Why the same sunlight makes different seasons
Earth’s tilt changes not only which hemisphere faces the Sun, but also how sunlight reaches the ground. When sunlight arrives more directly, its energy is concentrated on a smaller area. When it arrives at a slant, that same energy is spread over more ground, so it warms the surface less efficiently. Slanted rays also pass through more atmosphere before reaching the ground.
The tilt changes the length of the day, too. A hemisphere leaning toward the Sun gets more hours of daylight, giving the ground longer to absorb energy. In its summer, the combination of higher-angle sunlight and longer days helps produce warmer conditions. The opposite hemisphere is then tilted away and has shorter days and weaker sunlight.
The Sun’s height shifts through the year
As Earth travels around the Sun, its axis keeps pointing in nearly the same direction in space. The Sun therefore appears higher in the sky at midday for one hemisphere during its summer and lower during its winter. The change is gradual: at the equinoxes, neither hemisphere leans strongly toward the Sun, and day and night are close to equal in length around the world. At the solstices, one hemisphere gets its longest day and the other its shortest.
Seasons are not caused mainly by Earth being closer to or farther from the Sun. The distance changes over the year, but that change affects both hemispheres at once; it cannot explain why they have opposite seasons. The tilt does: while one half of Earth is set up for long, direct-light days, the other is receiving short, slant-light ones.
