Here we have a time lapse image of a full Moon rising over Skiathos Island, Greece. You will notice that the Moon subtends the same angle of the sky throughout its ascent and it does not change size from when it creeps over the horizon to when it takes centre stage in the sky. (It often APPEARS larger on the horizon and smaller overhead, just due to whether there are other things visible next to it for your brain to compare sizes to) What does change however, is the colour; from red to orange to yellow to white. The reason for this is accredited to changes in the path length of light. At “Moonrise”, when the Moon is low on the horizon, light must travel further through the atmosphere before it reaches your eyes- because of this increase in path length, more light is reflected and scattered. As a result, the colour of visible light appears to change. This occurs simply because the shorter wavelength colours like blue and green are being preferentially scattered from our view. What remain are the longer wavelengths colours (orange and red) so this is what we see. As the Moon climbs higher into the sky, the path length decreases and the shorter wavelength colours like blue and green come into focus and make the Moon appear yellow in colour. By the time the Moon has reached its pinnacle point in the sky, the atmosphere scatters almost an equal amount of long and short wavelengths of light- now the Moon appears white. Photographer: Nikos Paschalis
Posted on: Sat, 29 Jun 2013 12:36:54 +0000
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