mercoledì 4 novembre 2009

Le magie dell'arcobaleno notturno

Un fenomeno più unico che raro: l'arcobaleno notturno. L'ha fotografato l'inglese Chris Walker a Richmond, North Yorkshire. Di solito l'arcobaleno si forma di giorno per via dei raggi solari che colpiscono le goccioline d'acqua sospese per aria. Tuttavia può anche capitare di notte se, dopo abbondanti piogge, c'è la luna piena. Ecco il resoconto dell'avventura di Walker diffusa oggi dalle pagine del Daily Mail: This rare ghostly picture shows a full lunar rainbow arcing over the countryside in Richmond, North Yorkshire. Rainbows appear when sunlight is reflected from raindrops, but in this case the Sun had set hours before. Instead a particularly bright Moon created the impressive bow. Photographer Chris Walker captured the picture as he was driving through a storm around 7pm last Sunday. He said: 'I noticed something odd in the sky as I was driving home. 'A near full Moon was behind me and the wind was blowing a gale and rain was being driven from clouds on the horizon. 'The Moon was so bright that when I arrived home it was obvious that the object in the sky was a rainbow illuminated by moonlight. 'As moonlight is many thousands of times fainter than sunshine the bow is many times fainter and only seen when the Moon is near full.' Although 'Moonbows' often appear white to the beholder, the colours can be captured in a photograph by using a long exposure. In this case Mr Walker only needed a 30-second exposure on his Lumix digital camera to reveal the full range of colours.'The moonlight was so bright I could see red in the rainbow with my unaided eye,' he said. Lunar rainbows are always in the opposite part of the sky from the Moon and most easily viewed when the Moon is nearly full. They are relatively rare because they need a number of conditions to appear. The Moon must be less than 42 degrees high in the sky, the sky must be very dark and there must be rain falling opposite the Moon. Moon bows are more common near waterfalls. The phenomena should not be confused with Moon rings, which appear like circles around the Moon. These are caused by the refraction of light through ice crystals suspended in the upper atmosphere.

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