If this help, It's due to the tilt of the Earth's rational axis, which said to be is 23.4 degrees. During the June Solstice, the North Hemisphere is tilted at the sun, which causes Summer. Then when the Southern Hemisphere is tilted on the other hand, it causes Winter.
Hopefully this helps. If so, I don't mind a brainiest answer or 3 - 5 Star :)
