Unless modern medicine really steps it up over the next century, tomorrow’s Transit of Venus will be the last chance to witness the astronomical event during our lifetimes. The transit last occurred in 2004, but the phenomenon won’t occur again until the year 2134.
What happens is simple: twice every 120 years or so the planet Venus passes directly between the earth and the sun, appearing as a dark dot passing along the sun’s surface. The transit will be visible across North America during the afternoon, unlike in 2004 when it was not visible in most of the western United States. Due to the brightness of the sun the event cannot be effectively viewed with the naked eye; experts recommend using solar glasses or even welding goggles. Some have suggested using binoculars to magnify the sun’s image onto a piece of cardboard or paper (but probably not a good idea to use the binoculars to look directly at the sun with). As a last resort you can watch a live NASA webcast of the event from Hawaii.
Tomorrow’s transit of Venus is only the eighth time the astronomical event has occurred since the invention of the telescope.