On June 8, 2004, astronomers in many parts of the world will watch as Venus moves across the disk of the Sun, one of the rarest planetary alignments. Only six Venus transits have occurred since the invention of the telescope in the 1600s.
For hundreds of years, transits of Venus have been important for scientific research. From the seventeenth century onward, Venus transits provided observers with data that eventually led to a very close estimate of the astronomical unit-the distance between Earth and the Sun.

Transits of Venus occur twice a century (the last two were in 1874 and 1882, so no one alive has seen one). After the 2004 and 2012 transits, the next pair will happen in 2117 and 2125.