Question: why does the earth rotate on its axis

Jonathan Mound answered on 23 Mar 2026:

In some sense the Earth rotates on its axis because it always has.

All of the planets in our solar system formed from a big cloud of dust and gas. As parts of that cloud clumped together there were swirls and eddies. As the dust collected together into larger and larger clumps, they kept that spin. Even if you had two rocks that weren’t spinning as they came together, unless they hit exactly straight on, that collision would cause them to spin (in physics terms “linear momentum” can be turned into “angular momentum”).

Once the Earth formed and had its spin, it has just kept on spinning (in physics terms, its “angular momentum is conserved”). It would take some force from outside the Earth to stop its spin. In fact, there is a force that is slowing down the spin. The tides raised by the Moon actually end up slowing the Earth’s spin down a bit. This means that the day is getting slightly longer, one full rotation of the Earth takes about 2 milliseconds longer today than it did 100 years ago.