Verity Wood, of Ilam School, asks :-

Ryan McIntosh, of King's High School, asked:- Paul Manson, of Nayland College, asked:- Emma Malone, of Christ the King School, asked:- Room 6, of Balclutha Primary School, asked:- Emma Hendriks, of Sawyers Bay, asked:- The year 10 class at Marlborough Girls' College asked:-

Why is the sky blue?

Don Warrington, a physicist at Otago University, responded.

The sky on the moon is black but on earth is blue. The difference is that the earth is surrounded by an atmosphere of gas, that extends some 100 kilometres high, whereas the moon has no atmosphere. Hence the answer lies in the way light is scattered by our atmosphere.

Whenever we see anything it is because light is coming from that thing to our eyes. If something is very hot, like a fire or the sun, it is able to produce the light that our eyes can see, but most things we see (grass, people, flags etc) are not hot enough to emit their own visible light. We see them only because there is light from something else (usually the sun or a light bulb) shining on them and they reflect some of that light towards our eyes. The sky is like that. It is made up of air which is not hot enough to give out light of its own but which reflects (scatters) a little bit of the sunlight towards our eyes.

The light from the sun is white light which is a mixture of all colours, from blue through green and yellow to red. It turns out that blue light is scattered by the atmosphere much more than red light is so the sky appears blue. This also explains why the sun appears red at sunrise and sunset. Those are the times when the light from the sun directly to our eyes has to travel through the maximum distance of air and hence all the blue light has been scattered away leaving only the red light to come directly to our eyes.