Dec. 21, 2021, is the winter solstice in the northern hemisphere, meaning it is the day with the least amount of sunlight. It also marks the beginning of the astronomical winter. For those south of Earth's equator, the opposite is true; Dec. 21 is the summer solstice — the day with the most extended period of sunlight, which also brings with it the start of summer.
Earth experiences opposite seasons in its northern and southern hemispheres because the planet rotates on a tilted axis.
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