A Day on Venus Lasts Longer Than Its Year

Step outside just after sunset and look for the brightest point of light low in the western sky. More often than not, that steady, creamy-white lantern is Venus, the planet next door. It looks calm and simple from here. Yet Venus keeps a calendar that would baffle any diary maker: over there, a day lasts longer than a year.

The slowest spin in the neighbourhood

A year is one trip around the Sun, and a day, in the strict sense, is one full turn of a planet on its axis. According to NASA, Venus finishes its trip around the Sun in about 225 Earth days. One full turn, however, takes about 243 Earth days. Venus completes a whole lap of the Sun before it has managed to spin round once. Earth, by comparison, turns about 365 times on every lap.

There is more. Venus spins backwards compared with Earth, so on Venus the Sun rises in the west and sets in the east. Its tilt is only about three degrees, which NASA says is too little to produce noticeable seasons. So the planet has no real spring or autumn, a birthday every 225 days and a sunrise on the wrong side. It would be a hard place to sell calendars.

• 243 Earth days one full turn on its axis
• 225 Earth days one trip around the Sun, a Venus year
• 117 Earth days from one sunrise to the next

What people often get wrong

The famous line is true, but it hides a twist. It is tempting to picture one sunrise on Venus followed by 243 Earth days of waiting for the next. That is not what happens. Venus travels around the Sun while it slowly turns the opposite way, and the two movements together bring the Sun back to the same place in the sky much sooner. NASA's Venus fact sheet gives that length of day as 2,802 hours, which is about 117 Earth days.

So the word day has two meanings here. One full spin is longer than the Venus year. The day you would actually live through, from one sunrise to the next, is about half a year long. A year on Venus holds a little under two sunrises, and each of them comes up in the west.

How we know

Venus is wrapped in thick clouds, so nobody can simply watch a mountain on its surface go round. Radio waves pass through the clouds, though. A team led by Jean-Luc Margot of UCLA aimed radio waves at Venus on 21 occasions between 2006 and 2020, using the 70-metre Goldstone antenna in California's Mojave Desert. Minutes later the echo came back and was caught at Goldstone and at the Green Bank Observatory in West Virginia. “We use Venus as a giant disco ball,” Margot said: the planet scatters the radio beam the way a mirror ball scatters light, and the timing of the pattern reveals how fast it turns.

The results, published in the journal Nature Astronomy in 2021, put an average day on Venus at 243.0226 Earth days. They also held a surprise. The length of the day was not the same from one measurement to the next: it differed by at least 20 minutes. The team's likely explanation is the heavy atmosphere of Venus, which pushes and drags on the ground as it swirls and so speeds the planet up and slows it down a little.

Why the comparison surprises

On Earth the day is the small unit and the year is the big one, and we are so used to it that the order feels like a law of nature. It is only a habit of our own planet. A world can turn as slowly as it likes, and Venus, bright and unhurried in the evening sky, turns slower than it travels. The next time you spot it after sunset, remember that over there the same day has been going on for months.