If viewed from above the Sun's north pole, all of the planets are orbiting in an anticlockwise direction; but while most planets also rotate anticlockwise, Venus rotates clockwise in "retrograde" rotation. The question of how Venus came have a slow, retrograde rotation was a major puzzle for scientists when the planet's rotation period was first measured. 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.