The earth rotates around the sun. According to Newton's law of gravitation, the sun and earth exert equal and opposite forces on each other. So why does only the earth revolve? Consider two bodies moving at different velocities. The velocity of the center of mass of this system lies on the line joining their individual velocities. If the system is isolated, the center of mass can be taken as the origin. The velocities of the bodies can now be expressed relative to the velocity of the center of mass. Multiplying the velocities with their respective masses gives their momenta. The first term here denotes the reduced mass of the two-body system. The equation of the two-body system is now converted into a one-body system. In the earth-sun system, the center of mass velocity can be approximated to the velocity of the sun. So, the sun has negligible relative velocity with respect to its center of mass, whereas the earth possesses a relative velocity which allows it to revolve around the sun.