Earth's axis of rotation is tilted about 23.5° from the plane of its orbit around the sun. This tilt causes different parts of the earth to receive more or less direct sunlight at different times of the year.

When the Northern Hemisphere is tilted towards the sun, it receives more direct sunlight and experiences summer. At the same time, the Southern Hemisphere is tilted away from the sun and experiences winter. Six months later, the opposite is true: the Southern Hemisphere is tilted towards the sun and experiences summer, while the Northern Hemisphere is tilted away from the sun and experiences winter.