Why Do We Have Seasons?
QUICK ANSWER
Earth has seasons because its axis is tilted 23.5 degrees relative to its orbit around the Sun. As Earth orbits the Sun, different parts of the planet receive more or less direct sunlight at different times of year. The tilt, not the distance from the Sun, is what causes seasons.
Many people assume seasons happen because Earth is closer to the Sun in summer and farther in winter, but that's actually wrong. Earth's orbit is nearly circular, and we're actually closest to the Sun in early January (during Northern Hemisphere winter). The real cause of seasons is the tilt of Earth's axis. This tilt produces the most familiar yearly cycle on our planet.
What's actually causing seasons?
Earth's axial tilt of 23.5 degrees. According to NASA, Earth's tilted axis causes the seasons. Throughout the year, different parts of Earth receive the Sun's most direct rays. So when the North Pole tilts toward the Sun, it's summer in the Northern Hemisphere. When the South Pole tilts toward the Sun, it's winter in the Northern Hemisphere. The tilt of Earth's axis stays fixed relative to space (always pointing toward Polaris), but as Earth orbits the Sun, different hemispheres get more direct sunlight.
Why doesn't distance matter?
Because Earth's orbit is nearly circular. Earth's distance from the Sun varies only by about 3% over the year, far too little to cause major temperature changes. In fact, Earth is closest to the Sun (perihelion) in early January, when the Northern Hemisphere is in winter. If distance caused seasons, both hemispheres would have summer at the same time, but they have opposite seasons. Northern Hemisphere summer is Southern Hemisphere winter and vice versa. This proves the tilt, not distance, controls the seasons.
How does tilt produce temperature changes?
Through sunlight angle and day length. When sunlight hits a surface directly (high in the sky), it concentrates the energy on a smaller area, producing more heating. When sunlight hits at a glancing angle (low in the sky), the same energy spreads across a larger area, producing less heating. Earth's tilt makes the Sun appear higher in summer skies and lower in winter skies. The tilt also affects day length: summer hemispheres get more hours of sunlight per day, while winter hemispheres get fewer.
What about places near the equator?
They have weak seasons. Locations near the equator receive nearly direct sunlight year-round because Earth's tilt doesn't significantly change the Sun's angle there. Tropical regions don't have the temperature-based four seasons of higher latitudes. Instead, they typically have wet and dry seasons driven by atmospheric circulation patterns. The poles experience extreme seasons with continuous daylight in summer (midnight sun) and continuous darkness in winter (polar night). Higher latitudes have more extreme seasonal variations.
Earth has seasons because its axis is tilted 23.5 degrees relative to its orbit around the Sun. As Earth orbits the Sun, different parts of the planet receive more or less direct sunlight, producing summer when a hemisphere tilts toward the Sun and winter when it tilts away. The tilt, not distance from the Sun, causes seasons. Earth's orbit is nearly circular and we're closest to the Sun in January, during Northern Hemisphere winter.
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