Big question
Why does the Earth spin?
The Earth spins because it formed billions of years ago from a swirling cloud of gas and dust that was already turning, and nothing has ever stopped it. In space there is almost no friction to slow it down, so the planet has kept turning ever since.
Born From a Spinning Cloud
About 4.6 billion years ago, the Sun and all the planets, including Earth, formed out of a huge cloud of gas and dust floating in space. Clumps of matter inside that cloud were pulled together by gravity, and any tiny bit of turning motion already present in the cloud got concentrated as the material collapsed into a smaller, denser ball, similar to how a spinning skater speeds up when they pull their arms in. That effect is called conservation of angular momentum: the smaller something gets while its spinning matter pulls inward, the faster it tends to spin. This is why the early, forming Earth ended up rotating instead of sitting perfectly still.
Nothing to Slow It Down
On Earth, moving objects usually slow down because of friction, like a ball rolling across grass losing energy to the ground it rubs against, or air pushing back against it. Space is close to a vacuum, meaning there is almost no air, dust or anything else for the spinning Earth to rub against, so there is nothing to create meaningful friction and slow the spin. Once something starts spinning freely in space with nothing pushing back against it, physics says it keeps spinning at very nearly the same rate, which is exactly what has kept Earth turning for over four billion years.
A Spin That Is Slowly Changing
Earth's spin is not perfectly constant. The Moon's gravity pulls on Earth's oceans, creating tides, and the friction of that moving water very slightly drags on Earth's rotation, slowing it down by a tiny amount over long stretches of time. Scientists estimate that days were shorter in the distant past, perhaps only about 22 hours long around 600 million years ago, and are getting longer by roughly a couple of milliseconds each century. The change is far too small to notice in an ordinary lifetime, but occasionally timekeepers add a 'leap second' to official clocks to keep them matched with Earth's actual spin.
Try it yourself
Spin around slowly with your arms stretched out, then pull your arms in tight to your body while still spinning. You should turn faster, the same shrinking-and-speeding-up effect that helped set the young Earth spinning.
Questions children ask
Will the Earth ever stop spinning?
The Earth is not expected to stop spinning any time soon, since there is almost no friction in space to slow it down. Its spin is very slowly changing over millions of years, mostly because of tides caused by the Moon.
Why don't we feel the Earth spinning?
We do not feel the Earth spin because everything around us, including the air, is moving at the same steady speed together with the ground. There is no sudden jolt or change in speed for our bodies to notice.
How fast does the Earth spin?
At the equator, the Earth's surface spins at roughly 1,670 kilometres per hour to complete one full turn in about 24 hours. Near the poles the speed is much slower, since those points barely move as the planet turns.