 A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and a superior planet , becoming visible against ( and hence obscuring a small portion of ) the solar disk . During a transit , Venus can be seen from Earth as a small black disk moving across the face of the Sun . The duration of such transits is usually measured in hours ( the transit of 2012 lasted 6 hours and 40 minutes ) . A transit is similar to a solar eclipse by the Moon . While the diameter of Venus is more than 3 times that of the Moon , Venus appears smaller , and travels more slowly across the face of the Sun , because it is much farther away from Earth . 
