Part of the show Naked Science Questions and Answers
Donald in Essex asked:
What makes the Earth spin and will it ever stop?
The Earth has been spinning ever since it was created. It could have been because the Moon was created after a big collision with the Earth. If the collision was just off centre, then it would have spun the Earth really fast. When the Moon was created, it was really close to the Earth. The tides are actually slowing the Earth down. In fact, two hundred million years ago there were about 400 days in a year. This is actually a legacy of how all our planets formed because there was a big disk of material going around the sun and everything was spinning. As everything coalesced and formed planets, the conservation of momentum meant that they carried on spinning. This is the same principle as when an ice skater spins and brings their arms in and spins faster.