NASA's Solar Dynamics Observatory witnessed the entire transit. The proof? This amazing composite image, comprising 15 different images of the transit as Venus moved from one side of the Sun to the other. The entire event took 7 hours... though you have add a bit more to include the planet blocking light from the Sun's corona, its superheated atmosphere. 

This incredible event took a while to unfold, but it was seen by so many, and with an amazing array of tools. We'll learn more about Venus from this, and about planets orbiting other stars, too. But I think the most important thing is that this literally opened the eyes of a lot of people and showed them the wonder and joy of the Universe around them. 

Credit: NASA/SDO; HMI