A moonbow, or lunar rainbow, is a rainbow made by moonlight instead of sunlight. The optics are identical: light enters water droplets, bends, reflects off the back of each drop and comes out split into colours at an angle of about 42° from the point directly opposite the light source. The difference is that moonlight is roughly 400,000 times fainter than sunlight, so everything else has to go right.
What it takes
- A full or nearly full moon. In practice, the night of the full moon and one or two nights either side.
- A low moon. The bow forms opposite the moon, so if the moon is higher than about 42° above the horizon the bow is below ground. The best windows are the first hours after moonrise and the last hours before moonset.
- A clear sky between you and the moon.
- Lots of spray in the right place — which is why big waterfalls at high water are the most reliable places on earth to see one.
- Darkness. Artificial light washes it out.
Why it looks white
In dim light the eye switches to rod cells, which are sensitive but can't see colour. So most people see a moonbow as a pale, ghostly silver arc. Colour is there — a camera on a tripod with an exposure of 15–30 seconds will pick up the reds and violets clearly — and some people with dark-adapted eyes can see a faint tint in a bright one.
Where to see one
Victoria Falls, Zimbabwe and Zambia
The most famous and most reliable. The volume of spray at high water is enormous, and both national parks hold special late openings for a few nights around each full moon. The moonbow is strongest in the high-water months, when there is the most spray, though at the very peak the mist can be so thick it hides everything. Tickets for the lunar-rainbow openings are sold separately from day entry. See the Victoria Falls season guide.
Cumberland Falls, Kentucky, USA
Often described as the best place in the Western Hemisphere to see a moonbow with any regularity. The park publishes expected moonbow dates around each full moon, and on clear nights the bow forms in the mist below the falls, visible from the main overlooks.
Yosemite Falls, California, USA
In spring and early summer, when snowmelt is at its peak, moonbows form in the spray at the base of Lower Yosemite Fall on full-moon nights. They draw a crowd of photographers — go early for a spot on the bridge.
Iguazú Falls, Argentina and Brazil
The Argentine park runs guided full-moon walks to the Devil's Throat a few nights a month, and on clear, high-water nights a faint lunar rainbow sometimes appears in the spray. Check whether the Devil's Throat catwalk is open before you book — details in our Iguazú guide.
Planning around one
- Check the full-moon date for your month and plan to be there that night and the nights either side.
- Look up the moonrise time. You want to be in position shortly after the moon has cleared the horizon behind you.
- Pick high-water season, but not so high that the mist is a solid wall.
- Bring a tripod and a rain cover. Expect to get wet.
- Have a back-up night. Cloud is the most common reason people miss it.
For the other kind of rainbow — the daytime one — the rules are in our waterfall photography guide. For the months when each of these falls is running high, see when to go.
