Those who missed the solar eclipse two weeks ago may get an even rarer chance this week to see the sun dim when Venus sweeps in front of our home star Tuesday. Miss it and the next showing won't be for another 105 years, astronomers say.

"Just knowing that you're seeing something happen that won't be seen by anyone alive today again is a really neat experience," said Holly Gilbert, a solar physicist at the Goddard Space Flight Center in Maryland.

When viewed from the Bay Area, Venus will breach the sun's blinding edge at 3:04 p.m. and will cast a tiny silhouette for about five hours as it slowly crawls across the sky, until the fiery orb dips into the horizon with the planet still in its spotlight.

Sun gazers should use special solar sunglasses or telescopes with solar filters and never stare directly into the sun, Gilbert warned. There will also be real time data from NASA's Solar Dynamics Observatory, a telescope orbiting the Earth dedicated to observing the sun. The data will be available on the observatory's website: http://venustransit.gsfc.nasa.gov/data.

The transits of Venus come in pairs eight years apart. The 2004 transit was viewable in many parts of the world, but missed the West Coast of the United States.

This time around, the transit will be viewable in all U.S. states, and the best place to view the full 6½-hour show will be Hawaii.

Gilbert said she'll be traveling to the Mauna Kea Observatory on the Big Island to help NASA broadcast a live webcast of the event, 2.6 miles above sea level.

Scientists will also be using the opportunity to precisely measure the size of the sun by measuring exactly when the sun's brightness dips, Gilbert added.

Venus will appear be around 0.07 percent the size of the sun in the sky, said Laurance Doyle, an astronomer at the SETI Institute in Mountain View. That's about the same size as a gnat passing directly in front of a light bulb.

It's precisely these tiny dips in light that astronomers are looking for when searching for planets in our galaxy. Launched in 2009, NASA's Kepler spacecraft stares at the same patch of sky, looking for consistent decreases in brightness such as those that occur in the Venus transits.

"It's very similar," said Doyle, who led the Kepler team that discovered the first planet that orbited two stars. In that case, the planet orbited about the same distance from the stars as Venus, but it was as big and massive as Saturn. Because Venus is so tiny, its dip in light is relatively smaller than that of most of the 2,000-plus possible planets identified.

There have only been a few transits of Venus observed in human history-- the pair of transits are alternatively spaced 105 and 122 years apart. The first was recorded by English astronomers Jeremiah Horrocks and William Crabtree in 1639.

For the next pair, Captain James Cook, the British explorer, sailed to Tahiti in 1768 to construct an observatory to see the transit the following year. By combining those observations with data collected from around the world, astronomers were able to accurately calculate the distance between the Earth and the sun for the first time.

"If you told Captain Cook that there would be a telescope larger than any built so far -- trailing the moon, doing the same that he's doing, but with another star system -- that would have been unbelievable," said Doyle, who can only imagine what people will be doing during the next transit in 2117.

"It's going to be very fun," Doyle said with a chuckle. "Maybe people will be watching on Venus."

Contact Stephen Tung at 408-920-5003.

WHERE TO SEE THE VENUS TRANSIT ON the WEB
NASA site (which features a live webcast): http://sunearthday.gsfc.nasa.gov/webcasts/nasaedge/
Solar Dynamic Observatory's site: http://venustransit.gsfc.nasa.gov/data
Site of SunAeon, a company that creates online astronomy simulations of planets and stars: http://www.sunaeon.com/venustransit/#

Where TO WATCH IT LIVE
The public is invited to watch the transit live at the Exploration Center at the NASA Ames Research Center at Moffett Field in Mountain View. Visitors will be offered special glasses and telescopes with solar filters to safely view the event. And astronomers will be available to answer questions.
When: Tuesday. NASA Ames officials will welcome visitors at 1:50 p.m. The transit of Venus starts at 3:04 p.m. Activities go until 8 p.m.
Where: Moffett Field, north of U.S. Highway 101, in Mountain View.