What causes the seasons to change?

Seasons on Earth change in response to the regular change in the planet's axial tilt relative to the Sun. Earth is inclined at a 23-degree angle, and as a result it points first one, then the other, hemisphere toward the Sun as it orbits.

In June, the Northern Hemisphere is inclined toward the Sun. As a result, observers in the northern latitudes see the Sun rise earlier, traverse more of the sky and set later than observers in the Southern Hemisphere. By December, Earth has moved to the opposite side of its orbit, and the Southern Hemisphere is presented toward the Sun, causing summer temperatures in the south.