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HOW COME? Sun's changing face is all atmospheric

Why does the sun appear yellow sometimes and orange other times? asks Alex Peyser, of Bayside, Queens, N.Y.

The rosy pink sunlight streaming through your bedroom window at sunrise and the fiery red-orange at sunset don't seem to match the sun of midday. There are red stars in other solar systems, the bloated "red giants" hundreds of times the size of the sun. But our sun is a medium-size white star.

So why the fiery red sunrises and sunsets, followed by bright white noons? Looking at the sun through the Earth's atmosphere is like looking through a veil. Sunlight must pass through layers of air before it reaches our eyes, and is changed by its trip.

White light like sunlight is made of a hidden rainbow of colors -- red, orange, yellow, green, blue, indigo and violet. The rainbow colors are revealed when sunlight passes through a prism, different wavelengths (colors) of light going their separate ways. We also see the rainbow colors in an arc across the sky when the sun is at our back, raindrops falling in front of us.

When sunlight, traveling through the vacuum of space at 186,000 miles per second, enters Earth's atmosphere, some of the light zooms cleanly through, reaching the ground without encountering a single gas molecule. But since the Earth's air is made of zillions of gas molecules, some of the light will run into many on its way down.

And when it does, the light is scattered.

It is mostly the bluer end of light's spectrum that is scattered out of the beam of white light, creating blue skies. So by the time sunlight reaches our eyes, its blues and violets are diminished. This makes the sun's face look yellower to us than it actually is. The noonday sun appears closest to its true color. Why? When the sun is overhead, its light must pass through only the air above our heads, which becomes thinner and thinner higher up. Result: Much of the white sunlight reaches our eyes unscathed.

But at sunrise and sunset, when the sun is low in the sky, the color of its face changes more dramatically. Before it enters our eyes, light from the setting or rising sun must travel through the thickest layers of air near the ground, which extend from us to the horizon.

The sunlight streaming toward us is passing through an obstacle course of air molecules, dust and pollutants, so even more of the cooler, blue-violet end of the spectrum is scattered out of the white light beam. By the time the sunlight reaches us, it's mostly warm yellows, oranges and reds remaining in the beam. And the sun's face appears to blush rose, or blaze orange.

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