Posted by: Prospector PJ28 MAR 2012
A rainbow is a beautiful natural wonder, and a well established symbol of hope. It also has a rare night-time equivalent: a moonbow. This is a rainbow produced by light reflected from the surface of the moon rather than from direct sunlight. Because the light from moonbows is usually too faint to excite the eye’s cone receptors they often appear to be white.
Moonbows are most easily viewed when the moon is near to full and, for true moonbows, the moon must be low in a dark sky and rain must be falling opposite it. A few locations around the world, such as Victoria Falls and waterfalls in Yosemite National Park, provide ideal conditions for spray moonbows.
A coloured circle around the sun is not a moonbow, but a 22 degree halo produced by refraction through hexagonal ice crystals in cirrus cloud. Light passing through the ice prisms is deflected twice, producing deviation angles ranging from 22 to 50 degrees.
A moondog (or paraselene, meaning “beside the moon”) is a rare bright circular spot on a lunar halo caused by the refraction of moonlight. The moon must be bright and therefore full or nearly full to create a moondog, so this is another relatively rare phenomenon. The daytime equivalent — a sundog, or parhelion — is a much more common sight. Sundogs appear as a coloured patch of light to
the left or right of the sun, 22 degrees distant and at the same distance above the horizon as the sun. They can be seen anywhere in the world, but are not always bright or obvious. Sundogs are most conspicuous when the sun is low.
Sundogs have been recorded throughout history. Aristotle wrote that “two mock suns rose with the sun and followed it all through the day to sunset”. And in Shakespeare’s “Henry VI”, Edward says, “Dazzle mine eyes, or do I see three suns?”.