The seasons change according to the amount of sunlight reaching the surface of the Earth. The Earth’s axis is tilted as it revolves around the sun. Summer occurs when a hemisphere is tilted towards the sun. The Tropics of Cancer and Capricorn mark the regions of the Earth where the Sun reaches the zenith at least once a year.
Note: This animation takes a geocentric view of the Earth, and so the axis oscillates during the course of the year. Were a heliocentric view chosen, the Earth’s axis would have a fixed inclination throughout the year.