Actually not only the earth revolves but also the other planets, their satellites, the Sun, the other star systems and other Galaxies are also revolving. Nobody knows when and why those spins start.
However, the Earth, which was a huge cloud in the very beginning of its formation, started spinning as it became denser (as it was solidifying). The Science people report that the reason behind this is the conservation of the angular momentum. It is a bit hard to explain this for us but this example may ease the things up a bit. When we pour water to the sink, we see that the water flows in circles. And when we drain the water, the water goes down in spirals after pulling off the plug. And our Earth, which started spinning in this manner, continued to spin like this since there was literally no force in the space to stop it. And it still continues this movement. A basketball that we spun on a table stops spinning after a while. And why does it stop? Because unlike the outer-space all the empty space within our Earth is filled with the air. The air creates friction that would halt the spin of the ball. The force of friction in turn causes the loss of energy. You can feel this, if you are lucky, from the heating at the area that the ball spins on. Or if the ball spins fast, you can hear the sound of the airflow that the friction causes. There is no energy loss in space due to the lack of any effect that may slow down or stop the spin.
What if there was a force that would slow down the rotation of the Earth? The energy, generated as the result of the spin would boil the oceans, would melt down the rocks on the surface of the Earth or may have caused the volcanoes to erupt. When it finally came to a full stop, the side of the Earth facing the Sun would live an eternal day while the other side would be covered with an eternal night. Let’s hope that such events only occur in the scenarios of science fiction films. However, it is also reality that the rotation speed of the Earth is decreasing. Due to the gravitational force of the Moon, Sun and the other celestial bodies of the solar system, tides occur in the Earth. The frictional force as the result of the tides slows down the Earth. This deceleration is calculated to be 2 milliseconds per year. The geoscientists revealed that a year was 397 days as one day was 22 hours around 340 Million years ago. If we ever had a chance to travel to millions of years in the future, we would see that a day would be 48 hours.
As the Sir Isaac Newton depicts in its masterpiece “Mathematical Principles of Natural Philosophy” in 1687; if an object is in motion and if there is no external force that has an effect on the object, that object continues to move forever. Although it is not certain how the Earth got its initial spin move, since there is no external force that has any significant impact on it, it is certain that the Earth will continue its spin for a very long time.