The Earth spins on its axis but does not change direction of its axis as it revolves around the sun.This means that, for part of the year, the north pole is pointed away from the sun and so in perpetual shadow, while the south pole is pointed toward the sun, and thus in perpetual daylight. During the opposite time of the year, although the Earth's tilt hasn't changed, the position of the sun relative to the Earth has, so the situation is reversed. Away from the poles, the effects are less extreme, but nonetheless, the days are longer in the summer than in the winter and the sun is higher in the sky, and one hemisphere's summer is also the other's winter and vice-versa.

From March to September, the northern hemisphere gets more light, and the southern hemisphere gets less.From September to March, it's the other way around.