On the moon, the sky is black and the stars shine clearly even when the sun is high. But here on earth the clear daytime sky is blue and no stars can be seen through it. This was explained by John Tyndall, Director of the Royal Institution, in the 1860s as the scattering of sunlight in the upper atmosphere. He showed that small particles scatter light of short wavelengths more strongly than longer ones: blue more than red. That is why fine smoke and mist look blue. It is well known that distance, as depicted in a painting for example, involves a graded blue tinge. A. E. Housman asked ‘What are those blue remembered hills?’ and Thomas Campbell gave the answer ‘’Tis distance lends enchantment to the view, And robes the mountain in its azure hue’. Tyndall explained that even on clear days a very long propagation path will accumulate shorter wavelengths that have been scattered to form a blueish veiling.