Seasons

Seasons occur because Earth's axis (an imaginary line running through the center of the Earth from the North Pole to the South Pole) is tilted 23.5 degrees. During the planet's yearly orbit, this tilt always points in the same direction in space, so different parts of the planet receive varying amounts of direct sunlight throughout the year. Depending on which hemisphere (Northern or Southern) receives more direct sunlight, the seasons are defined by how much it contacts the Sun.

Summer is when your half of the Earth tilts toward the Sun, so the Sun's rays hit you straight on. This direct sunlight brings more heat, and the days last longer.

Winter is when your half of the Earth tilts away from the Sun, so the Sun's rays hit you at a shallow, slanted angle. This spreads the heat over a larger area, making days shorter and colder. 

Spring and Autumn occur halfway through the orbit; neither half of the Earth tilts toward or away from the Sun. Both the Northern and Southern Hemispheres receive equal sunlight, creating mild weather.

Common Misconception

Many people think the weather changes because the Earth gets closer to or farther from the Sun. In reality, Earth is closest to the Sun in January (winter in the Northern Hemisphere). The  tilt angle, not distance, controls our seasons.