Is the moon really blue?
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Did you know last night had the second brightest moonlight since 1993? The brightest was last month. We won’t see its like again until 2016. How cruel of fate to make this interesting astronomical event occur on a night when brass monkeys were wearing reinforced underpants. Still, The Ranger happened to be visiting one of his sites at 0130 this morning, with a camera and tripod… OK, I’ll admit I actually went up there specially for the purpose. So you didn’t have to. Standing on the self-same spot where I witnessed the solar eclipse in 1999, I took a picture by moonlight looking from Brading Down out over the English Channel, with Sandown Bay down below. The results were surprising:
It took four or five shots to get this one – and each shot took two minutes to capture and store (one minute exposure at F4, 100ASA, in case you camera-fetishists were wondering). So yes, my friends, I was standing there stamping my feet and huffing like an impatient horse for ten minutes. Blimey, it was cold. Sensible Cat sat in the car safely out of shot. The interesting thing about this result was how vivid the colours were. To our eyes, moonlight seems silvery-blue. I was hoping for a monochrome, luminous image of eerie beauty. Instead a full-colour picture emerged. I had taken a picture of something I could never see with my eyes. There’s a slightly other-worldly quality about the picture, and if you look carefully at the sky (try the larger picture) you can see some little streaks where the stars actually moved during the exposure. But otherwise it could almost be daylight. This served to remind me of how our eyes work, and why moonlight looks so silvery. The photoreceptors in our eyes (rod cells) that respond best in low light are actually not very good at distinguishing colours. The colour-sensitive receptors, the cones, need more light to work properly: apparently more than can be found by the light of the moon, even the brightest moon. But my digital camera knows no such distinctions. Colour exists in low light conditions, it’s just that we can’t see it. But the camera can, and that’s why the photograph was in colour. Out of interest, I spent some time trying to manipulate the image so it looked as it had to the naked eye. This proved surprisingly difficult, and I didn’t entirely succeed. Here’s the best I managed:
I think the key thing there is ‘just the right speed’. You’re gonna need a very steady hand either way!
I asked how a colleague had got a picture of comet Holmes without the stars being streaked like mine. He had built a contraption he saw in an astronomical magazine so that when he turned a screw it tilted the camera. By slowly turning the screw at just the right speed the camera was moved to counter to the earth’s rotation removing the blur.
I must admit I haven’t tried building such a device myself yet!