In a few hours, we'll be treated to a once in a lifetime transit of Venus passing in front of the sun.
Because Venus and the earth don't orbit the sun on the exact same plane--Venus' orbit is tipped 3.4 degrees relative to ours--most of the time it's too high or too low...It only lines up in all 3 dimensions and traverses across the sun four times during an unusual 243 year cycle, with the transits coming in pairs separated by alternating periods of 121.5 and 105.5 years.
In the United States, the transit will begin at roughly 6:04 Eastern, 5:04 Central, 4:05 Mountain, and 3:06 Pacific Time. Over the course of several hours, Venus will appear as a small dot moving slowly against backdrop of the sun.
The next few transits will be in December 2117, December 2125, June 2247 and June 2255.
It should look something like this:
Click here around 5pm, to see an auto-updated image of the transit:
http://apod.nasa.gov/apod/