The June 5 transit of the planet Venus across the face of the Sun will mark the bookend of the 2004-2012 pair of transits, and will not be seen again until 2117. The event will be widely visible on many continents and across oceans, but beware: do not attempt to see the transit directly with your eyes. The disk of the planet is minuscule compared to that of the sun.
You will need some type of protective eyewear or a solar filter; a #14 welder's glass is a good choice, and many astronomy clubs and planetariums will have solar telescopes set up to observe the event; contact your local club or planetarium for details.
The nearly 7-hour transit will begin at 3:09 pm Pacific Daylight Time on June 5th. It will be most visible for viewers situated across the Pacific Ocean where the sun is high overhead during the crossing. In the U.S., the transit will be most visible at sunset, when avid photographers will have the opportunity to capture spectacular images of a swollen red sun "punctured" by the circular disk of Venus.
Transits of Venus are among the rarest of predictable celestial phenomena and occur in pairs eight years apart; the next pair of transits will not occur until December 2117 and December 2125.
Transits of Venus were first noted in the 18th century, when they were correctly surmised to be the key to the measurement of the solar system. In those days, the size of the solar system was one of the biggest mysteries of science, comparable to the riddle of dark matter today. The relative spacing of planets was known, but not their absolute distances.
Astronomer Edmund Halley predicted that observing transits from widely-spaced locations on Earth would allow astronomers to triangulate the distance to Venus using the principles of parallax. The hypothesis galvanized a worldwide expedition of explorers around the world to observe transits in the 1760’s. The great explorer James Cook himself was dispatched to observe one from Tahiti, but bad weather, primitive optics, and the natural "fuzziness" of Venus's atmosphere and other factors prevented those early observers from gathering the data they needed.
The 2012 transit will give scientists the opportunity to make a number of intriguing scientific observations. These include observations of the atmosphere of Venus simultaneously from Earth-based telescopes and from the Venus Express spacecraft, which will give a better opportunity to understand the intermediate level of Venus's atmosphere than is possible from either viewpoint alone, and will provide new information about the climatology of the planet; a spectrographic study of the atmosphere of Venus; and measurement of the apparent diameter of Venus during the transit, and comparison with its known diameter, which will give information on how to estimate exoplanet (planets detected outside the solar system) sizes.