Moonbows, also known as lunar rainbows, are distinct from typical rainbows in that they are produced by the shimmering of the moon’s rays rather than the sun.
A moonbow, also referred to as a moon rainbow or lunar rainbow, is a rainbow that is created by the moonlight rather than the sun. Its formation is identical to that of a solar rainbow, with the exception of the light source; light is reflected in water droplets in the atmosphere caused by rain or a waterfall, for example. They are always situated on the side of the sky that faces away from the observer from the Moon.
Mentioned at least since Aristotle’s Meteorology (circa 350 BC), moonbows are much fainter than daytime rainbows, since the surface of the Moon reflects a smaller amount of light. For this reason, it is much more difficult for the human eye to discern colors in a moonbow, as the light tends to be too dim to activate the color receptors in our eyes. As a result, we usually see moonbows to be white, but their colors do appear in long exposure photographs.
The best times to see moonbows are during and immediately after a full moon when the moon is at or very close to its brightest phase and is not blocked by clouds. The Moon must be low in the sky (at an elevation of less than 42 degrees, preferably lower) and the night sky must be very dark for moonbows to appear. Moonbows, however, can only be seen two to three hours before sunrise or two to three hours after sunset because the sky is not completely dark during a rising or setting full moon. Of course, there must also be water vapor in the sky opposite the Moon, such as from rain or sprinkling.
This set of requirements makes moonbows much rarer than solar rainbows – they actually occur less than 10 percent as often as normal rainbows. Sometimes moonbows can also be observed during full moonrise during the winter months, when the sky is darker and rain falls at extreme latitudes. The definition of the colors is subject to the size of moisture drops present in the air: the smaller they are the less vivid the colors will be.
Moonbows can also be produced by spray, fog, or mist in addition to rain. Such bows can be seen near a number of waterfalls in the USA, including Yosemite National Park in California, Niagara Falls in New York, and Cumberland Falls near Corbin, Kentucky. Spray moonbows are another well-known feature of Victoria Falls, which is located on the border of Zimbabwe and Zambia.
How to spot a moonbow?
As noted above, moonbows are only visible for about 3 days around full moon, when viewed against a dark sky near the end of evening twilight, or before sunrise. In middle latitudes, the best time for moonbows is summer full moons, when the Moon spends more time low in the sky. In other seasons, moonbows may last only an hour.
In showery weather, always look for a pale moonbow when the Moon is low and bright. You probably won’t see much colors, but if you mount a camera on a tripod, you can capture the colors easily.