Why seasons happen — Astronomy, 7–10 years
Seasons happen because Earth's axis is tilted as Earth travels around the Sun. That tilt changes the angle and length of sunlight in each hemisphere during the year.
Tilted toward the Sun
Earth's axis is not straight up compared with its path around the Sun; it leans. When your hemisphere leans toward the Sun, sunlight arrives more directly and days last longer: it is warmer. When it leans away, sunlight is less direct and days are shorter: it is colder.
Explaining the yearly change
People noticed that daylight, temperatures, and living things follow a yearly pattern. Saying only that summer is hot does not explain why, or why the seasons are opposite north and south of the equator. Earth's tilt and journey around the Sun solve both questions.
Two hemispheres
Suppose it is June and the northern half of Earth leans toward the Sun. Northern places receive more direct sunlight and longer days, so they have summer. At the same time, the southern half leans away, with less direct light and shorter days, so it has winter. Six months later, the pattern reverses.
Not mainly the distance
A natural guess is that summer happens when Earth is closer to the Sun and winter when it is farther away. Distance sounds important because the Sun is enormous and hot. But the two hemispheres have opposite seasons at the same time, so the tilt and sunlight angle must be the main cause.
Living with seasons
Seasonal sunlight affects what farmers plant, when animals migrate, and how people prepare homes and clothing. It also helps astronomers predict when the Sun will rise earlier or set later at a chosen place. The pattern is useful, but local weather can still vary from day to day.
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