Venus puts on the show of the century

Where do astronomers work? The answer seems easy. Astronomers work in observatories, right? They arrive at sundown. They open the big revolving dome, point the telescope at the heavens, and spend the night looking at the stars. As the sun begins to rise, they close up the dome and head home, satisfied with a good night's work.

Not exactly. Only a few astronomers work with telescopes at night. Most work during the day. They sit at desks, think about the stars, and try to understand how the universe works. They try to solve puzzles. Only when they think they have solved one do they run to their telescopes to see if their solution is correct.

Here's an astronomical puzzle for you: We know that the sun is at the center of our solar system. Around the sun circle the planets. Mercury is closest to the sun. Venus is next, followed by Earth and Mars. The big planets are beyond Mars, after the asteroid belt: Jupiter, Saturn, Uranus, and Neptune. Pluto is usually farthest from the sun.

Most drawings of the solar system show the planets orbiting in the same plane, as if they were sitting atop the same table. Here is the puzzle: Is that picture of the solar system correct? And if it is, how do you know for sure?

Think about it. Picture the planets rolling around the sun on the same tabletop. If that model is correct, then every now and then Mercury and Venus would pass between Earth and the sun. This is what happens a few times every year with our moon. When the moon passes between us and the sun, we call it a solar eclipse, but you could also say it was a "transit of the moon." Neither Mercury nor Venus could blot out the sun partially or fully, the way the moon can. They are too far away. They would just look like little black dots on the sun.

But have you ever heard about Venus going across the sun? If our tabletop solar system is correct, then a little mathematics would show that Venus should pass between Earth and the sun every 586 days or so. Yet for the entire 20th century, this did not happen. Now we know part of the answer to our puzzle: The "tabletop" solar system is not quite right.

So how do Earth and Venus keep missing each other? Earth must be a little higher than Venus, or perhaps Venus is a little higher than Earth. If this is so, then the orbit of Earth and the orbit of Venus must be tilted a little. Even though the orbits are tilted, the planets must line up now and then so that Venus is directly between the earth and the sun. Again, using some mathematics, we can show that a "transit of Venus," as it's called, will happen only twice every 120 years or so.

This puzzle was first solved in the 17th century by German astronomer Johannes Kepler. Kepler, working at his desk, reasoned his way through the problem just as we have done (though with more mathematics). He computed that a transit of Venus would occur on Dec. 6, 1631. Kepler died in 1630, and the transit occurred when it was night in Europe. No European astronomer observed it.

The next transit, on Dec. 4, 1639, was seen by English astronomer William Crabtree. Other transits occurred in 1761 and '69, and again in 1874. The most recent transit occurred on Dec. 6, 1882. The next transit of Venus is a week from Tuesday, on June 8. Venus will start passing across the face of the sun just about the time the sun rises in the Middle East. It will take about six hours to cross in front of the sun. If you live in the eastern half of the United States you might be able to see some of the transit from sunrise until about 7:25 a.m. ET.

Transits of Venus occur in pairs, eight years apart. If you miss this one, the next one is June 6, 2012. But after that, you'll have to wait until 2117.

When all is said and done, next week's transit of Venus will have little impact on our daily lives. To astronomers of the 18th century, however, the event was a once-in-a-lifetime chance to try to compute Earth's distance from the sun and so determine the scale of the solar system.

Seen at the same moment from widely separated observation posts on Earth, Venus would appear to be in slightly different positions relative to the sun's disk. (This position change is due to "the parallax effect." You can demonstrate it: Stretch out one arm with your thumb raised. Close one eye, and cover a distant object with your thumb. Now close that eye and open the other. The object under your thumb seems to jump to one side.)

With a little geometry and a little mathematics, you can use parallax and a transit of Venus to figure out the distance from Earth to the sun. Once you know that distance, you can use Johannes Kepler's (1571-1630) laws of planetary motion to calculate the orbits of all the other planets. An accurately observed transit of Venus would therefore provide a windfall of scientific knowledge.

In 1677, English astronomer Edmond Halley (of Halley's comet fame) first proposed that a transit of Venus could be used to find the distance from Earth to the sun. Halley died before his theory could be tested. But the Earth-to-sun figure astronomers came up with during the 1761 transit - about 95 million miles - was pretty close to the figure we have today: 92,955,807.267 miles.

Never look directly at the sun to see the transit of Venus. The Sun is too bright, anyway, and Venus too small. Do not try binoculars with sunglasses, either - no sunglasses are safe enough. Instead, make a pinhole camera out of a shoe box.

Get a shoe box. Remove or discard the top. Using the point of a nail, punch a hole in the center of one of the narrow ends. Glue or tape a white piece of paper onto the inside of the other narrow end. With your back to the sun, hold the box over your shoulder with the hole pointing at the sun. Look into the open top of the box.

An image of the sun - a small white dot - will be projected onto the white paper. Venus will be an even tinier black dot. The longer the box, the bigger the projected image will be. Test this device before you use it to view the transit.

Note: Pinhole images can be dim and small. A more advanced method of viewing a projection of the transit calls for binoculars, duct tape, white cardboard, and a tripod. For instructions, go to: www.exploratorium.edu/venus/ question2.html

The safest and most surefire way to see the transit is via live webcast. See information box, below, for Web addresses.

On June 8, Venus will be crossing the sun from 1:13 a.m. to 7:25 a.m., ET. Obviously, you can see the transit only after sunrise (about 5 a.m. ET). It will be visible to those living in the eastern United States. The best views will be from Asia and Europe.

But anyone can see the transit via live webcast on the Exploratorium museum's site: www.exploratorium.edu/venus. It will be broadcast from the Penteli Astronomical Station near Athens. High-resolution images will be updated every 15 minutes.

The Exploratorium's website also has histories of past transits, facts about the immensely inhospitable planet, and curriculum material for teachers.

NASA is providing a list of transit-of-Venus webcasts, history, and activities, as well as celebrations in your area. Go to: sunearth.gsfc.nasa.gov/sunearthday/2004/index_vthome.htm. There you'll also find exact transit times for hundreds of cities.