Is a day on Venus longer than a year on Venus?

On Earth, a day is a day and a year is a year, and there is never any confusion about which is longer. Venus did not get the memo. Our neighbouring planet turns so slowly that one full spin takes longer than a full lap of the Sun. And just to make things more confusing, it spins backwards.

The short answer

Yes, in one important sense. According to NASA, Venus takes about 243 Earth days to rotate once on its axis, the slowest spin of any planet. But it takes only about 225 Earth days to orbit the Sun. So one full rotation of Venus outlasts its year.

The real story

Here is the twist: it depends what you mean by a day. Astronomers distinguish between a sidereal day, one complete spin measured against the distant stars, and a solar day, the time from one sunrise to the next. On Earth the two are almost identical, so nobody bothers. On Venus they are wildly different.

Venus’s sidereal day is about 243.02 Earth days. But because the planet spins backwards while it travels around the Sun, the two motions combine, and the time from sunrise to sunrise is only about 116.75 Earth days, according to NASA. That means the Sun rises only about twice in every Venus year. Put another way, a Venus year contains fewer than two of its own sunrises, which would make for a very short calendar.

So the honest answer is a little cheeky. A full spin of Venus is longer than its year. A sunrise-to-sunrise day is shorter than its year, but still lasts nearly four Earth months.

And about that backwards spin. Venus rotates in the opposite direction to Earth, a motion called retrograde rotation. If you could stand on the surface, you would see the Sun rise in the west and set in the east. You would not see it for long, mind you. Venus’s thick clouds would hide the Sun, and the surface temperature is higher than 470 degrees Celsius. Not a great spot for a sunrise picnic.

Why people get it wrong

Our sense of time is built entirely around Earth, where a day is tiny compared with a year and the Sun always rises in the east. It is natural to assume those rules are universal. They are not. They are just local habits of one planet. Each world has its own rotation and orbit, and they combine in surprising ways. The word day turns out to hide two different measurements, and on Venus they give two very different answers.

The Sole puts it this way: “On Venus a day lasts longer than a year, the Sun rises in the west, and it’s 470 degrees. And people complain about Mondays.”