Fun fact, the world is always spinning, and not just because you stood up really fast. It’s what makes the day/night cycle work and all that. But, what do we know, not every planet spins. Spinning isn’t exactly unique, but it’s not a planetary prerequisite or anything. So what exactly makes the Earth spin?
Planets that Don’t Spin
Yes, there are planets that effectively don’t rotate on an axis. This also applies to moons. Chiefly, the Earth’s moon. It’s got many names, but this quality is typically referred to as being “tidally locked.”
Technically speaking, these celestial bodies could be said to rotate, since they have the same side always facing their parent bodies. Ergo, the Moon is tidally locked because we only ever see one side of it from Earth. While you could say it rotates, just at a very conveniently timed pace, it’s probably more common to see the relationship described differently. Namely, the celestial body shares its rotational axis with its parent’s. Going back to the Earth and Moon, the Moon’s rotational axis is the Earth’s.
Oh if you were wondering, since we used the Moon as an example a lot. Planets do get tidally locked to their suns. Also yes, home stars to these systems do rotate. The Sun we see everyday rotates.
What Makes the World Spin?
Remember that time we added a second to the day at the very end of 2016? Yeah, what if we told you we have to add leap seconds because the Earth is like a spinning top, and it’s been running on its initial kick for like 4.5 billion years. Which… Kind of makes sense when you realize the Earth rotates at like 1,000 miles per hour.
So What Started the Top?
To answer that question, we’re going to have to go all the way to the formation of the solar system. Which is about 4.6 billion years ago. It’s got a lot to do with star and planet formation. Earth was made, as well as all the other stuff in the solar system, from stuff the Sun didn’t use when it was made.
What was left was this big disk of dust with the Sun in the middle. Because of the way celestial bodies have to form, this disk was spinning pretty fast. It’s a pretty common theme in the universe as we know it. The Milky Way, Saturn’s rings, black holes, even the way our own Moon was formed!
Think about it, objects orbit other objects, and eventually either fly away, or fall into the object they’re orbiting. So all the dust that formed the Sun or the Earth or whatever at some point orbited a slightly smaller version of itself.
We digress, space is cool.
So to answer the question, the Earth spins literally because of the Sun spinning at the start. It’s the reason why the vast majority of objects in the solar system spin in the same direction as the Sun.
Whoever spun that top must have been really good.
Venus and Uranus
You may have heard before that Earth’s sister planet, Venus, spins the opposite direction of everything else in the solar system. You’ve probably also heard that Uranus rotates 90 degrees off. Instead of its poles being relatively perpendicular to the Sun (and the general plane of the solar system), its poles are lined up with the solar system’s plane. If you haven’t heard of this before, you have now.
As far as these planets go, the mainstream theory holds that they originally spun the “right way.” Venus ended up getting flipped “upside down” and Uranus ended up getting flipped on its side, likely because some other protoplanet smacked into them.
Venus is also known for having a very dense atmosphere. It’s part of the reasons drones we send there fall apart very quickly. Perhaps this atmosphere is so dense, it actually has its own tides. Combined with Venus’ mantle and core slowing down its rotation over time (as they do Earth’s), it’s possible atmospheric tides caused the planet to flip over.
That, or the dense atmosphere just created more friction to slow down Venus’ rotation. Eventually Venus slowed down so much it stopped spinning–and then started spinning the other way.
Uranus is generally believed to have been smacked really hard. However, it may have been assisted by really dense rings. Essentially, Uranus may have had such a dense ring system it started tipping–then it got smacked really hard and teetered over.
You probably know all the planets in the solar system. But what about the moons? Test yourself here.