Vanishing Act

Mark your calendar: Feb. 26 is the Western Hemisphere's last total eclipse of the sun until August 2017.

The best place to watch it will be from the northern tip of South America or from a handful of islands in the eastern Caribbean. That's where you'd see all of Ol' Sol slip behind the moon. But the eclipse still promises to put on a good show in many parts of North America.

To watch what one astronomer calls "one of the seven wonders of the universe," you'll need to follow two simple rules.

Rule No. 1: Never, never look directly at the sun with the naked eye, even if the sun is shining through clouds.

Rule No. 2: See Rule No. 1.

So, you ask, how am I supposed to watch it?

Alan MacRobert, an associate editor at Sky & Telescope magazine, has some great ideas. The easiest way to look, he says, is to make a simple pinhole camera. Use a pin or thumbtack to poke a small hole in a piece of cardboard. Hold the cardboard so the hole faces the sun. Let the light from the hole shine on a piece of white paper. Slowly move one piece or the other back and forth until you get a good image of the sun.

The best image won't be very big - perhaps 3/16ths of an inch across. But it will allow you to watch safely.

Grab a No. 14 welder's filter

If you are indoors, you can try another method. Tightly cover a small mirror with a piece of paper that has a 1/4-inch hole punched in it. Place the mirror in a south-facing window. Angle the mirror so that light from the 1/4-inch opening shines on a far wall. The image will be only 1 inch across on a wall 9 feet from the window. You may want to darken the room as much as possible. You might even try testing holes of different sizes to see which one gives you the sharpest, brightest image.

If you have a telescope, or even binoculars, you can aim them toward the sun and let the image fall on a piece of paper. (NEVER look into a telescope or into binoculars aimed at the sun unless the lenses are covered with sun filters designed to fit them.) Bring the image into focus either by moving the paper closer or farther from the telescope or by twisting the focus knob on the 'scope. This method will give you the largest image of the sun.

Finally, if you use a special kind of darkened glass (a No. 14 arc-welder's filter), you can look at the sun directly. Some people may tell you that it's OK to look through a CD, through several layers of fully exposed film, through soot-covered glass, or even through the silvery envelope that covers Pop Tarts. Don't believe them!

A scientist in Canada tested all kinds of "filters" people have used. None of them was safe except the arc-welder's filter and a special kind of solar filter made with mylar. Arc-welder's filters may be easier to find. All five of the welding-supply stores we contacted in Boston had them. Depending on the size of the filter (2 by 4 in., or 4 by 5 in.), the prices ranged from $1.50 to $5 each. A couple of salespeople said their filters were selling fast. Word is out about the eclipse and the filters' safety.

Whichever viewing method you pick, you can be sure that you will be joined by scientists around the world. They are traveling to where the eclipse will be total.

For one solar scientist, Shadia Habbal of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., this month's event is payback time. Last March, she traveled all the way to Mongolia to study a total eclipse of the sun. At 3 a.m. the morning of the event, snow started falling. The foul weather that day ruined her experiment. This year's eclipse allows her to swap the snowy plains of Mongolia for the sunny sands of Guadeloupe in the Caribbean.

The mystery of the too-hot corona

Dr. Habbal studies the sun's corona, a very hot expanse of gas that extends far beyond the sun's surface. The sun's surface layer, called the photosphere, is responsible for the sunlight we see. And it's hot - about 8,500 degrees F. But the corona is even hotter: about 2 million degrees F. That's weird if you think about it. You get cooler when you walk away from a radiator or a campfire, not hotter.

"Why is the corona so hot?" Habbal asks. "That's a fundamental issue."

High-priced satellites can study the sun's corona nonstop from space. But watching from Earth still has some advantages. When the moon blocks the sun, scientists' high-tech cameras can see parts of the corona that are very close to the photosphere. They can't look as deeply when using mechanical disks on satellites or telescopes to block the sun. And eclipses allow some scientists, who are designing new sun-staring satellites, to tweak their instruments cheaply on earth, rather than launching test models into space.

This eclipse also will be the first time that scientists have used radio signals from a distant spacecraft to study the corona. Researchers will measure radio signals from Galileo, which is orbiting Jupiter and its largest moons, as the signals pass through the corona on their way to Earth. This will give scientists one more tool to use for unlocking the corona's secrets.

Coming Soon To a Sky Near You

A total eclipse of the sun occurs when the moon - in an amazing bit of planetary proportion - exactly covers the disk of the sun.

In an annular eclipse, the moon does not cover the sun completely. Observers see a bright 'ring of fire' around the moon. This is because the size of the moon shrinks in relation to the sun when the moon is farther from the earth because of the moon's orbit, or the earth is closer to the sun.

This illustration shows the path that the darkest part of the moon's shadow (called the umbra) will trace on the earth's surface during a solar eclipse. The shadow, about 170 miles wide, travels 2,000 miles an hour (0.6 miles per second) and is only visible for 2-1/2 to 7-1/2 minutes in any spot.

For more information, visit Sky & Telescope's Web site at:

skypub.com

Click on their "Eclipse Page."

The Exploratorium, a San Francisco science museum, will sponsor a live 'webcast' of the Feb. 26 eclipse from Aruba, beginning at 9 a.m. Pacific time. (Totality is 10 a.m. PST.) See: exploratorium.edu/eclipse

What Happens During a Solar Eclipse

The ancient Chinese thought an eclipse was a dragon swallowing the sun. Chaldean astronomers in the Mideast discovered that eclipses occur in an 18-year cycle they named a saros. But it wasn't until the 1500s that people understood the mechanics of an eclipse.

Solar eclipses occur when the moon passes between the earth and the sun, casting a shadow that falls on the earth. The darkest part of the shadow, the umbra (Latin for shade, shadow), is relatively narrow. A much wider, lighter shadow is called the penumbra.

Solar eclipses would happen all the time if the earth, moon, and sun orbited in the same plane. But the moon's orbit around Earth is slightly tilted in relation to Earth's orbit around the sun, so all three objects don't line up as often.