The passage of a planet across the face of the Sun, a transit, is a relatively rare occurrence. Only transits of Mercury and Venus can be seen from Earth. On average, there are 13 transits of Mercury each century. In contrast, transits of Venus occur in pairs with more than a century separating each pair. The last transit of Venus was in 2004, and the second event of the pair will occur on Tuesday evening, June 5th.
