We All Know Rainbows But Have You Ever Seen a Moonbow?

Moonbows, also known as lunar rainbows, are different from regular rainbows in that they are created by the shimmers of moon rays rather than the sun.

A moonbow (alsoknown as a moon rainbow or lunar rainbow) is a rainbow created by moonlight rather than direct sunlight. Apart from the light source, its formation is identical to that of a solar rainbow: light is reflected in water droplets in the air as a result of rain or a waterfall, for example. They are always positioned on the opposite side of the sky from the Moon relative to the observer.

Moonbows, which have been mentioned at least since Aristotle’s Meteorology (circa 350 BC), are much fainter than daytime rainbows because the surface of the Moon reflects a smaller amount of light. As a result, it is much more difficult for the human eye to distinguish colors in a moonbow because the light is too dim to activate the color receptors in our eyes. As a result, moonbows are usually white, but their colors can be seen in long exposure photographs.

Lunar rainbow over Kihei, Maui, Hawaii, US. Image credit: Arne-kaiser

Moonbows are best seen during and near full moon, when the Moon is at or near its brightest phase and is not obscured 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. However, because the sky is not completely dark on a rising/setting full moon, moonbows can only be seen two to three hours before sunrise or two to three hours after sunset. And, of course, there must be water droplets (from rain or spray) in the sky opposite the Moon.

Moonbows are much rarer than solar rainbows due to these requirements; they occur less than 10% as oftenly as normal rainbows. Moonbows can also be seen at full moonrise during the winter months, when the sky is darker and rain falls at high altitudes. Color definition is affected by the size of moisture drops in the air: the smaller they are, the less vivid the colors will be.

A night rainbow on Fraser Island, Queensland, Australia. Image credit: Garry

Moonbows can be caused by spray, fog, or mist in addition to rain. Such bows can be seen around various waterfalls in the United States, including Niagara Falls, New York, Yosemite National Park in California, and Cumberland Falls, near Corbin, Kentucky. Victoria Falls, located on the border of Zambia and Zimbabwe, is also well-known for its spray moonbows.

Rob Ratkowski captured this shot of a moonbow from Hawaii in May 2004. The bright point of light near the center of this image is not the Moon, but instead the planet Venus. By necessity, the Moon is behind the photographer in this shot.

How to spot a moonbow?

Moonbows, as described earlier, 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. Summer full moons are the best time for moonbows in the middle latitudes because the Moon spends more time low in the sky. Moonbows may last only an hour during other seasons.

When the Moon is low and bright, look for a pale moonbow in showery weather. You probably won’t see much colors, but if you mount a camera on a tripod, you can capture the colors easily.

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