The earth’s shadow (blue) and the Belt of Venus (orchid) behind Left and Right Mitten Buttes in Monument Valley. Click here to see larger image. Photo by Jeff Mitton.
By Jeff Mitton
We sit on a spinning top, for the Earth rotates each day on its axis, causing it to appear that the sun rises in the east, traverses the sky, and sets in the west, when in fact the sun is relatively stationary but our view of the heavens is changing by 0.25 degrees per minute.
The apparent rising and setting of the sun may sometimes seem to be prosaic, everyday events. But when we have time to watch and appreciate, sunrises and sunsets are fascinating and entertaining interactions of light and atmosphere providing daily reminders of our place on a spinning planet.
Light emanating from the sun passes through space with little distortion, but once it enters the atmosphere, it is selectively filtered by air. The degree of filtering, from a fixed position on our spinning planet, is a direct function of the position of the sun in the sky.
The rising band of blue is the portion of the atmosphere not longer lit by the sun. It is literally the shadow of the earth, and it continues to spread up into the sky until night is full, when we view the sky through the shadow of the earth."When the sun is at its highest point, sunlight takes its shortest path through the atmosphere to reach the ground. However, at sunrise or sunset, sunlight traces a much longer path through the atmosphere to reach the same point. This is because the depth of the atmosphere is a thin skin in comparison to the diameter of the earth.
The majority of the atmosphere is within 10 miles of the surface, but when we see the sun rise above the horizon, that horizon might be 50 to 100 miles away. When the sun is at the horizon, it penetrates 40 times as much air as it does at noon and consequently dawn’s light is softer and has different properties.
The visible light from the sun is electromagnetic radiation with wavelengths from 400 to 700 nanometers. From shortest to longest, wavelengths are perceived as violet, indigo, blue, green, yellow, orange and red, but the full mixture of these is perceived as sunlight or white light.
When I was 7, my maternal grandfather, who was also my optometrist, showed me how a triangular piece of glass could refract or differentially bend sunlight to reveal its component colors.
Our atmosphere scatters light, filtering short wave lengths more than long wavelengths. Consequently, we see more of the long wavelengths—red, orange, yellow—at dawn and dusk, when light makes the longest transit through the atmosphere.
Immediately after the sun drops below the horizon in the west, a deep blue line forms on the eastern horizon, and in the next several minutes the band of blue rises higher into the sky. The rising band of blue is the portion of the atmosphere not longer lit by the sun. It is literally the shadow of the earth, and it continues to spread up into the sky until night is full, when we view the sky through the shadow of the earth. Just before dawn, the shadow of the earth shrinks to the horizon in the west.
When the air is clear and clouds do not occlude the sky, a zone of orchid to lavender is evident above the shadow of the earth. The common name for this band of color is the Belt of Venus, perhaps because the planet Venus often appears brightly lit near the horizon before dawn and after dusk.
The Belt of Venus is immediately above the shadow of the earth, and it is the portion of the sky where sunlight makes its longest transit through the atmosphere. The red light is back scattered by fine dust high in the atmosphere, and the orchid to lavender is the composite color of the backscattered red and the blue sky seen through it.
Alpenglow is a rosy glow most apparent on snow-covered mountains. The glow is an indirect illumination from light backscattered by snow, water, or ice particles low in the atmosphere and it is apparent on the horizon away from the sun just before sunrise or after sunset.
Jeff Mitton, mitton@nhllivebetting.com, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.
March 25, 2016