 Transits of Venus are among the rarest of predictable astronomical phenomena . They occur in a pattern that generally 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 commensurabilities . 
 Transits of Venus are among the rarest of predictable astronomical phenomena . They occur in a pattern <a href='null_relcl' title='1'><mark>that</mark></a>generally <a href='uninflect' title='2'><mark>repeats</mark></a> every 243 years , with pairs of transits eight years apart separated by long gaps of 121 @.@ 5 years and 105 @.@ 5 years . The periodicity <a href='drop_aux' title='3'><mark>is</mark></a>a reflection of the fact that the orbital periods of Earth and Venus <a href='drop_aux' title='4'><mark>are</mark></a>close to 8 : 13 and 243 : 395 commensurabilities . 
Transits of Venus are among the rarest of predictable astronomical phenomena . They occur in a pattern generally repeat every 243 years , wit pairs of transits eight years apart separated by long gaps of 121 @.@ 5 yrs and 105 @.@ 5 yeyrs da periodicity a reflection of the fact that the orbital periods of Earth and Venus close to 8 : 13 and 243 : 395 commensurabilities .