Why Earth has seasons — Earth science, 11–13 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 in each hemisphere, rather than Earth simply becoming much nearer to or farther from the Sun.
Tilt changes sunlight
Earth’s axis is tilted about 23.5 degrees. As Earth orbits the Sun, one hemisphere leans towards the Sun and receives more direct light for longer days; six months later, it leans away and receives less direct light during shorter days.
Why explain seasons?
People noticed repeating changes in daylight, temperature, plants and animal behaviour, but needed to explain why they occur at opposite times north and south of the equator. Earth’s tilt solves this pattern: when the north has summer, the south has winter, and vice versa.
Comparing two hemispheres
Imagine Earth in July, with the northern half tilted towards the Sun. At 45° north, sunlight arrives more directly and the day may last about 15 hours. At 45° south, sunlight arrives at a lower angle and the day may last about 9 hours. The two places receive opposite seasons.
It is not mainly distance
A reasonable guess is that summer happens when Earth is closer to the Sun, because warmth usually increases near a heater. But Earth’s orbit is nearly circular, and the hemispheres have opposite seasons at the same time. Sunlight angle and daylight length are the important changes.
Planning with the seasons
Seasonal sunlight affects when farmers sow crops, when some animals migrate and how buildings need heating or cooling. Weather can still break the usual pattern on a particular day. The tilt explains the broad yearly rhythm, not every day’s temperature.
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