a vivid spectrum. ^[1][2]

Why the colours fade

Human colour vision depends on cone photoreceptors, which need a certain level of light to work well. In dim conditions, the more sensitive rod cells take over, and rods do not distinguish colour. So even when a moonbow's light does contain the full spectrum, your eye often registers it as grey or white. ^[2]

Where and when to look

Moonbows need the same geometry as rainbows: the Moon behind you, raindrops in front of you, and the Moon low in the sky. They are most often reported near waterfalls, where spray provides a constant curtain of droplets, and around full Moon, when reflected sunlight is strongest. ^[1][3]

• Look opposite the Moon, with your back to it and rain or spray ahead. ^[1]
• A long-exposure photograph can reveal colours your eye misses. ^[3]
• Cumberland Falls, Kentucky, is nicknamed for its frequent moonbows. ^[3]

A rainbow by any other light

Moonbows are not a separate optical phenomenon; they are rainbows formed when the Sun's light has taken a detour via the Moon. The same 42-degree geometry applies, and the same drop physics applies. The difference is purely one of intensity and how our eyes respond to it. ^[1][2]

That makes moonbows a useful reminder that perception is not the same as physics. The colours are there in the light; they are just below the threshold your colour vision needs. Cameras, with their ability to accumulate light over seconds, can sometimes show what the naked eye cannot. ^[2][3]