The influence of Earth's tilt on seasons.  Day and night lengths vary throughout the year because of the orientation of Earth's tilted spin axis in relationship to the Sun.  Associated changes in solar energy input to each hemishere cause seasons.  In the northern hemiphere, summer begins at the summer solstice when the north pole is titled toward the Sun.  Winter in the northern hemisphere begins when the north pole is oriented away from the Sun, around December 21.  In both hemispheres, day and night lengths are equal on two days each year: the autumnal and vernal equinoxes.  [Source: NASA, Jet Propulsion Laboratory]