The main reason for the seasons on Earth is the tilt of its axis relative to its orbital plane around the Sun. This axial tilt, approximately 23.5 degrees, causes different parts of the Earth to receive varying amounts of solar radiation throughout the year. During summer, the hemisphere tilted towards the Sun experiences longer days and more intense sunlight, while the opposite hemisphere experiences winter with shorter days and less sunlight. This variation results in seasonal changes in temperature and climate.