Seasons and Earth’s tilt — Astronomy, 11–13 years
Seasons happen because Earth is tilted as it travels around the Sun. Astronomy, 11–13 years.
The idea
Earth’s axis is tilted about 23.5 degrees instead of standing straight up relative to its orbit. As Earth goes around the Sun, each hemisphere takes turns leaning towards the Sun and then away from it. Leaning towards the Sun brings longer days and more direct sunlight: summer; leaning away brings winter.
Why we need this idea
People noticed that daylight, shadows and temperatures change through the year, and that the seasons are opposite in the Northern and Southern Hemispheres. Earth’s distance from the Sun alone cannot explain that pattern: both hemispheres are nearly the same distance away. The tilt explains the changing sunlight and the opposite seasons together.
A worked example
Use a tilted globe and a lamp. Keep the globe’s axis pointing the same way while moving it around the lamp. When the Northern Hemisphere leans towards the lamp, its light is spread over a smaller area and its days are longer: northern summer. Half an orbit later, it leans away, sunlight is less direct and days are shorter: northern winter.
The common trap
A reasonable mistake is to think summer happens because Earth is much closer to the Sun. Earth’s orbit is not a perfect circle, so its distance does change a little, which makes the idea tempting. But that small change cannot produce opposite seasons at the same time; the axis tilt is the main cause.
Where it is used
Understanding the seasons helps farmers, gardeners and communities plan for changing daylight and temperatures. Solar panels and building designs can also take account of the Sun’s changing height in the sky. It matters when comparing places: a warm summer in Australia happens during the Northern Hemisphere’s winter because the hemispheres tilt in opposite directions.
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