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Ask Astro: What are moonbows and how often do they occur?

Like normal rainbows, moonbows are an optical phenomenon caused by light being reflected and refracted by water droplets in the atmosphere.
RELATED TOPICS: MOON | ATMOSPHERIC PHENOMENA
Hawaii moonbow

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.

Rob Ratkowski

I saw a rainbow caused by moonlight. How often does this occur?

Ed Sterbenc
Palo Alto, California

Moonbows — also known as lunar rainbows — occur less than 10 percent as often as normal rainbows. Like regular rainbows, moonbows are an optical phenomenon caused by light streaming through the atmosphere and being reflected and refracted by water droplets in the air. However, because the moonlight that produces moonbows is much fainter than direct sunlight, moonbows require a few additional conditions to form. And that's why rainbows around the Moon are so rare.

Despite being well known, rainbows themselves are not very common. Most locations on Earth see fewer than six regular rainbows per year. However, regions that experience frequent, brief showers mixed with clear skies and bright sunshine see rainbows much more often. The Isle of Skye on Scotland's west coast and Hawaii, for instance, are both places with frequent rainbows.

To make a rainbow, we need bright sunshine and a screen of rain opposite the Sun. But even that's not enough. Both rainbows and moonbows require almost perfectly spherical raindrops to form — distortions of only 1 percent are sufficient to prevent the formation of either type of bow. 

Also, the light source (whether the Moon or the Sun) must be at the right altitude. Rainbows and moonbows are centered directly opposite the Sun or Moon. The higher the light source's altitude, the lower the bow's arc. So, the Sun or Moon must be low enough that their bows appear above the horizon.

However, moonbows need something more. While lunar rainbows form in exactly the same way as with the Sun, the Moon is bright enough to make visible moonbows only for about 3 days around each Full Moon.

Even when the Moon is bright enough to form a lunar rainbow, moonbows are so faint they must be viewed against a dark sky near the end of evening twilight. If you wait too long, the Moon will rise so high that any moonbow it creates lies below the horizon.

At middle latitudes, the Full Moon is best placed for moonbows in summer, when it spends more time low in the sky. And in other seasons, the window of opportunity for catching a moonbow may last only an hour.

In showery weather, you should always look for a pale moonbow when the Moon is low and bright. A moonbow actually is colored like its daytime counterpart, but its light is mostly too dim to activate the color sensors in our eyes. Therefore, you might not detect a distinct separation of colors in a moonbow. A camera mounted on a tripod will capture the colors easily, but our eyes and brain alone are all that are needed to experience the wonder of the Moon's own rainbow! 

Les Cowley
Norfolk, England


Editor's note: This article was updated Nov. 9, 2022.

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