A murky moonrise (and why it appears red)

Moonrise over Bristol 27 May 2010. [Click for full size] Credit: Will Gater

Last night there was a lovely moonrise (image above) over Bristol. The conditions were relatively good for viewing it too, as there was only a small amount of low-level haze and not too much cloud around.

It appears that wonderful orange/red colour because, when the Moon is low, it is shining through more of the Earth’s atmosphere. The gases in our atmosphere are particularly good at scattering blue light (which is why our sunny skies are blue). This means that, as the light from the Moon travels through a thick slice of the atmosphere, the bluer wavelengths of light are essentially ‘filtered’ out. The end result…predominantly redder light reaching us watching on the ground and so we see a gorgeous red/orange Moon.

You’ll also notice that the effect gradually wears off as the Moon rises, with the Moon becoming less and less red the higher in the sky it climbs. That’s simply because the amount of the atmosphere that the light is travelling through, with respect to us observing on the ground, gets smaller. So as the amount of atmosphere that the moonlight has to pass through to get to our eyes reduces, there’s less ‘filtering’ of the bluer wavelengths.

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