The earth spins as a result of things colliding with each other when the Solar System was formed. Some scientists believe that the Earth started spinning after a direct collision with the Moon. Kept moving by the force of momentum, the Earth takes one day to make one full rotation.
It can’t be a coincidence. Look down on the Earth from above, and you’d see that it’s turning in a counter-clockwise direction. Same with the Sun, Mars and most of the planets.
It’s the conservation of angular momentum. Think about the individual atoms in the cloud of hydrogen. Each particle has its own momentum as it drifts through the void. As these atoms glom onto one another with gravity, they need to average out their momentum. It might be possible to average out perfectly to zero, but it’s really unlikely.
Which means, there will be some left over. Like a figure skater pulling in her arms to spin more rapidly, the collapsing proto-Solar System with its averaged out particle momentum began to spin faster and faster. As the Solar System spun more rapidly, it flattened out into a disk with a bulge in the middle. We see this same structure throughout the Universe: the shape of galaxies, around rapidly spinning black holes, and we even see it in pizza restaurants.
Over the course of a few hundred million years, all of the material in the Solar System gathered together into planets, asteroids, moons and comets. Then the powerful radiation and solar winds from the young Sun cleared out everything that was left over. Without any unbalanced forces acting on them, the inertia of the Sun and the planets have kept them spinning for billions of years.
And they’ll continue to do so until they collide with some object, billions or even trillions of years in the future. The Earth spins because it formed in the accretion disk of a cloud of hydrogen that collapsed down from mutual gravity and needed to conserve its angular momentum. It continues to spin because of inertia.
Picture Credit : Google