Rainbows usually belong to the daytime. Sunlight passes through raindrops and shows its familiar colors in the sky, and people have grown used to associating rainbows with daylight hours that shift between sun and rain. On certain quiet, dark nights, however, an arc very much like a rainbow can also appear in the sky. This time the light that makes it comes not from the Sun but from the Moon.
This rare natural phenomenon is called a “moonbow.” The Moon does not send out colored light of its own. A moonbow is a rainbow formed by refraction after moonlight passes through raindrops, mist, or the water droplets thrown up by a waterfall. In physical terms, a moonbow is no different in kind from a daytime rainbow. The one great difference is that the starring role has passed from the Sun to the Moon, and it is precisely this that makes the moonbow such an uncommon sight in the night sky.
An invisible rainbow hides opposite the Moon
When a moonbow appears, the position of the Moon matters a great deal. As with an ordinary rainbow, a great many fine water droplets must be present. Rain, the spray thrown up by a waterfall, and even fog in the air may all become the conditions for a moonbow to form.
Light refracts as it enters the droplets and finally forms an arc-shaped band of light. One feature, however, is easy to overlook: a moonbow always appears in the part of the sky opposite the Moon. So when the Moon stands in one half of the sky and dense rain, mist, or waterfall spray happens to fill the other, a moonbow may be seen where the moonlight and the droplets meet, once the conditions align. This rainbow, though, is usually far fainter than a daytime one. The reason is simple. The Moon does not shine by itself; the moonlight we see at night is mostly sunlight reflected from the lunar surface, and the amount of light that reaches the ground is far less than in daylight.
A moonbow is really there, but the human eye can make out only a faint white arc.
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Why does a moonbow look colorless?
This may be the most puzzling thing about moonbows. If they form on the same principle as daytime rainbows, why does the eye so often see a band of white light? The key is that the light is too weak. The human visual system has cone cells that identify color, but the light of a moonbow is usually not enough to stimulate them effectively. So on a pitch-dark night, the eye more readily sees a whitish arc than clearly separated bands of red, orange, yellow, green, blue and the rest.
This also creates a curious visual effect. Seen from a distance, a pale white rainbow seems to hang in the night sky, and without knowing beforehand what causes it, a viewer might find it hard to believe it really is a rainbow. A camera, however, can capture something else. In long-exposure photographs, the colors that are hard to detect by eye can be recorded, so a moonbow in a photograph often looks more vivid than the one people see. A moonbow, in other words, is not colorless. Rather, the eye at night can hardly make out its colors.
A full moon is an important time to wait for
Seeing a moonbow with your own eyes takes more than looking up and waiting for the Moon. First, the Moon must be bright enough, which makes the time of a full or nearly full moon the easiest for observing one. Second, the Moon must not be too high. Sources on the subject note that its elevation needs to be below 42 degrees, and the lower the better. At the same time, the Moon must not be hidden by cloud, and the sky opposite the Moon must hold raindrops, mist, or other fine water droplets.
Finally, one more condition is essential: “darkness”. This is part of why moonbows are rarer than ordinary rainbows. When a full moon rises or sets, the sky is often not fully dark, so the window suitable for observing a moonbow is quite narrow. Generally the better times are two to three hours after sunset, or two to three hours before sunrise. To see a moonbow, you have to keep watch when most people are already asleep, or before the sky has begun to lighten. Night, water vapor, moonlight, and angle: not one of them can be missing.
The smaller the droplets, the whiter a moonbow may look
The size of the droplets themselves also affects what people see. How vivid the colors of a moonbow are is related to the size of the water droplets in the air, and the smaller the droplets, the less distinct the colors usually become. When a droplet is less than a five-hundredth of an inch across, light of different colors blends together readily, and the result looks all the closer to white.
The fine mist that hangs around a waterfall therefore tends to produce an effect quite unlike a rainy-day rainbow. Rather than the strong colors of a daytime rainbow, it forms a hazy, silver-white arc of light suspended between the dark sky and the spray.
Waterfalls are good places to look for moonbows
Although moonbows can form from raindrops, some of the world’s famous waterfalls have become places to observe the phenomenon because they hold so much spray in the air for so long. Niagara Falls in New York state, Yosemite National Park in California, and Cumberland Falls near Corbin, Kentucky, have all drawn attention for their moonbows. At Victoria Falls, on the border of Zambia and Zimbabwe, the water dropping from a great height throws up a huge volume of mist, and the falls are known for their spectacular spray moonbows.
There is a particular relationship between waterfalls and moonbows. By day, sunlight entering the spray that flies around a waterfall can form a rainbow. At night, as long as the position and brightness of the Moon and the surrounding conditions happen to line up, that same spray may become a natural canvas on which moonlight forms a moonbow.
Moonbows hide in Costa Rica’s night skies too
Moonbows are not confined to the area near waterfalls. The cloud forests of Costa Rica are another place to observe them. Mountain towns such as Monteverde and Santa Elena are affected by mist from the Caribbean. When the winds of a particular season carry that mist into the mountains, great numbers of fine water droplets can hang in the air, and when the Moon is close to full, those droplets may form a moonbow in the night sky.
Locally the mist has a Spanish name of its own, “cat hair,” and in sunlight it forms flowing, streaked patterns. In this part of Costa Rica, moonbows may appear during the full moon between December and the following February, and they need not wait for the hours before dawn: as long as the Moon is at or near full, there is a chance of seeing one after sunset as well.
Hawaii’s moonbows have a stage of their own
Moonbows can likewise appear in some of the humid parts of Hawaii, and Kauai is one such place. When light rain falls and the Moon rises in the east, the right combination of moonlight and water droplets may form a moonbow in the night sky.
The island of Hawaii sees a similar phenomenon. What makes these places suitable for observation has to do with the humid environment and the frequent rain and mist. When moonlight and water droplets meet at the right time and the right angle, an arc that the eye can barely detect may appear in the night sky.
People noticed them more than 2,000 years ago
The moonbow is not a curiosity discovered only in modern times. The moonbow was already recorded at least as early as Aristotle’s Meteorology, written in 350 B.C. In ancient Greek culture, the moonbow was also linked with Selene, the goddess of the Moon, which made it seem more remarkable still.
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