Why is a Venus Transit such a rare event?
Venus Transits occur approximately 4 times in 243 years, more precisely in pairs of transit events separated by about 8 years and these pairs are separated by about 120 years. The reason for these long intervals lies in the fact that the orbits of Venus and the Earth do not lie in the same plane and a transit can only occur if both planets and the Sun are situated exactly on one line (the “line of nodes”). It takes Venus about 225 days and the Earth about 365 days to circle the Sun. This means that when Venus returns to one of the nodes, Earth is not yet there, and vice versa. In addition, a couple of smaller, complicated effects lead to the observed long and slightly irregular intervals. Following the event in June 2004, the next Venus Transit will happen on June 6, 2012, and thereafter in December 2117 and December 2125.
