A moonbow is a rainbow produced by moonlight instead of sunlight, usually appearing ghostly white to the naked eye.
Not attributable - a transparent English compound (moon + bow, on the model of rainbow). It is not an old word, though: the OED's earliest evidence is from 1871, in George MacDonald, and Wiktionary's earliest quotation is James Wells, Travel-pictures from Palestine, 1896.
// Where it sits
The spectrum is all there. The failure is at your eye, not in the sky.
What it really means
A moonbow is a rainbow made of moonlight, and it is almost always described as colourless — a pale ghost arc, all the drama of a rainbow with none of the spectrum.
The colours are there. Every one of them. Moonlight is sunlight that bounced off the moon, so it carries the same spectrum, and the physics inside each raindrop is identical: refract on entry, reflect off the back, refract on exit, and split. A long-exposure photograph records the full colour.
What fails is your retina. Colour vision uses cones, which need reasonable light levels; in the dark you switch to rods, which are far more sensitive and completely colour-blind. So the arc is genuinely there in full colour and you are genuinely unequipped to see it. The whiteness isn't a property of the moonbow — it's a specification of the observer.
The conditions are unforgiving. The moon must be below about 42 degrees, nearly full, with rain or mist opposite it and a genuinely dark sky. That last requirement is doing more work every year: Ernest Sternberg of the University at Buffalo wrote in 2018 that light pollution now obscures the moonbow once visible in the mist at Niagara Falls. The waterfall is still there. The optics still work. There's simply too much artificial light for anything that faint to register.
Which makes moonbows one of the few natural phenomena you can lose without anything physical changing.
Where it comes from
'Bow' here means the arch shape, not a ribbon or a weapon — the same element hiding inside rainbow and elbow.
Myths & misconceptions
Moonbows are white — they have no colours.
All the spectral colours are physically present. The limitation is your retina: in low light you see with rods, which aren't colour-sensitive, so the arc reads as white. A long-exposure photograph records the full colour.
Any full moon will do.
The geometry is strict — the moon must be low, no more than about 42 degrees above the horizon, the sky must be genuinely dark, and the rain or mist must be opposite the moon. Light pollution alone is enough to take one away: at Niagara Falls the natural conditions are still in place, but artificial light now obscures the moonbow in the mist.
Compare & contrast
Moonbow vs rainbow
Identical optics, roughly 400,000 times less light. The only real difference is intensity — and intensity is exactly what determines whether your eye uses colour-blind rods or colour-sensitive cones. The physics doesn't change; your retina does.
| // | Moonbow | Rainbow |
|---|---|---|
| Light source | Moon | Sun |
| Colours present | Yes | Yes |
| Colours visible | No | Yes |
| Seen with | Rods | Cones |
How it connects
- Earthshineboth are things you can only see once the sun is out of the way.
- Green Flashboth are real optical effects widely assumed to be folklore.
Tell it apart
Questions people ask
- Why do moonbows look white?
- Human colour vision is poor in low light. Night vision is rod vision, and rods don't detect colour — so the colours genuinely present in the arc simply go unseen. It isn't that the moonbow lacks them.
- Can you photograph the colours?
- Yes. Long exposures record the full spectrum that the eye can't resolve, which is why photographs of moonbows look far more colourful than the experience of standing under one.
- Where can you reliably see one?
- Waterfall mist during a full moon is the classic setting — Victoria Falls, Yosemite Falls, Wallaman Falls. Niagara no longer works: the University at Buffalo reported in 2018 that artificial lighting obscures the moonbow that was once visible there.