What causes Earth's seasons?

Earth's seasons are caused by the 23.5° tilt of Earth's rotational axis relative to its orbital plane around the Sun. As Earth orbits, the tilt causes hemispheres to receive different amounts of direct sunlight at different times of year. Summer in a hemisphere: that hemisphere is tilted TOWARD the Sun, receiving more direct sunlight (higher in sky) and longer days. Winter: tilted AWAY from the Sun, less direct sunlight (lower in sky) and shorter days. Common misconception: many people think Earth's distance from Sun causes seasons. Earth's orbit is slightly elliptical, but the distance variation is small (~3%); Earth is actually CLOSEST to Sun in January (Northern winter) — proving distance doesn't cause seasons. Key dates: (1) March 20-21: Vernal Equinox — equal day/night globally; Northern spring begins, Southern fall begins; (2) June 20-21: Summer Solstice — longest day in Northern Hemisphere, shortest in Southern; (3) September 22-23: Autumnal Equinox; (4) December 21-22: Winter Solstice — shortest day in Northern, longest in Southern. At equator, less seasonal variation. Polar regions: 24 hours daylight in summer (midnight sun), 24 hours darkness in winter. Tropics of Cancer (23.5° N) and Capricorn (23.5° S) mark where Sun is directly overhead at solstices.