The imaginary line around which the Earth spins is called the axis of rotation. Because the Earth's axis points at an angle of 23.5 degrees away from the axis of the Sun's rotation, the Earth experiences seasonal changes in weather.
During the summer months, the Earth's North Pole tilts toward the sun, creating warmer weather in the Northern Hemisphere. During the autumn, both hemispheres receive roughly the same amount of sunlight, but when the North Pole tilts away from the sun, the Southern Hemisphere experiences the warmer weather. However, the Earth spins on its axis similar to the movement of a top, and the dates for shifting seasons changes very gradually, on a cycle that lasts 25,800 years. Eventually, the summer and winter seasons for the two hemispheres will switch.