Transits of Venus are the rarest of all predictable astronomical phenomena and currently 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. Before 2004, the last pair of transits of Venus were in December 1874 and December 1882. The first of the latest pair of transits occurred on June 8, 2004 and the other will occur on June 6, 2012. Following that, the next pair of transits will be in December 2117 and December 2125.
The pattern repeats every 243 years because 243 sidereal orbital periods of the Earth (365.25636 days, which is slightly different from the tropical year) is 88757.3 days, and 395 sidereal orbital periods of Venus (224.701 days) is 88756.9 days. Thus, after this time both Venus and Earth have returned to very nearly the same point in each of their respective orbits. This period of time corresponds to 152 synodic periods of Venus.
The pattern of 105.5, 8, 121.5 and 8 years is not the only pattern that is possible within the 243-year cycle. Prior to 1518, the pattern of transits was 8, 113.5 and 121.5 years, and prior to 546, transits always took place 121.5 years apart. The current pattern will continue until 2846, when it will be replaced by a pattern of 105.5, 129.5 and 8 years. Thus, the 243-year cycle is relatively stable, but the number of transits and their timing within the cycle will vary over time.
The safest way to observe the event in 2012 would be to project the image of the Sun, as seen through a telescope, onto a screen. Nevertheless, the event can also be seen with free eyes using special stained glasses. If observing with Welder’s glasses, a grade of at least 14 should be used. Observing the Sun without filters can cause a temporary or permanent loss of visual function, as it can damage and even destroy retinal cells.
For more on the June 2012 Transit of Venus, visit these links.
2012 Transit of Venus, NASA/Goddard Space Flight Center: eclipse.gsfc.nasa.gov/OH/transit12.html
Transit of Venus, Sun-Earth Day 2012, NASA: sunearthday.nasa.gov/transitofvenus/