A moonbow, or lunar rainbow, is a rainbow produced by moonlight rather than sunlight. The physics is identical — light refracts entering a water droplet, reflects off the back, refracts again on the way out, and emerges concentrated at about 42° from the antisolar point, which for a moonbow is the anti-lunar point directly opposite the Moon. The only difference is brightness. Full moonlight is roughly 400,000 times fainter than direct sunlight, which puts a moonbow below the threshold at which human colour vision operates, so most people see a pale grey or silvery-white arch rather than a spectrum. The colours are genuinely there, and a camera making a fifteen- or thirty-second exposure records them clearly. Because the light budget is so tight, moonbows require four separate conditions to coincide, and there are only a handful of places on Earth where they do so predictably.
Two Waterfalls That Sell Tickets For It
Most moonbows are chance encounters during night rain. Two places instead produce them on a published schedule, because both supply their own water.
Victoria Falls, on the Zambezi between Zambia and Zimbabwe, is the world's premier moonbow site. The falls throw up a spray plume that on a high-water day rises hundreds of metres and is visible from tens of kilometres away — the local name Mosi-oa-Tunya, the smoke that thunders, describes exactly this. That plume is a permanent, reliable supply of suspended droplets, so the only variable left is the Moon. Both the Zambian and Zimbabwean parks extend opening hours for three nights around each full moon during the high-water season, roughly February to July, with the strongest displays when the Zambezi is in full flood between March and May. The falls themselves are covered in the Victoria Falls guide.
Cumberland Falls in southeastern Kentucky is the only place in the Western Hemisphere with a comparable reputation, and the American moonbow tradition is built almost entirely around it. The falls are modest by Victoria Falls standards — about 20 m high and 38 m wide — but the geometry is unusually favourable: the gorge below runs in the right direction, the surrounding forest is dark, and the spray is dependable. The state park publishes moonbow dates for the two nights either side of each full moon, year-round, and the display is best in winter and spring when flow is highest and the Moon rides high.
Other sites with genuine track records include Yosemite Falls in late spring, when snowmelt is at its peak and the spray is heaviest; Waimea on Kauai; and the Waterfall Bluff area of South Africa's Wild Coast, which has the unusual distinction of a waterfall dropping directly into the sea.
Four Conditions, and the Nights They Coincide
The reason moonbows are rare is not that any one requirement is difficult, but that all four have to hold simultaneously, and two of them are in tension.
- A bright Moon: in practice within a day or two of full. A half Moon supplies a quarter of the light and is generally not enough.
- A low Moon: below about 42° altitude, and ideally much lower. Above that the entire arc falls below the horizon, exactly as a rainbow does when the Sun is high. This is the awkward one, because it means the Moon must be bright and low — which restricts the window to the hours just after moonrise or just before moonset.
- Water droplets opposite the Moon: rain, spray or mist in the anti-lunar direction, with the observer between the Moon and the water.
- A dark sky behind it: clear in the direction of the Moon, and free of artificial light. A moonbow is faint enough that any streetlight within view will erase it.
The tension between the second and fourth conditions is what makes the whole thing scarce. A full Moon is opposite the Sun, so it rises around sunset and sets around sunrise, meaning the low-Moon windows fall in early evening and early morning twilight — precisely when the sky is least dark. The best moonbow nights are therefore not the exact night of full Moon but the night or two after, when the Moon rises an hour or so into full darkness and spends a useful period low in a genuinely black sky.
One further variable is often overlooked: the Moon's altitude at full phase varies enormously over the year, and it runs opposite to the Sun. A full Moon in midwinter rides high, exactly as the summer Sun does, which is bad for moonbows; a full Moon in midsummer stays low, which is good. Northern-hemisphere observers therefore get their best geometry in June and July, while Victoria Falls, in the southern tropics, gets its best combination of low full Moon and heavy spray during the March-to-May flood — one of the reasons that site works as well as it does.
Why Your Eyes See Grey and the Camera Sees Colour
Human retinas carry two systems. Cones handle colour and detail and need reasonable light. Rods work at very low light, are far more sensitive, and are effectively colour-blind. Moonlight sits in the range where rods dominate — scotopic vision — so the world at night is rendered in shades of grey, and a moonbow with it.
Nothing about the moonbow itself is grey. The droplets disperse moonlight into the same spectrum they would disperse sunlight into, in the same order, at the same angles, with red on the outside of the arc and violet on the inside. The information reaching the eye is complete; the eye simply cannot decode the colour channel at that intensity. A camera has no such limit, and a long exposure on a tripod at a high ISO will return a fully saturated arc that looks like a daylight rainbow shot at night.
This produces a predictable moment of disappointment for first-time moonbow travellers who have seen photographs, and it is worth setting expectations before the trip rather than after. What the eye sees is a distinct, luminous, pale arch — genuinely impressive, and unmistakably an arch — but not a colour photograph. Some observers with good dark adaptation report catching a hint of red at the outer edge on the brightest displays, which is consistent with red being the wavelength at which the cones hang on longest.
Aristotle Counted Two in Fifty Years
The earliest surviving description is in Aristotle's Meteorologica, and it is notable both for being correct and for quantifying the rarity. Aristotle records that the rainbow can occasionally be produced by the Moon, that it happens only at full Moon, and that in more than fifty years of observation only two had been seen. He also noted that it appears white rather than coloured — an observation that stood unexplained for two thousand years, until the physiology of rod and cone vision supplied the reason.
The count is a useful corrective to the modern assumption that moonbows are essentially unobtainable. Aristotle was watching for rain-produced moonbows in the Aegean, without knowing about the waterfall sites that make the phenomenon schedulable, and two in fifty years is about right for passive observation in a dry Mediterranean climate. Standing in the spray at Victoria Falls on the right night converts a once-in-a-generation accident into a near-certainty, which is the whole reason those two parks built visiting hours around it.
The American record runs through Yosemite before it runs through Kentucky. John Muir wrote at length about the lunar spray bows at Yosemite Falls, describing arcs standing in the moonlit spray at the base of the fall and the way they shifted with the wind — an account that remains one of the better descriptions of what the effect actually looks like from close range, as opposed to what it looks like in a photograph.
Spray Bows, Fogbows and Other Relatives
Several near-relatives are worth being able to tell apart, since they turn up in the same conditions and are regularly mislabelled.
Spray bows are what most waterfall moonbows technically are: the droplets come from spray rather than falling rain. They behave identically, but because spray droplet sizes are more uniform than raindrops, spray bows can be unusually crisp.
Fogbows — also called white rainbows — form in fog, where droplets are so small that diffraction smears the colours together into a broad white band. They are common on coastal hills and, at night under a bright Moon, produce a ghostly white arch that is often reported as a moonbow. The distinction is width and colour: a fogbow is broad and white; a moonbow is narrower and, on camera, coloured. The same small-droplet diffraction physics produces the rings of a glory around a Brocken spectre.
Secondary moonbows exist, sitting at about 51° with reversed colour order, produced by two internal reflections instead of one. They carry roughly a tenth the light of the primary, which puts them beyond the eye entirely, but long exposures at Victoria Falls have recorded them.
If you want a single recommendation: go to Cumberland Falls or Victoria Falls on the second night after a full Moon, bring a tripod and a lens that opens to f/2.8, and look at the spray, not at the Moon. And accept that the best view of the night may well be the one on the back of the camera — the same trade that governs aurora photography, where the sensor consistently outperforms the eye.