Why do the Northern and Southern hemispheres have opposite seasons?

The Northern and Southern hemispheres experience opposite seasons primarily due to the tilt of Earth’s axis, which is inclined at approximately 23.5° relative to its orbital plane.12

As Earth orbits the Sun, this axis maintains a nearly constant orientation in space. Because of this tilt, each hemisphere leans toward or away from the Sun at different times of the year. When one hemisphere is tilted toward the Sun, it receives more direct sunlight and experiences longer days, resulting in summer. Simultaneously, the other hemisphere is tilted away, receiving less direct solar radiation and experiencing shorter days, which results in winter.21

These conditions reverse over the course of the year as Earth moves along its orbit. Consequently, when the Northern Hemisphere experiences its summer solstice, the Southern Hemisphere is experiencing its winter, and vice versa. This seasonal cycle is driven by the variation in the angle and duration of sunlight, rather than Earth's distance from the Sun.2314