We have all seen rainbows during the day but what about rainbows at night? Thomas Thies of Gearhart, Oregon photographed this one after the sun had set on December 8th 2003.He says, "The rainbow, which lasted for about 20 minutes, was out over the ocean with the moon rising behind us." The several bright yellow lights near the horizon are crab boats.
The moon is the key, just as sunlight produces rainbows during the day, moonlight can produce rainbows at night. This is a lunar rainbow or 'moonbow'.
Moonbows are rare because moonlight is not very bright. A bright moon near to full is needed, it must be raining opposite the moon, the sky must be dark and the moon must be less than 42ยบ high.
Put all these together and you do not get to see a moonbow very often! To the unaided eye they usually appear, as in the small image, without colour because their light is not bright enough to activate the cone colour receptors in our eyes. Nonetheless colours have been reported and might be seen when the moon is bright.
Moonbow and Venus imaged by Rob Ratkowski (photography) in May 2004. Moonbows are very rare. The moon must be bright and not too high, the sky must be dark else the faint bow will not be visible and there must be rain in the direction opposite the moon.
Orion was setting when Rob took this spectacular image, Venus is the bright object inside the bow.
Mars is above it to the left and Saturn is above Mars and outside the primary. Capella is the bright star to the right (north) of Venus.
Do not expect to see much colour with the unaided eye because moonbows are usually too faint to excite the colour sensors in our eyes. We therefore mostly see them as a series of grey shades.
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