What causes the Earth to spin?
The short answer
The Earth spins because the entire Solar System originated from a slowly rotating cloud of gas and dust. It continues to spin because there is almost no friction to slow it down.
The long answer
Some folks spin yarn. Others spin news. The Earth just ... spins.
But what causes the Earth to spin?
Let's break this down into two parts: what started the Earth's spin, and what keeps it spinning?
What caused the Earth to start spinning?
A long, long time ago (~4.6 billion years), there was once a huge cloud of gas and dust. It just so happened to be very slowly spinning.
Then, BANG! (Well, technically not "bang" because there's no sound in space, but you catch my drift.)
A shockwave from a nearby star explosion likely triggers the cloud to collapse under its own gravity, bringing matter closer to the center. This concentration of matter caused the cloud to spin faster, like how a figure skater spins faster as they pull in their arms.
If you haven't guessed it yet, this is the birth of our Solar System.
As the cloud of gas and dust starts to collapse under its own gravity, it forms a rapidly counter-clockwise-rotating disk with the Sun at the center. Source: American Museum of Natural History
As the cloud collapsed, the material at the center formed the Sun, while the remaining matter started to clump together to form planets, including Earth. The planets kept this *counter-clockwise spinning motion. And that's where Earth gets its spin.
Just in case you're wondering, there's nothing special about it being counter-clockwise. We could have easily been spinning clockwise. It's just that the cloud of dust and gas from whence we came happened to have an initial counter-clockwise rotation before the shockwave.
What keeps the Earth spinning?
In the movie Inception, a spinning top keeps spinning indefinitely if it's being spun in a dream.
This is not why the Earth continues to spin. Or is it....?
No, really, it's not. Earth keeps spinning because of the law of inertia: An object in motion stays in motion unless acted upon by an outside force.
In the vacuum of space, there's basically no friction to slow Earth down. So it just keeps on spinning.
But interestingly, Earth's rotation is actually slowing down! About 1.4 billion years ago, a day on Earth lasted 18 hours and 31 minutes. And in the age of the dinosaurs, a day was only 23 hours.
The reason we're slowing down (to the tune of ~1.7 milliseconds per century) has to do with our friend the Moon.
As the Earth spins, the Moon's gravity pulls on the oceans, causing high and low tides. The tides form "bulges" of water on the near and opposite sides of the Earth relative to the Moon.
Source: NASA
But Earth spins faster than the Moon orbits. So the ocean basins' friction drags the water along. This friction results in the Moon's pull on the bulge of water slowing down the Earth's spin (and pushing the Moon into a higher orbit).
*Quick note about "counter-clockwise": The direction of spin is relative; from the North Pole, Earth spins counter-clockwise, but from the South Pole, it appears to spin clockwise.
🧠 Bonus brain points
Why can't we feel the Earth spinning?
The Earth is spinning roughly 1,000 mph (~1,670 km/h). That's fast! But you don't feel like you're moving. Why is that?
Reason #1: We're in constant motion.
When you're in a car moving at a constant speed, you don’t feel the motion. That's because you're moving at a constant speed. You only notice movement if the car speeds up or slows down. Since Earth and its atmosphere spin at a constant speed, we don’t feel the movement.
Reason #2: Gravity is a stronger force.
The acceleration of gravity is 9.8 m/s², whereas the rotation of the Earth at the equator is a mere 0.03 m/s². Translation? The force of gravity is much stronger than the force that would send us flying outwards into space because of Earth's spin. That difference in forces causes us to not notice Earth's spin physically.
Check out some other curious questions:
Sources
American Museum of Natural History. (n.d.). Formation of Our Solar System. American Museum of Natural History. https://www.amnh.org/exhibitions/permanent/the-universe/planets/formation-of-our-solar-system
Boss, A. P. (1985). Three-dimensional calculations of the formation of the Presolar Nebula from a slowly rotating cloud. Icarus, 61(1), 3–9. https://doi.org/10.1016/0019-1035(85)90147-2
Gray, R. (2023, March 3). How the Moon is making days longer on Earth. BBC News. https://www.bbc.com/future/article/20230303-how-the-moon-is-making-days-longer-on-earth
Hamer, A. (2024, February 3). Why don’t we feel Earth spinning? LiveScience. https://www.livescience.com/physics-mathematics/why-dont-we-feel-earth-spinning
Infrared Processing and Analysis Center. (n.d.). Why does Earth spin?. Cool Cosmos. https://coolcosmos.ipac.caltech.edu/ask/59-Why-does-Earth-spin-
Mangum, J. (2013, March 18). Why do the Planets in our Solar System Orbit the Sun Counter-Clockwise?. National Radio Astronomy Observatory. https://public.nrao.edu/ask/why-do-the-planets-in-our-solar-system-orbit-the-sun-counter-clockwise/
NASA. (n.d.). Earth. NASA. https://science.nasa.gov/earth/facts/
National Oceanic and Atmospheric Administration. (2005, December 1). Tides and Water Levels. National Ocean Service. https://oceanservice.noaa.gov/education/tutorial_tides/tides03_gravity.html