You have to admit, it doesn't feel like you're spinning around the center of the Earth at hundreds of miles an hour, so it's not hard to cut our scientific forebears some slack for assuming the Earth was stationary and that the sun rotated around it. Thankfully, Copernicus set the record straight with his heliocentric model, and we now know that the Earth spins on its axis as it revolves around the sun. But why does our planet spin in the first place?
Remember Newton's first law of motion? It states that an object remains in whatever state of motion it's in unless another force acts upon it. The Earth is rotating because it's been doing that as long as it has existed.
Before there were planets in our solar system, there was a spinning, nebulous cloud of dust with our sun at the center. Over time, these dust particles collided into one another and began to stick, forming larger and larger rocks and ultimately planets through a process known as accretion. But remember, the cloud of dust -- or accretion disc -- was rotating from the start. As the particles that formed the Earth began to stick together, that momentum was conserved, causing the growing planet to spin faster and faster, much the way a figure skater does when he pulls his arms in toward his body. By the time the Earth had formed, it had all of the angular momentum it would need to keep spinning to this very day. Just how fast is that anyway?