Why is the 21st of December the shortest day?

The 21st of December marks the winter solstice in the Northern Hemisphere, when the North Pole is tilted farthest away from the Sun. This tilt results in the Sun taking a shorter, lower path across the sky, leading to the least amount of daylight and the longest night of the year. Conversely, in the Southern Hemisphere, this date corresponds to the summer solstice, where the day is the longest.