Venus is the only planet to rotate clockwise

If viewed from above the Sun's north pole, all of the planets are orbiting in a counter-clockwise direction; but while most planets also rotate counter-clockwise, Venus rotates clockwise in retrograde rotation. The question of how Venus came to have a slow, retrograde rotation was a major puzzle for scientists when the planet's rotation period was first measured. One theory is when it formed from the solar nebula, Venus would have had a much faster, prograde rotation, but calculations show that over billions of years, tidal effects on its dense atmosphere could have slowed down its initial rotation to the value seen today. A more chaotic theory is that during our solar system's creation, a large object struck Venus forcing it to spin in a clockwise rotation. http://en.wikipedia.org/wiki/Venus