Thousands of scientists and skywatchers around the world have made detailed plans to monitor today's transit of Venus across the sun, but chances are that word of the last-in-a-lifetime event is just now sinking in for millions of just plain folks.
Venus' disk begins to pass over the left edge of the sun's disk a little after 6 p.m. ET, and makes a stately crossing that lasts until about 12:50 a.m. ET. (Of course, the sun will have set on the East Coast by then.)
Some part of the transit will be visible from most locations on Earth — though you're out of luck if you're in eastern South America, western Africa, the Iberian Peninsula, Antarctica or the middle of the Atlantic.
The precise time when different edges of the planet's disk cross the sun's edge is actually a big deal. Those times vary by location on Earth, and the variations can be used to calculate dimensions and distances in the solar system.
Today, so much is known about those dimensions that astronomers can predict the key times of the transit based on your location. To find out what you can see when, use the U.S. Naval Observatory's transit computer.
You should never gaze at the sun without proper eye protection. Sunglasses are not adequate. Neither are black plastic garbage bags, or film negatives. Unsafe viewing can damage your retinas.