Solar irradiance

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.