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Mercury, July/August 1997 Table of Contents

James C. White II
Middle Tennessee State University

Earth's simmering sibling burns through our early evening skies in July and August. Let's follow its encounters with celestial crabs, lions, and virgins.

About 10 years ago, Weekly World News, that august publication, detailed the discovery by scientists of a replica of Dodge City on Venus. Not a contemporary Dodge City, either. No, scientists had stumbled upon a reproduction of the city we all know from westerns, complete with saloons, men in large cowboy hats, and women strolling down Main Street in crinolines.

That fantastic story about Venus, the second planet out from the Sun, played upon images of the planet introduced by science-fiction writers in the 19th and early 20th centuries. There were large oceans lapping at sandy shores, tall palms, and beautiful, intelligent Venusians (that looked like us, of course). Nice place for Spring Break, if only it were true.

When you actually observe Venus, referred to as "Earth's twin" because its size and density are roughly the same as our planet's, the striking feature is that no features are striking. Venus's bland, dense, cloudy atmosphere does not allow us to see the surface. Astronomers in the 1930s, looking at the sunlight reflected from Venus, discovered that its atmosphere is rich in carbon dioxide.

Now, unless you have been in a cave the past 20 years, that carbon dioxide should ring alarm bells. Like the panes of glass in a greenhouse, carbon dioxide in an atmosphere allows sunlight through to the ground, but when the ground heats up and emits infrared radiation, the carbon dioxide will not let this heat radiation leak back into outer space. Consider how hot your car gets on a sunny day when you leave the windows up. The seats and dash absorb sunlight, heat up, and emit invisible infrared radiation. But the glass windows prevent this radiation from escaping, so the interior of your car swelters.

Venus's atmosphere, 96 percent carbon dioxide, is a car with the windows rolled up tight. In Earth's atmosphere, the fraction of carbon dioxide is only 0.03 percent, so the greenhouse effect is modest -- for now. Most experts think that rising levels of carbon dioxide, from cars, power plants, and rain-forest destruction, will mean a higher temperature on Earth, too.

The misery of Venus is compounded by the weight of its atmosphere -- 90 times that of Earth's. People stupid enough to be standing on Venus's surface would feel about 1,300 pounds per square inch on their bodies. Naturally, they would be crushed by this incredible pressure, just as though they were 3,000 feet (900 meters) under the sea.

In the 1970s a robotic Soviet spacecraft made it to Venus's surface and lived a few seconds to tell about it. The probe's descent through the atmosphere was treacherous, not least because it had to pass through clouds of sulfuric acid. Upon reaching the surface, the probe held out against the metal-crunching pressures just long enough to transmit back the temperature reading: 900 degrees Fahrenheit (500 degree Celsius). That's almost twice as hot as your oven broiler.

Now we know what it is really like on Venus. A bright day would be about as bright as an overcast day here on Earth. Because of the sulfuric-acid droplets in the atmosphere, the sky would be a hazy yellow. The temperature and pressure would be oppressive; well, deadly actually. Most certainly not a suitable place for building replicas of old western towns.

Observing Guidelines

Venus will be spectacular during July and August, almost as if it's putting on its best show because it knows mighty Jupiter will soon enter from stage east. For this Guest Observer project, let's concentrate less on Venus and more on the general area of the sky it will pass through. The planet's motion, individual stars, star clusters -- these are our targets. And for this mission, should you choose to accept it, you'll need paper, pencils, binoculars, plenty of sweet beverages, and if you are, like me, a telescopaholic, your handy telescope.

Over the next two months, Venus will journey through three constellations. For a part of the trip, tiny, fleet-footed Mercury will jog along. I encourage you to refer to Mercury's SkyCharts or to Sky & Telescope's or Astronomy's evening sky maps for help in charting the planets' progress.

As July arrives, Venus sits in the western evening-twilight sky amid the stars of Cancer. It is the brightest object in the sky and impossible to miss. Mercury will be in Gemini then, hugging the horizon down and to the left of Venus, directly beneath Gemini's bright stars Castor and Pollux. From night to night, note how fast the two planets move relative to the background stars; how their altitude, measured at a specific time of day, changes; and how their brightness changes.

Keep a written record of the stars and clusters along the way. How would you describe Pollux's color compared to that of, say, Regulus or Spica? Oh, and goodness me, but don't forget the Moon! Draw Luna as you see it, growing from a thin crescent the first week of July.

Please submit your completed report by Sept. 30, 1997, by regular mail to Jay White, Attn: Guest Observers, Department of Physics and Astronomy, Middle Tennessee State University, Murfreesboro, Tenn. 37132. The selection committee will evaluate the reports and choose the Guest Observer for a future issue.

JAMES C. WHITE II is a professor in the Department of Physics and Astronomy at Middle Tennessee State University in Murfreesboro. Don't tell anyone, but he's really a Venusian. His email address on Earth is jwhite@physics.mtsu.edu.