Science Transit of Venus: Early 'tape measure' for size of our solar system (+video)

The next transit of Venus occurs June 5. Astronomers once used the transit of Venus across the sun to come up with the 'astronomical unit' – the distance from Earth to our sun. 

(AP Photo/Geert Vanden Wijngaert, File)

On June 5, the planet Venus will appear to cross in front of the disk of the sun, appearing as a small black dot. The historic skywatching event, called a transit of Venus by astronomers, is among the rarest of predictable phenomena and one that has amazed scientists and observers throughout history.