Why does the Earth rotate on its own axis?

Answer

Every celestial body be it a planet, a moon, a star or an entire galaxy revolves around its axis. The earth, together with the other planets and the sun, originated from an interstellar cloud of matter, which also rotated very slowly. When that cloud contracts, the physical law of conservation of angular momentum comes into play.

The angular momentum of a body is as follows : suppose that a mass m moves in a circle with a radius R and with an angular velocity ω. Then the angular momentum is mR².ω

That ω is therefore the angular velocity, so the number of radians that the body travels per second. You see that if this product is conserved, and R decreases (because the star or planet contracts under its own gravitation), then ω must increase to keep the whole thing constant. If the radius decreases by a factor of 1000, ω will increase by a factor of one million. During the contraction of such an interstellar cloud, the minute angular acceleration is thus increased to such an extent that the final star acquires a clearly measurable rotation with a rotation period of typically a number of days.

Planets form from the clumping together of smaller chunks orbiting the forming sun, and “inherit” sufficient angular momentum through these chunks, just like the moons orbiting these planets. Therefore, with few exceptions, everything in the solar system also revolves in the same sense. If you position yourself high above the North Pole, high enough to see the entire solar system, you will notice that everything revolves counterclockwise, the sun on its axis, the planets and moons on their axis, the planets around the sun, the moons around their planet (with few exceptions).