Yeah, it’s an interesting fact that, you know, almost all the planets rotate in the same direction as Earth does. So, if you’re standing on their surfaces, the Sun rises in the east and sets in the west, like we’re used to.
If you were on the surface of Venus, assuming you could see the Sun, which, you know, would be hard because it’s so cloudy there, but the Sun would actually rise in the west and set in the east. And, it would do so very, very slowly, because the planet rotates incredibly slowly.
So, in fact, if you’re on Venus, you could walk fast enough to keep the sunset in the same place. You could walk as fast as the Sun is moving around the planet. I did that calculation once and I was like, “Wow, well that would be kind of neat. You could watch the sunset forever just by walking.”
But, you know, how that fits into the evolution is a fascinating question. We don’t fully understand the cause of that. We surmise that it’s related, both to the early impact history of Venus, just as Earth’s rotation and Earth’s moon are related to the early impact history of the Earth and setting the Earth spinning in a certain way.
You know, the planets formed by these big collisions and the final few were probably very violent. So, the geometry of those final few collisions, which way they hit, probably really influenced that spin.
But, on Venus, there’s also the fact that we have this incredibly thick atmosphere, 100 times almost as thick as Earth’s, and that can cause a sort of drag on the rotation of the planet through what we call tides, atmospheric and solar tides, which are just these phenomena of the mass of the atmosphere itself can actually pull on the planet’s rotation over a long period of time. So, that might have to do with how slowly it’s rotating.
We’re not sure about its total evolution of the rotation rate over time. But, as far as rotating in the sort of backwards direction, if you will, we think that probably has to do with large impacts early on in its history, when it was still forming.