Answer: The tilt of the Earth's axis of rotation.
Explanation:
The seasons on the planet Earth are as a result of the tilt of the Earth's axis of rotation. When the Earth's tilt in axis causes the northern hemisphere to face the sun more directly those parts experience summer and when the Earth's axis tilt causes the Northern hemisphere not to face the Sun directly, those parts experience winter. Most parts of the United States are found in the Northern hemisphere.
