Why seasons happen — Earth science, 14–17 years
The seasons are caused mainly by Earth’s tilted axis as it travels around the Sun. The tilt changes the angle and length of sunlight, rather than Earth simply moving much nearer to or farther from the Sun.
Tilted sunlight
Earth’s axis is tilted about 23.5° compared with the plane of its orbit. As Earth travels around the Sun, each hemisphere takes turns leaning towards the Sun. A hemisphere then gets more direct light and longer days: its summer; the other has winter.
The problem it explains
People noticed a repeating pattern: daylight length, temperatures and the Sun’s height change through the year, while opposite hemispheres have opposite seasons. A distance-only explanation fails because both hemispheres cannot be closest to the Sun at the same time. Tilt explains both patterns together.
A simple comparison
Suppose two equal patches receive 100 units of sunlight. In one case the light is concentrated on 1 m²; in another it is spread over 2 m², so each square metre gets 50 units. Step by step: lower Sun angle spreads energy, longer days add exposure, and the surface warms more in summer.
The distance mistake
It is reasonable to think summer happens when Earth is closest to the Sun: warmth often increases when we move closer to a heater. But Earth’s orbit is only slightly non-circular, and the Northern Hemisphere’s summer occurs when Earth is actually a little farther away. Sun angle and daylight length matter more.
Where it is used
Seasonal sunlight guides farming calendars, solar-panel planning and estimates of heating demand. It also helps explain why a midnight Sun can occur near the poles in summer while polar night occurs in winter. Near the equator, seasonal temperature changes are often smaller.
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