The phenomenon that explains why we see different seasons on Earth is the tilt of the Earth's axis. As the Earth orbits the Sun, its axial tilt (approximately 23.5 degrees) causes varying angles of sunlight to hit different regions at different times of the year. This results in seasonal changes in temperature, daylight hours, and biological activity. For instance, during summer in the Northern Hemisphere, sunlight is more direct, resulting in warmer temperatures, while in winter, sunlight is less direct, leading to cooler weather. This axial tilt is fundamental to life on Earth.