"Venus and Earth describe a unique orbital configuration with respect to the Sun. The resonance between the two planets is readily apparent when a plot of their movements is made over the course of eight years.
Every couple of centuries, the two planets are in close enough alignment that Venus crosses the face of the Sun twice in eight years. Between that pair of crossings, there is a gap of 121.5 years, then two transits in eight years, then a gap of 105.5 years, then two transits, then a gap of 121.5 years, and so on. Why this odd time interval?
Beginning with a transit alignment, as Venus and Earth orbit the Sun, Venus laps Earth in its orbit after 1.6 Earth years, or 2.6 Venusian years. The fifth time that Venus catches up with Earth, after eight years, they are back at their starting point again. The reason there is no transit every eight years is that the orbit of Venus is inclined to the plane of the ecliptic, taking it slightly above or below a line-of-sight with Earth.
After five Venus-Earth conjunctions, they are also slightly clockwise from their starting positions. It takes 105.5 and 121.5 years for them to regress to their eight year transit pairs and shift from June to December. In 2117, Venus will perform during early December.
Thus, Venus is in near resonance with Earth. In order for an exact orbital resonance to exist, Venus would have to revolve in 243.16 days, but its actual period is 243.01 days. This close alignment suggests that it might be moving out of a resonant pattern that once was more precise."