I hadn't, until an article on the always-interesting Environmental Graffiti blog caught my eye. An excerpt:
When the moon is near its fullest, and barely a cloud veils its face, certain locations on earth treat observers to the scarcely seen light phenomenon known as the lunar rainbow, or moonbow. Like daytime-occurring rainbows, moonbows are formed when rays of light bounce off water droplets suspended in the air – the vapour of a raincloud, say, or the spray from a thundering waterfall – though of course they are caused not by the direct light of the sun but by that which is reflected by the moon.
Like their diurnal counterparts, moonbows always appear in the part of the sky opposite the celestial body that provides their light source, with the moon thus positioned behind the viewer. Except for those lunar bows whose medium is the mist of waterfalls, a rare combination of a low moon and a dark sky are needed to create this spectacular sight – not to mention rainfall up ahead.
Even with the moon at its brightest, moonbows are faint compared to typical rainbows due to the low quantity of light shone down by our only satellite. The glow is too weak to stimulate the colour receptors of the naked eye – meaning moonbows are often seen as being white – so it’s lucky long-exposure photography has stepped in, enabling us to see all the colours of the moonbow.
Yosemite is such a hotspot for viewing moonbows that a team of astronomers at Texas State University were inspired to develop a computer programme which can reliably predict when moonbows are likely to appear at the falls of America’s famous national park – other factors such as a clear sky permitting.
People have been watching moonbows since Aristotle’s day – and doubtless long before – but this research is the first time anyone has calculated precise dates and times for their appearance. Now it is hoped we may better appreciate this incredible yet elusive natural wonder.
There's more at the link, including more photographs. Fascinating reading, and highly recommended.