What makes the earth spin?

We were driving N. to camp this morning when A. exclaimed "The sun keeps following us!" N. corrected him and explained that the earth was revolving around the sun, and spinning at the same time.

A: "Oh." (pause) "What makes the earth spin?"

Me: "Wow, that's a good question. I know that the earth revolves around the sun because of gravity (we fall around the sun), but I don't know why it spins. We'll have to look that up when we get home."

A: "Okay." (another pause) "Mom? Why don't cars and people spin off the earth?"

Man I love this kid! He asks such awesome questions for a 4 year old! Such insatiable curiosity, and a natural love of learning. He's gonna make a terrific homeschooler, and I'm going to have trouble keeping up with him!

Why does the Earth orbit the Sun?

The basic reason that the Earth orbits the sun is because the Sun's gravity keeps it in place. Just as the Moon orbits the Earth because of the pull of Earth's gravity, the Earth orbits the Sun because of the pull of the Sun's gravity. The Earth travels about 595 million miles around the sun in 365 days (a year). Like all planets in our solar system, Earth travels around the Sun in an elliptical orbit.

Why does the Earth rotate?

Our everyday experience teaches us that an object must be "pushed" by a force in order to keep it moving. Otherwise, it will slow down and eventually stop. But this intuition is absolutely wrong. If an object is moving, then a force is required *to slow it down or stop it*, not to keep it moving. (Hence, "Objects in motion tend to stay in motion. Objects at rest tend to stay at rest.") In our everyday experience, it's the force of friction that tends to stop Earth-bound objects from moving forever. But for the Earth rotating on its axis, there is no force working to counteract the rotation (except the tidal effect of the Moon, but that's working very slowly), so you don't need to have any input energy to keep it spinning.