IMAGINE, if you can a bright sunny day on the moon, with not a single cloud to mar the lunar sky. There would be one striking difference compared to an equivalent summer's day on Earth although what Yeats called the "golden apple of the sun" would be very much in evidence, the rest of the sky would be quite black.
The reason for the difference is that we on Earth are lucky enough to have an atmosphere, while the moon does not.
But even here on our own planet, the blueness of the sky can vary significantly from place to place. The skies over the Greek islands are a deep, deep blue, and those over Ireland on a fresh showery day are an even richer shade of the same colour, but skies over continental Europe are paler by comparison, a study in pastels quite anaemic to our Irish eyes.
To understand why we must recall why the sky is blue in the first place. Light coming from the sun is white light, a mixture of lights of all the colours of the spectrum. But light, as it travels through the atmosphere is subject to a process known as scattering it is obstructed by any tiny particles in its way, and even by the molecules of the air itself, and deflected away from its original path.
The effectiveness of this scattering process depends both on the colour of the light and the sizes of the scattering partides. When the atmosphere is very clean, the tiny molecules of air scatter only blue light the red and orange parts of the solar spectrum, and indeed all the other colours, surge straight through, virtually unaffected.
So when you look upwards at an Irish sky, away from the sun, what you see is sunlight that was originally heading in an entirely different direction, but has been scattered towards you by the atmosphere. And since only the "blue" light is affected in this way, you see the whole sky, except for the sun itself, as blue.
Now imagine yourself transported to the Continent. The air over Europe contains a much greater concentration of impurities from industrial and other sources than does ours particles which are many times larger than the molecules of the air itself. These larger particles are capable of scattering, not just the blue light, but many of the other constituent colours of sunlight as well.
As a result, the mixture of scattered light which reaches our eyes is a mixture of many colours and has a tendency towards white, making the continental sky a much paler shade of blue than that which we see at home.