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Earth's seasons are primarily determined by two factors: the axial tilt of Earth and its revolution around the Sun. Earth's axis is tilted approximately 23.5° and maintains its orientation (pointing toward Polaris) throughout Earth's orbit.
During the summer solstice (around June 22), the Northern Hemisphere tilts toward the Sun, and the Sun is directly overhead at the Tropic of Cancer. The Northern Hemisphere receives more direct sunlight and longer daylight hours, making it summer. The Southern Hemisphere tilts away, experiencing winter.
During the winter solstice (around December 22), the situation reverses. The Northern Hemisphere tilts away from the Sun, and the Sun is directly overhead at the Tropic of Capricorn. The Northern Hemisphere receives oblique sunlight and shorter daylight, making it winter. The Southern Hemisphere experiences summer.
During the equinoxes (March and September), Earth's axis tilts sideways relative to the Sun, and the Sun is directly overhead at the equator. Day and night are equal length worldwide, and both hemispheres receive similar amounts of sunlight, marking the beginning of spring or autumn.
Interestingly, the variation in Earth's distance from the Sun has minimal effect on seasons. In fact, the Northern Hemisphere's winter occurs near perihelion (when Earth is closest to the Sun), demonstrating that axial tilt is the dominant factor in seasonal changes.