Kepler's laws approximate two-body orbital motion about a common gravitating center. For planets orbiting the Sun, T²/a³ is approximately constant in a consistent unit system, with corrections from planetary masses and gravitational perturbations. Earth travels faster near perihelion (early January) and slower near aphelion (early July). Northern Hemisphere winter is not caused by Earth being farther from the Sun; Earth is actually near perihelion then. Nevertheless, Earth–Sun distance still affects incoming solar radiation. Axial tilt, through changes in solar angle and day length, is the main driver of the seasons.