Where it sits
The spectrum is all there. The failure is at your eye, not in the sky.
Tell it apart
| Aspect | Moonbow | Rainbow |
|---|---|---|
| Light source | Moon | Sun |
| Colours present | Yes | Yes |
| Colours visible | No | Yes |
| Seen with | Rods | Cones |
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.
What it doesn't mean
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.
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.
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
English — 'bow' from Old English boga, 'arch, bend'
'Bow' here means the arch shape, not a ribbon or a weapon — the same element hiding inside rainbow and elbow.
- optics
Formed exactly as a rainbow is — moonlight refracts entering a droplet, reflects off the back, and refracts again on exit.
- 2018
Ernest Sternberg of the University at Buffalo documents that light pollution now obscures the once-visible moonbow in the mist at Niagara Falls.
In real life
- Standing in the spray below a waterfall on a full-moon night, you see a pale white arc in the mist — a moonbow, its colours too faint for your eyes.
- A long-exposure photograph of the same arc shows bands of red, green and blue that nobody standing there could see.
- Waterfalls with heavy spray, such as Victoria Falls, are the classic places to see moonbows around the full moon.
Questions
Q01Why 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.
Q02Can 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.
Q03Where 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.
Sources
- Light pollution obscures glow of Niagara Falls moonbow — University at Buffalo.
- Moonbows: What's a Lunar Rainbow? — Penn State.
- moonbow (etymology) — Wiktionary.