The Elusive Beauty of Lunar Rainbows

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Image: Meng Tang

Moonbow, Yosemite Lower Falls

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.


Image: G a r r y

Driving through a night time archway: Moonbow on Fraser Island, Australia

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.

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Image: Pierre Lessage

Gold at the end of the moonbow: Captured over the Pacific Ocean in Tahiti

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.

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