Publications Topics:

Books

ASP Conference Series

Monograph Publications

IAU Publications

Books of Note

Purchase through the AstroShop

Journals

Publications of the ASP (PASP)

Magazines

Mercury Magazine

Archive

Guidelines for Authors

Order Mercury Issues

Mercury Advertising Rates

Newletters

The Universe in the Classroom

ASP E-mail Newsletters

Special Features

Astronomy Beat

Contact Us

Mercury, January/February 1997 Table of Contents

James C. White II
Middle Tennessee State University

Sunsets can pull us out of our easy chairs and into the chill of winter or the balm of summer, just for a better look. Let's give them a little more scrutiny.

This month's project: coloring sunsets

"Mother," asked the child, "why is the Sun soooo red?" Driving west into the light from a Sun dropping beneath the horizon, the woman thought for a moment and replied, "Because it's put on its red pajamas for sleep just like you do!" The child squinted as she faced the Sun's dying light and thought: Mom doesn't know why the Sun is red. Explosions of reds and oranges hang over us at the borders of night and day; turtles of white nothingness crawl along the cerulean ceiling. The sky and its many hues are just some of the many benefits of living at the bottom of a sea of air.

Made of nitrogen, just enough oxygen to allow us to breathe easily while running up stairs, and a few other gases, air also carries dust, pollen (Gesundheit!), and other small particles. All these airy constituents deflect light. Consider the effect of dense fog on your car headlight beams: The beams penetrate only a short distance into the fog because the tiny water droplets of the fog scatter the light in all directions.

Atmospheric gas molecules do a similar thing to sunlight. They scatter it into new directions, and do so more efficiently for light at short wavelengths (blue) than at long ones (red). The result: nice blue skies when the Sun is up. The blue sky is simply sunlight deflected by the gas molecules all around us. Compare this to the sky on the airless Moon: no blue, just black everywhere except in the direction of the Sun.

As the day advances and the Sun approaches the horizon, its light must travel ever greater distances in Earth's atmosphere to reach us. On its way, most of the blue light is scattered out, so we only see what's left: the red. To our west, people still see blue sky, while the red sunbeams whiz over their heads and land on us (see box). There is additional scattering by those feisty airborne particulates, removing all but the longest visible wavelengths of light. We are left at the end of daylight with a tired, red Sun swathed in a colorful sunset. Red pajamas?! Hey, ol' mom knows just about everything!

Observing Guidelines

What is it that makes one sunset merely pretty, and another worthy of dragging your honey outside to share it with you? Your sunset observations will try to find out. They will involve not only observing the setting Sun, but also estimating the humidity and atmospheric visibility.

The amount of water vapor in the air and the concentration of airborne particulates affect the amount of sunlight scattering. You can obtain humidity levels from your local weather source, such as the nightly weather report, or your very own hygrometer. You can estimate atmospheric visibility, a measure of the particles in the air, in the following fashion: Pick a western line-of-sight you can use each evening which shows you landmarks a good distance away; for example, an east-west street. Looking down the street, you'll notice on some evenings that you can see that big elm tree a mile away, but on other evenings you'll barely be able to see it because of the stuff in the air. Thus, you'll be able to make a relative determination of visibility; say, "good visibility to 1.5 kilometers this evening."

Begin your observations when the Sun is low in the western sky, but don't begin too early, because the Sun will be too bright; never look at the Sun if it is uncomfortable to do so. The first thing you'll want to determine is the Sun's altitude (its apparent distance above the horizon, measured in degrees) when you can first see reddening. Again, don't stare at the Sun; glance. To measure the altitude, use your outstretched arm. A fist marks off 10 degrees; your pinky, 1 degree. Record how the altitude of first reddening changes from night to night.

Characterize the changes in color as the Sun sinks further. How does the color at an altitude of 10 degrees differ from that at 5 degrees? What colors do you see? Where do you see them? Are there clouds? Describe the effects of the scattered sunlight on cloud colorings.

Over several evenings, you will become more comfortable in your observations. "Red" this evening will not be the same as "red" last evening. Combine your written descriptions with your knowledge of the humidity and atmospheric visibility, and consider the effect of prevailing weather. How does the sunset appear on a evening with, say, good visibility but strong wind?

Finally, sketch or photograph what you see. Use pastels, watercolors, or crayons to reproduce the sunset; your choice of color on the page will enhance your interpretation of color in the sky. Photographing sunsets is a little more difficult. You need a wide view of the horizon, so put that telephoto lens away. The best exposure time will probably be between 1 and 20 seconds; it requires experimentation on your part.

Please submit your completed report (see previous issues for details) by March 31, 1997 by email to jisles@voyager.net or by regular mail to John Isles, Attn: Guest Observers, 1016 Westfield Drive, Jackson, Mich. 49203¯3630. The selection committee will evaluate the reports and choose the Guest Observer for an upcoming issue.

Sunset in a Jar

• With a jar, some water, a little powdered milk, and a light, you can make your own blue skies and reddish sunsets. Fill a 1500-milliliter beaker or a quart Mason jar with water. Place it on an overhead projector if you have one; if not, just put the beaker in front of a slide projector or powerful flashlight. Stir the water a little to get it swirling, and add about a quarter teaspoon of powdered milk. As the milk dissolves, watch the beaker begin to glow blue ­ the blue component of the white light gets bounced in all directions, while the red and orange sail right on through.

• If the beaker is on an overhead projector, look at the color of the light projected on the screen: first it is grayish, then ruddy, then a dirty orange. If your beaker or jar is in front of a slide projector, look directly through the water at the light source. See how orange it is?

JAMES C. WHITE II is a professor in the Department of Physics and Astronomy at Middle Tennessee State University in Murfreesboro. The activity is courtesy of White's colleague, Victor J. Montemayor.