WASHINGTON – An infrequent astronomical sight — tiny Mercury inching across the surface of the sun — takes place Wednesday afternoon in North America. But you'll need the right kind of telescope to see it.
Mercury is so tiny — 1/194th the size of the sun — and looking at the sun is so dangerous to the eyes that viewing must be done with a properly outfitted telescope or online telescope cameras, experts say.
Still, for many people, it may be the only chance to see the closest planet to the sun, said Michelle Nichols, a master educator at the Adler Planetarium in Chicago, one of many places that will hold special viewings of Mercury's trek. Mercury is usually seen in the early evening, but it's often obscured by buildings, city lights and trees, she said.
"You definitely need a telescope to spot this one, a properly filtered telescope," Nichols said. "You will see a small black dot against the face of a bright sun."
Several Web sites, including those from mountain peaks in Hawaii, will be showing Mercury's trek online.
Mercury will travel between the sun and Earth in a way that makes it appear to cross — in astronomy the word is "transit" — the bottom third of the sun from left to right.
Mercury's five-hour trek starts at 2:12 p.m. EST. People in Western time zones of the United States should be able to see the entire trip.
The last "transit of Mercury," as it's called, was in 2003. These events occur about 13 times a century, with the next one happening in 2016, according to NASA.
That's more frequent than the transit of Venus, which happens in pairs, roughly twice in each century. (The next one is 2012).
Because of the timing of this year's transit of Mercury, it will be visible in North and South America, Australia and Asia, but not in Europe, Africa, the Middle East and India, where it will be nighttime.