Earth's rotation period is about 24 hours, or one day. The time it takes for a planet or other celestial object to complete one spin around its axis is called its rotation period. So, each planet has a North and South Pole, the points where an axis meets the planet's surface. Each planet in our solar system rotates on its axis. Ocean tides shift the center of mass, although not enough to radically shift the planet's axis.
An object's center of mass is a point where an outside force acting on the object acts as if the object were located at just that point-where the object appears 'balanced.' Earth's center of mass actually varies. In either case, an object's axis runs through its center of mass, or barycenter. Or it could be a star with the mass of a thousand suns. The object can be a tiny particle, smaller than a single atom. An axis is an invisible line around which an object rotates, or spins.
