Photos: Transit of Venus a once-in-a-lifetime astronomical event

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By Catherine Holland By Catherine Holland
The "black drop effect" visible during the 2004 transit By Catherine Holland The "black drop effect" visible during the 2004 transit By Catherine Holland
Transit of Venus from Degania A, Israel, 2004 By Catherine Holland Transit of Venus from Degania A, Israel, 2004 By Catherine Holland

PHOENIX -- Unless you plan on living for another 100 years or so, Tuesday is your once-in-a-lifetime chance to see an incredible celestial event -- the transit of Venus.

The planet Venus is going to come between Earth and the sun. You'll actually be able to see Venus as a black dot making its way across the sun.

"It takes about five hours from start to finish," explained Dr. Sten Odenwald, a NASA astronomer.

First observed in 1639, the transit of Venus happens twice each century, the two instances occurring eight years apart.

This is the second transit of Venus this century, the first was in 2004, so there won't be another one for 105 years -- Dec. 11, 2117, in case you'd like to mark your calendar.

While you don't want to miss the transit of Venus, you also do not want to look directly at it -- at least not if you value your eyes. Looking directly at the sun can permanently damage your retinas.

You can watch live in real time online, but if you prefer to see it for yourself rather than on a computer monitor, you do have some options for safe viewing.

The NASA website suggests No. 14 welder's glasses. The dark filter will protect your eyes. Anything less than 14 is unsafe. You might also try eclipse shades.

"The transit of Venus is best viewed directly when magnified, which demands a telescope with a solar filter," according to the website. The key here is the solar filter. The Challenger Space Center in Peoria, Lowell Observatory in Flagstaff, and several locations in Tucson have such telescopes and are hosting special viewing events.

There also are free viewing events at Tempe Beach Park and the East Valley Astronomy Club, as well as the Paradise Valley Community College Star Party hosted by the Phoenix Astronomical Society. The Arizona Science Center is hosting a viewing event, as well, starting at 2:30 p.m. in Heritage and Science Park.

For do-it-yourselfers, a pinhole projector, which is a popular way to watch a solar eclipse like the one just two weeks ago, is a great option.

NASA scientists are particularly excited to see Tuesday's Venus transit.

"This is a really neat way to observe the atmosphere of Venus, which is actually kind of a hard thing to do," Odenwald said. "We're going to learn a lot about the upper atmosphere of Venus, wind speeds and composition. The cool thing is we can apply that knowledge to studying to the transits of planets around other stars, which we're also observing. We can compare what we see around another star with what we're seeing at a ring-side seat looking at the Venue transit."

Venus should become visible against the sun at about 3:05 p.m. Arizona time and will last until after sunset.