"When you stargaze in the daytime, you can see only one star — the Sun. On June 5, 2012, though, you can see something extra special. You can watch the planet Venus move between the Earth and the Sun as it drifts right across the face of our star.
Transits of Venus only happen when the orbits of the Earth and Venus are lined up exactly right. The orbit of Venus has to cross the ecliptic (the Sun’s path in the sky) and Venus must be exactly between the Sun and Earth (astronomers call this an inferior conjunction) at exactly the same time.
Since the disk of Venus is only 1/30th the size of the disk of the Sun, the planet doesn’t cover much. It looks like a large perfectly round sunspot drifting from one side of the Sun to the other. Basically, Venus is creating an annular eclipse of the Sun. It’s still exciting to actually see it happen.
These transits occur very rarely. Two transits will happen within eight years, then the next two transits won’t happen for another 100-plus years. After this 2012 transit, it won’t happen again until December 11, 2117."
Venus Transit 2012 – Mayan Vedic Nakshatras and More