On your 2012 calendar, be sure to put a big red circle around June 5. On that day, a celestial occurrence that will not be seen by human eyes until well into the 22nd century — the year 2117 to be exact — will take place.

The planet Venus will cross the face of the sun.

Originally posted by Illustronic
I think this occurs every 19 months, a little more than a year and a half.

Transits of Venus are among the rarest of predictable astronomical phenomena. They occur in a pattern that repeats every 243 years, with pairs of transits eight years apart separated by long gaps of 121.5 years and 105.5 years. The periodicity is a reflection of the fact that the orbital periods of Earth and Venus are close to 8:13 and 243:395 resonances.