sky, apparent dome over the earth, background of the clouds, sun, moon, and stars. The blue color of the clear daytime sky results from the selective scattering of light rays by the minute particles of dust and vapor in the earth's atmosphere. The rays with longer wavelengths (the reds and yellows) pass through most readily, whereas the shorter rays (the blues) are scattered. An excess of dust, especially in large particles, causes scattering of many rays besides the blue, and the sky "fades" and becomes whitish or hazy. The sky thus is clearest in winter, in the morning, after a rain, over a mountain, or over the ocean. Leonardo da Vinci experimented with light and attempted an explanation of the sky's blue color. The work on light and its behavior by Sir Isaac Newton, Lord Rayleigh, and other physicists provided explanations of rainbows, sky color, mirages, and other atmospheric phenomena.