Earth's tilted spin axis causes the seasons, according to the National Weather Service. Earth spins on its axis and is tilted to its orbital plane while orbiting around the sun in an ellipse.
The tilt of Earth's axis, which is 23.5 degrees, allows the sun to hit certain parts of Earth more directly. When it is winter for the northern hemisphere, it is summer for the southern hemisphere. At this time of the year, the sun shines more directly on the southern hemisphere, according to NASA. Without the tilt of Earth's axis, the equator and immediate surrounding areas would get the most sun, and the northern and southern hemispheres would be either hot or cold with the same temperature year-round. Earth's distance from the sun does not affect the seasons. Instead, Earth's elliptical orbit means the planet is closer to the sun during the northern hemisphere's winter.
The winter and summer solstices refer to the times of the year when Earth's axis points towards or away from the sun, according to the National Weather Service. Some scientists believe that a large object, called Theia, hit Earth when the planet was still young, causing Earth to tilt and sending dust and rubble into orbit, states NASA.