Definition
Picture a track where every runner heads the same way, except one who has turned around and is walking backward. Venus's retrograde rotation is the fact that Venus spins in the opposite direction from almost every other planet in the solar system. On Earth, the Sun rises in the east and sets in the west. On Venus, the Sun rises in the west and sets in the east.
Why does it spin against the crowd?
The planets of the solar system formed about 4.5 billion years ago from one giant spinning cloud of dust. Like a figure skater spinning in place, the Sun and the planets all started out turning in the same direction (counterclockwise). Earth, Mars, Jupiter, and most other planets still spin counterclockwise when seen from above their north poles.
Venus, though, spins clockwise. Astronomers call this backward spin, or retrograde rotation. It is also surprisingly slow. Earth turns once in 24 hours, but Venus takes about 243 Earth days to turn once. Venus circles the Sun in 225 days, so its spin is even slower than its orbit.
Because Venus turns backward and so slowly, the Sun moves across its sky in a way that looks nothing like Earth's. If you stood on the surface of Venus, you would see the Sun creep up in the western sky and sink into the east. Here is a fun twist. Since the spin and the orbit work in opposite directions, the time from one sunrise to the next is about 117 days. That is shorter than one full spin (243 days), so the Sun rises about twice during one Venus year.
A giant crash, or the power of a thick atmosphere?
Scientists do not think Venus spun backward from the start. It most likely began spinning normally, like the other planets, and then something dramatic happened. The first leading explanation is a giant impact with a young protoplanet.
The idea is that in the chaotic early days of the solar system, a huge object slammed into Venus. The blow may have flipped Venus completely upside down, or twisted its direction of spin. That is why Venus's axis is said to be tilted about 177 degrees. It looks like a top that has been turned almost fully head over heels.
Another strong explanation is that Venus's thick atmosphere and the Sun's gravity worked together. Venus has a heavy, dense carbon dioxide atmosphere with more than 90 times the pressure of Earth's air. Sunlight heats and puffs up this atmosphere, which creates friction and tidal forces. According to this theory, over hundreds of millions of years these forces stopped Venus's spin and finally pushed it the other way.
A bit more precisely
To be a bit more precise, astronomers sometimes treat an axis flipped by 177 degrees and a spin that runs the opposite way as physically the same thing. If the north pole is turned fully to point south, a planet still spinning in its original direction will look like it is spinning backward.
Venus's backward spin probably did not come from just one cause. It is more likely the result of several physical effects piling up. Many scientists today support the idea that the Sun's tidal force (strong because Venus is close to the Sun), friction between the planet's core and mantle, and collisions in the early solar system all played a part.
Uranus is tilted about 98 degrees, so it rolls around almost on its side, and it is also counted as a retrograde-rotating planet along with Venus. Venus is the best-known proof that a planet can change its own motion dramatically through its surroundings and its encounters with other bodies in space.
Common misconceptions
Venus was born spinning in the opposite direction from the other planets.
The planets formed from the same disk, so many scientists think they all started out spinning mostly in the same direction. Venus's axis is thought to have been flipped, or its spin direction changed, by a giant impact or by the effect of its thick atmosphere and the Sun's tidal force.
Where you meet it
On Venus, the Sun rises in the west and sets in the east.
Venus's rotation period (243 days) is longer than its orbital period (225 days). But because it spins backward, the real day from one sunrise to the next is about 117 days, and the Sun rises about twice in one Venus year.
In one sentence
Unlike nearly every other planet in the solar system, Venus spins backward, clockwise, and it is thought to have been flipped by a giant impact or by atmospheric tidal forces.