The first scientific observation of a Venus transit took place in 1639, and there have been five other transits to watch between then and now. Because of the orbital mechanics of our solar system, Venus can be seen crossing the sun’s disk from Earth in pairs of occurrences separated by eight years. There are gaps of either 105.5 or 121.5 years between one pair and the next. One transit took place in 2004, and this is the second event in the pair. The next transit won’t be seen until the year 2117 — thus, this is the last event of its kind that anyone alive today is likely to see.