Why Earth has seasons — Astronomy, 14–17 years
Seasons are caused mainly by Earth’s tilted axis, not by Earth moving much closer to or farther from the Sun. The tilt changes the angle and length of sunlight in each hemisphere during the year.
Tilt changes sunlight
Earth’s axis is tilted about 23.5 degrees as Earth travels around the Sun. When your hemisphere tilts toward the Sun, sunlight arrives more directly and days are longer, so more energy reaches the ground. Six months later it tilts away: sunlight is spread out and days are shorter.
Why explain seasons with tilt?
A simple model was needed to explain why opposite hemispheres have opposite seasons and why daylight changes through the year. Earth’s orbit is slightly oval, but that small distance change cannot explain the pattern. The tilted-axis model fits both observations and also predicts solstices and equinoxes.
Comparing noon sunlight
Draw two equal squares on paper. Spread the same 100 sunlight units over one square at a steep angle; nearly all stay in that square. At a low angle, the same units stretch across two squares, so each square receives about 50 units. This is why lower winter sunlight warms the ground less, even before shorter days are considered.
The mistake: summer means closer
It is reasonable to link warmth with distance: a hand feels hotter near a heater. But when it is summer in the Northern Hemisphere, Earth is actually near its slightly more distant part of the orbit. The decisive changes are the sunlight angle and the length of daylight; at the same time, the Southern Hemisphere has winter.
Seasons outside astronomy class
The tilt model helps predict daylight, solar-panel output and the best time for some crops. Architects use sun angles when planning shade in summer and useful sunlight in winter. It also matters in space missions: a lander’s heating, power and communication can change with the season at its destination.
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