The true colurs of the Moon
The guest speaker at Mays monthly meeting of Keighley astronomical society was Mr Kevin Kilburn. If you think about the Moon in black and white, Mr Kilburn’s presentation entitled, ‘Not fifty shades of grey’. Explained all
You can’t see colours at night because our visual systems are not designed to see colours when there isn’t very much light in a scene. We actually have two visual systems that work in parallel to help us survive in the world. When there is plenty of light, we use our cone photoreceptors. There are three types of cones roughly sensitive to red, green, and blue light and we can compare the images captured with these three systems to perceive the colours in the scene. We can also see fine detail with our cones.
However, the ability to see colours and detail with our cone system means that the cones cannot be very sensitive to light. As the light levels decrease at night, we reach a point where our cones can no longer respond because there simply is not enough light for them to produce a response. In this situation, our visual system automatically switches to a second set of photoreceptors known as rods. There is only one type of rod receptor, so that means we can only see in shades of grey when our rods are working and our cones are not. The rods also gang up together to capture light over relatively large areas. This helps them to be very sensitive to the small amounts of light available at night, but it means that they cannot possibly allow us to resolve fine details.
Thus, it is our switch from the colour-sensitive, but light-insensitive, cone system to the colour-insensitive, but very light-sensitive, rod system that causes us to loose our colour vision at night. Or as it was once written by the rock band, The Moody Blues:
Cold hearted orb that rules the night
Removes the colours from our sight
Red is grey and yellow, white
But we decide which is right
And which is an illusion
Another part of the explanation is that the moon is generally covered in dust. Fine powders often appear white whatever they are made from, as the grains scatter light randomly in all directions, rather than being able to selectively absorb or reflect different colours.
The colour varies slightly from region to region due to differences in mineral content, but the actual shade/brightness of the moon is pretty dark. The average albedo of the moon is 0.136, which means it reflects %13.6 of the light that hits it, about the same as worn asphalt.