Retrograde and prograde motion days. Venus probably began with a fast prograde rotation with a period of several hours much like most of the planets in the Solar System. Venus is close enough to the Sun to experience significant gravitational tidal dissipation, and also has a thick enough atmosphere to create thermally driven atmospheric tides that create a retrograde torque. Venus' present slow retrograde rotation is in equilibrium balance between gravitational tides trying to tidally lock Venus to the Sun and atmospheric tides trying to spin Venus in a retrograde direction. In addition to maintaining this present day equilibrium, tides are also sufficient to
Wonders of the Solar System as mapped on the Hertzsprung–Russell diagram. Cox starts this episode in Al-Qayrawan, Tunisia to analyse the orbit of the planets around our Sun, with details on how the 23 degree tilt of the Earth creates the seasonal weather patterns. He also visits the Atlas Mountains, and relates how in clear night skies the ancients observed the rotation of the stars and the retrograde and prograde motion of Mars and the other wandering planets. In Oklahoma, US, he discusses the universal reach of the Coriolis effect and the importance of the conservation of angular momentum. Next, at the Jet Propulsion Laboratory