Science Transit of Venus: Early 'tape measure' for size of our solar system

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.