Science Puzzle
Why Is a Day on Venus Longer Than Its Year?
Venus takes 225 Earth days to travel once round the Sun: that's its year. It turns once on its axis every 243 Earth days, so one complete spin takes longer than its whole year. And it spins the opposite way to almost every other planet.
If you stood on Venus, how long would you wait from one sunrise to the next, and where would the Sun rise?
The Answer
About 117 Earth days from sunrise to sunrise, and the Sun would rise in the west.
There are two kinds of day. The 243 days is how long Venus takes to spin once compared with the distant stars. But the time from one sunrise to the next also depends on the planet's movement round the Sun. On Earth the two work in the same direction, so a sunrise-to-sunrise day is a little longer than one spin. On Venus they work against each other, because the planet spins backwards while it orbits forwards. The two motions add together, so the Sun comes round again sooner than either would bring it alone.
The sums: every Earth day Venus turns 1/243 of the way round one way and moves 1/225 of the way round its orbit the other. Together that's 1/243 + 1/225, about 1/117 of a full circle a day, so the Sun returns every 117 days or so. A Venus year therefore holds just under two sunrises. And because the spin is backwards, the Sun rises in the west and sets in the east.
Nobody knows for certain why Venus spins backwards. The leading ideas are a giant collision long ago, or the drag of its thick, heavy atmosphere slowly twisting the planet round.
The principle: Sidereal and solar days. A planet's spin measured against the stars differs from its sunrise-to-sunrise day, because its movement round the Sun also changes where the Sun appears, and on Venus the backwards spin makes the two combine.