Kepler's third law (the harmonic law), stating that the square of the orbital period is proportional to the cube of the semi-major axis, applies specifically to bodies in stable, periodic (elliptical) orbits around a common central mass, such as planets around the Sun, moons around their host planets, or artificial satellites around Earth. It does not apply to non-periodic comets, which follow open (parabolic or hyperbolic) trajectories and never return, since such trajectories have no well-defined, repeating orbital period or semi-major axis to relate via the law.