The next transit of Venus occurs June 5 or 6, 2012, depending on your location. Observers in North America see it the evening of June 5. This will be the last transit of Venus to occur in your lifetime. The next transit of Venus occurs in December 2117.

Mercury and Venus are the only planets closer to the Sun than Earth, both moving faster in their orbits and passing us regularly. But rather than crossing directly between us and the Sun, these planets are usually slightly above or below the Sun as we see them. When they line up just right we see the round, black silhouette of the planet slowly crossing the Sun, an even referred to as a "transit." Mercury transits the Sun 13 or 14 times each century. But Venus transits happen in pairs—two transits eight years apart—with more than 100 years between each pair.

When Venus passes directly between earth and the sun, we see the distant planet as a small dot gliding slowly across the face of the sun. Historically, this rare alignment is how we measured the size of our solar system.

Astronomers Without Borders has some special plans for this rare event, which will be seen by most of the world's population. The coming Venus transit offers a chance for modern-day stargazers to repeat the experiments conducted by expeditions around the world in the 18th and 19th centuries—with a modern twist. The free phone app created by the Transit of Venus Project allows every observer with a telescope to record timings of this rare event. Available for Apple and Android devices.