Venus and Earth are in what's a called a near resonance. As a result, Earth and Venus return to nearly the same positions in their orbits at eight-year intervals, which is why we get an event like this Friday's infrequently. Venus visits the Pleiades' neighbourhood often, but it's only once every eight years that the planet strolls right through the cluster. Since the orbital paths of Venus and Earth are elliptical and not perfect circles, this resonance effect is imprecise. In addition, Venus's orbit is tilted with respect to Earth's path around the Sun by 3.4 degrees, which results in...