The cycle of seasons is caused by Earth's tilt toward the sun. The planet rotates around an axis. At different times during the year, the northern or southern axis is closer to the sun. During these times of the year, the hemispheres experience spring and autumn
Since the earth is tilted at 23.5°, therefore at some point during revolution the southern hemisphere is closer to the sun and experiences summer while northern hemisphere experiences winter. At some other point the southern hemisphere is away from the sun and experiences winter while the northern hemisphere experiences summer.
