Venus puts on the show of the century

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.

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