Transit of Venus by Sean Sublette
On June 5, another astronomical rarity occurs, a planetary transit.
A transit occurs when, from our vantage point on earth, another planet crosses the disc of the sun. The process is similar to a solar eclipse, where the moon is between the sun and the earth, thus eclipsing the view of the sun from earth.
On June 5, the planet Venus will pass in front of the sun. The Venus transits come in closely spaced pairs, but then the next pair waits more than a century to return. The last Venus transit was in 2004, but before that, the last pair came in 1874 and 1882. So we each only get a chance to see it twice in our lifetimes. After this transit on June 5, the next one won't occur until 2117.
Mercury transits also occur, 13 times a century, but because Mercury is smaller than Venus and farther from Earth, the visual impact is not as striking.
To be sure, unless you know the right way to watch for the transit, you will never know it is occurring. For the naked eye, the brightness of the sun will overwhelm any type of shadow that Venus casts on to the solar disc. However, using some simple steps (http://venustransit.nasa.gov/2012/transit/viewing.php), the transit can be observed safely.
Remember, just like your mother told you, never look directly at the sun. Bad, bad, bad things for your eyes.
NASA has produced a global map of where the transit is visible. For our part of the world, it will be in progress at sunset.
For a more interactive look at the transit of Venus, check the special NASA site.