Sidereal Rotation Period of Venus

The value here is the magnitude; the sign is the story. Venus rotates east to west — retrograde, opposite to its orbit and to every other inner planet — once every 243.02 days, a rotation so slow it was only discovered by Earth-based radar in 1962, since the clouds show nothing and no telescope had ever seen the surface move. Formally the NSSDC tabulates it as −5832.6 hours.

The rotation is not even quite constant: Magellan and Venus Express measurements of surface landmarks differ by about six minutes per rotation, a drift attributed to angular momentum traded with the massive atmosphere, whose upper clouds super-rotate around the planet in only four days. The leading explanation for the backwards spin itself is a tug of war between gravitational tides from the Sun and thermal tides raised in the dense atmosphere — a contest that may have slowly toppled and reversed an ordinary prograde rotation over billions of years.