Page:Astronomy for Everybody.djvu/142

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THE SUN, EARTH, AND MOON

tropics a beautiful effect may be noticed, which can rarely or never be seen in the thicker air of our latitudes. It arises from the unequal refraction of the rays of light by the atmosphere. Like a prism of glass the atmosphere refracts the red rays the least and the successive spectral colours, yellow, green, blue, and violet, more and more. The result is that, as the edge of the sun is disappearing in the ocean, these successive rays are lost sight of in the same order. Two or three seconds before the sun has disappeared, the little spark of its limb which still remains visible is seen to change colour and rapidly grow paler. This tint changes to green and blue, and finally the last glimpse which we see is that of a disappearing flash of blue or violet light.

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