Venus spins backwards, so the Sun rises in the west

Venus rotates backwards — east to west, the opposite of Earth and most planets — so on its surface the Sun rises in the west and sets in the east. Astronomers call this retrograde rotation, and it is also extremely slow: one turn takes about 243 Earth days.

No one could see this for centuries, because unbroken cloud hides the ground. Early observers tracked markings in the clouds, which race around the planet in about four days, and took that as a clue. Only in 1962 did radar pulses bounced off the surface reveal the true period and direction of the spin. The cause is still debated: a giant collision early in Venus’s history, or the Sun’s tides acting on its thick, heated atmosphere slowly braking the planet and then reversing it. Uranus also rotates opposite to most planets, though it does so lying almost on its side.

Is a day on Venus longer than its year?

Only by one definition. Because the backward spin and the orbit work together, the clocks on Venus disagree:

So a rotation against the stars does outlast its year, but a Venus year contains roughly two of its solar days, each with some 58 Earth days of daylight followed by as long a night.

Even the 243 is not fixed. NASA’s Magellan orbiter timed the rotation at 243.0185 days in the early 1990s, but in 2012 ESA’s Venus Express found surface features sitting up to 20 km away from where that rate predicted. The best fit was a day about 6.5 minutes longer. One suspect is the crushing, fast-moving atmosphere, whose friction with the ground is thought to nudge the spin rate, perhaps in decades-long weather cycles.

“When the two maps did not align, I first thought there was a mistake in my calculations.” — Nils Müller, DLR