Jessica Davidson, Balclutha Primary School, asks :-

Why does the Moon look red sometimes? What does it mean?

John Campbell, a physicist at the University of Canterbury, responded.

It means that the air between you and the Moon is dirty.

Firstly, consider the Sun. It emits white light and this light travels through empty space to reach the top of the Earth's atmosphere as white light. Our atmosphere extends only a few tens of kilometres above the surface of the Earth, getting less dense as you go up. For example, jet airliners, which need the oxygen in air to burn the kerosene which fuels their engines, fly at about 10km high, whereas satellites, which want to be above the friction of the air so they continue to orbit un-powered, are over 100km high.

Because of micro-fluctuations in the density of the air, and large impurities in the air, the white light is scattered. This is why we don't have a black sky like we would see if we were on the Moon, because it has no air layer. White light, the tiny region of the electromagnetic spectrum which our eyes register, ranges from red light to blue light. The amount of scattering depends very strongly on the wavelength (think the equivalent of the distance between wave crests at the beach). Blue light is only half the wavelength of red light so is scattered 16 times more. That is why on a clear day our sky is blue. The light from the Sun is scattered in all directions into our eyes and because the scattered light is mostly blue that is the sky colour we see.

If the Sun is overhead, the path of the light is the shortest possible and because not much blue has been scattered away the Sun still looks white. There are many ways we can increase the scattering by increasing the number of tiny clusters in the air. For example, large forest fires put lots of smoke (carbon) particles high in the air. Another cause is volcanic eruptions throwing tiny rock particles high into the atmosphere. Or in humid countries where very tiny water droplets form on dirt particles in the air. For all of these, the increase of blue light scattered away can make the Sun look red.

The Moon reflects enough of the Sun's white light that it can be seen as white at night, and near dawn and dusk when the sky isn't very bright. To see the Moon as red, look at it when it is low on the horizon or when there is a lot of smoke or volcanic ash in the air. To save waiting, you could add a few drops of milk (i.e. some large fat molecules) to a glass or bottle of water. Look at a white surface through it and it will appear reddish. Look at light scattered from it and it will be blueish.