Why Does the Earth Have Seasons?

Earth axis of rotation is tilted approximately 23.5° from the perpendicular to its orbital plane around the Sun. As Earth orbits over the course of a year, this tilt causes each hemisphere to alternately lean toward and away from the Sun. When the Northern Hemisphere tilts toward the Sun (around June), it receives more direct sunlight and longer days, producing summer; simultaneously, the Southern Hemisphere experiences winter. Six months later, the situation reverses. The tilt, not the distance from the Sun, is the primary driver — Earth is actually closest to the Sun (perihelion) in early January, during Northern Hemisphere winter.