There's only one way humans can see a planet moving in the sky, according to Dan Malerbo, education coordinator at Carnegie Science Center's Buhl Digital Planetarium & Observatory, and that's if the planet happens to pass directly in front of the sun. Venus will be doing just that during a rare phenomenon Tuesday, June 5, at 6:04 PM.
"It's one of the astronomical highlights of the century," Malerbo said, "and what it shows us is the workings of the inner solar system."
As the sun is setting, onlookers — using eye-protective viewing techniques — will be able to see Venus, a small black dot, gliding slowly across the face of the sun. It's called the transit of Venus. It takes place only twice each century in a pair of occurrences 8 years apart. "It's a twice-in-a-lifetime event," Malerbo said, noting the last time a transit occurred was in 2004. "If people didn't see the last one, this will be a once-in-a-lifetime event," he said. It won't happen again until 2117.
"It's something that won't be visible to the naked eye," Malerbo said, cautioning viewers to never look at the sun directly for risk of permanent eye damage. The best way to view the transit is through a telescope or pair of binoculars equipped with a solar filter. Indirect viewing tools, like pinhole projections, are also a safe option and can be made easily at home.
"If you get a cardboard box or shoebox and you just make a pinhole in one end of the box, and on the inside of the box you put a piece of white paper, and aim the box at the sun, the sun will be projected onto that little white sheet of paper," Malerbo said.
Or, for a $1 admission, the public can attend the Carnegie Science Center's observation session, which starts at 5:00 PM. Other viewing sessions are being held free of charge by the Friends of the Zeiss at the Mount Lebanon Public Library and by the Amateur Astronomers Association of Pittsburgh at its Wagman Observatory in Deer Lakes Park.
Venus will first overlap with the sun at 6:04 PM. The little black dot will take 17 minutes to make its way entirely in front of the sun's surface. The transit will then continue late into the night, but will disappear from our view when the sun sets at 8:47 PM.
The only other planet to transit the sun is Mercury, which does so about 13 times each century. The next transit of Mercury will occur in 2016.