Transits of Venus are among the rarest of predictable astronomical phenomena . They occur in a pattern that generally repeat 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 commensurabilities .