The phenomenon that causes the Earth's seasons is the tilt of Earth's axis relative to its orbit around the Sun. This axial tilt is approximately 23.5 degrees. As Earth orbits the Sun, different parts of the planet receive varying amounts of sunlight throughout the year, resulting in seasonal changes. For instance, when the Northern Hemisphere is tilted toward the Sun, it experiences summer, while the Southern Hemisphere experiences winter, and vice versa. These seasonal variations influence climate and ecosystems.