Cardinal Light Phenomena

The light phenomena associated with the cardinal directions are white light in the east, associated with dawn, blue light in the south, associated with midday, yellow light in the west, associated with evening twilight, and darkness in the north, associated with the night. These associations are easily understood by examining the diurnal changes in the atmosphere of the earth and the light phenomena that then result.

The atmosphere is coolest in the early morning. The lack of heat input during the night has, in general, led to a decrease in the winds driven by temperature differences in the atmosphere and thus much of the dust that was stirred up by winds and human activity during the day has settled out of the atmosphere. This fact leads to the difference in color seen between dawn and evening twilight. The color of the sky is entirely attributible to the Rayleigh scattering undergone by the sun's light as it passes through the earth's atmosphere on the way to the earth's surface. The light rays are scattered by the particles in the atmosphere: molecules of air, water drops, ice crystals and dust particles. The shorter the wavelength of the light (i.e. the bluer the light ray) the more it is scattered. Therefore the blue light is scattered over a greater range of angles than is the red light. This is the reason that the sky appears blue in all areas away from the sun itself. Near the sun there is a greater contribution from the light rays of other colors. Very close to the sun, where all colors are present, the sky will appear white. This explains the blue color associated with the midday sky.

Why does the sky not appear the same color at morning and evening twilight, when the sun is low in the sky? The light path is similiarly long in both cases. As I mentioned before, the dust in the atmosphere, except in exceptional conditions, will settle out during the night. This means that most of the larger particles, those that most efficiently scatter the light of longer wavelengths, are not in the atmosphere in the morning and can be there in abundance in the evening, after the dust has been stirred up by human activity and winds caused by thermal gradients. For this reason, the evening twilight will usually appear yellow-orange and sometimes even distinctly red. Because of the very long path length of the light through the atmosphere, all of the short wavelength light will have been scattered out of the path to the viewer. However, at dawn, most of these large particles are absent. Therefore, much less light is scattered out of the light path. For this reason the sky appears blue away from the sun, when there is enough light to see, and white near the sun, in the east, where little light is scattered out of the light path, while in the evening a large area around the sun will appear red-orange to red.

Everyone will easily accept darkness as properly associated with night, when the sun is down. However, the atmosphere still scatters starlight and moonlight. Astronomers have measured the brightness of the night sky as a function of wavelength (color) and show that the sky between the stars still has much more blue light than red light. It is just so faint that the human eye is not sensitive enough to make the color discrimination, and thus sees the sky as dark in all colors. In star atlases many bright stars are said to have very distinct colors. However, many people are unable to see these colors because the variations in sensitivity of the human eye are great.

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