Final answer:
The existence of Earth's seasons is explained by the tilt of Earth's axis and its orbit around the Sun.
Step-by-step explanation:
The existence of Earth's seasons is explained by two characteristics of Earth's relationship to the Sun. First, Earth's axis is tilted relative to its orbital plane. This tilt causes different parts of Earth to receive different amounts of sunlight throughout the year, creating the seasons. Second, Earth orbits around the Sun, completing one orbit each year. As Earth moves in its orbit, different parts of the planet are tilted toward or away from the Sun, resulting in the changing seasons.