A rainbow, the multi-coloured bow shaped pattern that is often seen in the sky during or shortly after rainfall is caused by the refraction[1] and internal reflexion[2] of sunlight as it comes into contact with droplets of moisture in the atmosphere. Less commonly known is the fact that the refraction and reflexion of moonlight, as it comes into contact with droplets of moisture in the atmosphere, also cause similar bow-shaped patterns to be formed in the sky. Such a bow is called a moonbow[3].
Moonbows always occur in the part of the sky that lies opposite to the part where the moon is and, by comparison to ordinary rainbows, moonbows are much fainter. This is not surprising when one considers that the intensity of light that comes to us as moonlight is much less than the intensity of the light that comes directly to us from the sun. Indeed, so faint is the light that a moonbow gives out that the cone cells[4] in man’s eyes are not sensitive enough to detect the colours in the bow and the human eye normally sees the a moonbow as white bow shaped object in the sky[5]. However, scientific advancement has come to the rescue; long-exposure photographs have confirmed that moonbows are just as colourful as the traditional rainbow. But although his eyes are not efficient enough to make out the colours of a moonbow, the phenomenon has been known to man for centuries; in his Meteorology, written around 350BC, the ancient Greek philosopher, Aristotle, described the phenomenon.
Care should be taken, if one is viewing the sky in search of a moonbow, not to mistake some other phenomena for it; and there are quite a few such phenomena that might easily be mistaken for a moonbow. For instance, at sunset, sunrise as well as in twilight conditions, rainbows can sometimes be seen in the sky. These are ordinary rainbows produced by direct sunlight. There is yet another fairly common phenomenon that might be mistaken for a moonbow. Sometimes, it will be noticed that the moon has something that looks like a halo surrounding it. This halo-like phenomenon happens as moonlight is diffracted[6] when the light hits the edges of hexagonal ice crystals that are found in cirrus clouds.
Because the amount of light that is reflected off the moon’s surface is comparatively limited (as compared to direct sunlight), moonbows are best viewed when there is a full moon and the moon is at its brightest and on nights when the sky is very dark. Additionally, the moon has be hanging low in the sky, no more than 42 degrees, and the rainfall must be on the opposite side to the moon. It will be appreciated that this combination of factors required for the production of moonbow makes the phenomena much rarer than rainbows.
The likeliest way, in fact, to see a moonbow does not require any rainfall at all. As noted above, the phenomena is caused by the interaction of light with moisture particles in the atmosphere and there are other ways than rainfall for there to be sufficient moisture particles in the atmosphere for the formation of moonbows. The great water falls and sprays of the world provide the required moisture and, if one is prepared to forgo some sleep, a night out at such places when moon conditions are right provide the most promising chances. Great moonbow sites include the Victoria Falls in East Africa, Kentucky’s (US) Cumberland Falls, and many of the falls that lie in California’s (US) Yosemite National Park.
[1] That is, deflection.
[2] That is, turning, bending or folding aside, backward or downward.
[3] Of course, moonbows are actually formed as a result of the refraction and reflexion of sunlight since the moon gives off no light of its own but merely reflects the light of the sun that falls upon its surface.
[4] Cone cells are light sensitive cells that allow us to detect colours.
[5] White rainbow is another name for the rainbow.
[6] Diffraction is the spreading of light rays as a result of the rays hitting the edge of an opaque object.
