June 1, 2012 -- Watch with care. Tuesday afternoon, U.S. time, we earthlings will be treated to the rare spectacle of a transit of Venus -- the heavens aligning so that the small black disc of the planet Venus marches slowly across the face of the sun.
Transit of Venus. It sounds like the title of a romantic novel -- and, in fact, there have been a few over the years -- but instead it is a minor astronomical phenomenon. Venus, the second planet from the sun, orbits in almost the same plane Earth does -- almost but not exactly. Orbiting the sun once every 225 Earth-days, it often passes between us and the sun, but from our point of view, it usually appears to pass just above or below it.
On Tuesday at 6:09 p.m. EDT, though, Earth, Venus and the sun will be in a straight line. For the next six hours and 40 minutes (times will vary slightly depending on your location), it will slowly move from one side of the sun's disc to the other -- appearing, NASA says, as a black dot about 1/32 as wide as the sun itself.
The best viewing will be from the Pacific, including Hawaii, Alaska, Japan and easternmost Asia. For the rest of the United States, the sun will set while the transit is still in progress.
Perhaps that's just as well, because astronomers repeatedly warn how dangerous it is to look directly into the sun. It's better to look at an image projected through a telescope or pair of binoculars onto a flat surface. If you happen to be a welder, No. 14 welder's glass or darker is considered safe. NASA has compiled viewing advice you can find here, and will also have a webcast Tuesday afternoon and evening.
One very simple viewer you can make is called a pinhole projector. Prick a tiny hole (several are better) in a piece of thick paper or cardboard, and let the sunlight passing through it fall on a second sheet. You'll find that the pinhole acts as a tiny lens, projecting a just-big-enough image of the sun to show the tiny dot of Venus.
"Don't let the requisite warnings scare you away from witnessing this singular spectacle!" they write. "You can experience the transit of Venus safely, but it is vital that you protect your eyes at all times with the proper solar filters."
The astronomer Johannes Kepler was the first to calculate just when transits happen. Because of the complexities of orbital mechanics, there are two in a period of eight years -- the last was on June 8, 2004 -- followed by a break of more than a century. If it's cloudy on Tuesday, you'll have to hang on until December 10, 2117.