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为什么一年里白昼长度会变化

Earth orbits the Sun while tilting at about 23.5 degrees on its axis.
This tilt stays pointed in the same direction as Earth travels around the Sun.
When the North Pole tilts toward the Sun, days grow longer in the Northern Hemisphere.
During that time, the South Pole tilts away, causing shorter days and winter there.
The equinoxes occur when neither pole tilts toward the Sun, making day and night nearly equal.
Solstices mark the longest and shortest days when tilt is most extreme toward or away.
This consistent motion explains why summer has more daylight than winter at mid-latitudes.
Seasonal changes in daylight also affect plant growth, animal behavior, and human energy use.
Without axial tilt, every place on Earth would have almost the same day length year-round.
Astronomers use these patterns to calibrate calendars and predict climate-related seasonal shifts.