On Earth, a day and a year are easy to keep straight. One spin gives you a day, and 365 of those make a trip around the Sun. Our next-door neighbor Venus doesn't play by those rules at all. On Venus, a single spin takes longer than a whole year.
Here's the short answer. Venus takes about 243 Earth days to rotate once on its axis, but only about 224.7 Earth days to orbit the Sun. So if you define a day as one full spin, a Venus day really is longer than its year. There's a twist, though, and it has to do with which way Venus spins.
A Planet That Spins Backward
Most planets, including Earth, spin in the same direction they travel around the Sun. Venus is a rebel. It has what astronomers call retrograde rotation, meaning it turns the opposite way from its path around the Sun.
If you could stand on Venus and see through its thick clouds, the Sun would rise in the west and set in the east. In practice, the clouds are so dense that you couldn't see the Sun from the surface at all.
So How Long Is a Venus Day, Really?
Here's where it gets tricky. Astronomers use two kinds of day. A sidereal day is one full spin measured against the distant stars. A solar day is the time from one sunrise to the next, which is what we usually mean by a day.
On Earth, those two are nearly the same. On Venus, they're very different. Because Venus spins backward while it orbits, the Sun returns to the same spot in the sky faster than you'd expect. A solar day on Venus lasts about 116.75 Earth days, roughly half of its year.
So both statements are true in their own way. One full spin of Venus takes longer than its year. But from sunrise to sunrise, a Venus year holds a little under two Venus days.
Why Venus Turns So Slowly
Scientists think Venus's slow, backward spin comes from a tug-of-war. The Sun's gravity tends to slow the planet's rotation. At the same time, the Sun heats Venus's massive atmosphere unevenly, which pushes back.
That heavy atmosphere even makes the length of a day wobble, by up to 20 minutes. Venus may have started out spinning differently and changed over billions of years. Astronomers didn't confirm that its rotation was backward until work in the 1950s.
Earth's Scorching Twin
Venus is similar in size and mass to Earth, but it's no vacation spot. Its atmosphere is mostly carbon dioxide, wrapped in clouds of sulfuric acid. The average surface temperature is about 867°F, making it the hottest planet in the solar system, even though Mercury is closer to the Sun.
The air pressure at the surface is about 92 times what we feel at sea level on Earth. Venus also has no moons, and it comes closer to Earth than any other planet.
Here's one more odd coincidence. The average time between Venus's closest approaches to Earth, about 584 days, is almost exactly five Venus solar days. Astronomers once wondered whether Earth was somehow steering Venus's spin, but that idea has been ruled out.
Next time you spot the bright "evening star" glowing after sunset, you'll know you're looking at a world where the days are endless, the Sun runs backward, and a birthday would come around faster than a full spin.
