The image below shows a moonbow arcing across a sky bedazzled by aurora light over a village in northern Sweden. Moonbows are rare and quite faint. But what is a moonbow, and how does it form?
Moon bows, like rainbows, form when light enters a raindrop in such a way as to cause the incidental white light to “split” into its component colors. As we can see from the graphic below, the rainbow forms at an angle that is 42 degrees opposite from the position of the light source. Light waves experience this splitting, properly called refraction, because the speed of light changes in different media, such as glass and water. When the light enters the rain drop (which we assume to be spherical, but isn’t), the wavelengths propagate through the water as speeds proportional to their wavelengths. Red moves more slowly than the violet, as the latter is of slightly higher energy than the former. As a result, these different color waves follow divergent paths in the droplet so that when they reflected out of the drop, they appear as a continuum of colors.
A moonbow is more rare than a rainbow because not only does the moon have to be lower in the sky, it also has to be bright and therefore either full or in a late gibbous phase. Moreover, when a moonbow does form, it will be quite faint because even a full moon will be significantly dimmer than the Sun. (Even though the full moon is nine times brighter than the quarter moon and is much closer to us than the Sun, its angular diameter is equal to that of the Sun and it reflects only about 7% of the Sun’s light.)
When a moonbow does form, it will generally appear white because our eyes cannot perceive distinctive colors at low light levels. (For this reason, we don’t see colors in most stars.) The colors we saw in the first image were noticeable because that moonbow was captured in a time exposure photograph.
As the moon is passing through the waning crescent phase today and will be about 25% illuminated, we won’t expect to see a moonbow anytime soon. However, if the moon is at least 75% illuminated and is low on the horizon when the sky is clearing after a rainstorm, gaze toward a region opposite that of the moon to find a faint arc of white light: a rare, but beautiful, lunar rainbow.
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