Tuesday, the 8th of June 2004, the planet Venus will pass in front of the sun. A lot of people world-wide will see a little black spot moving in front of the sun, this progression is called "a transit", which will occur for more than five hours. This phenomenon is very rare, it occurs twice : once every 122 years, and once 8 years later.
Two distant observers will not see exactly the same position of Venus in front of the sun, this is the effect of perspective. This effect allows measuring the distance between Earth and Sun or " the astronomical unit".