The Northern Hemisphere's procession of dwindling days is about to reach its nadir. The winter solstice is the year's shortest day, but the start of winter also launches the sun's steady climb toward the long, warm days of summer.

This year, the northern winter solstice occurs on Wednesday, December 21, at 5:44 a.m. ET (10:44 UTC). It happens at the same moment no matter where you live, but because we've divided Earth into 24 times zones, people around the world will observe it at 24 different times of day.

Why does the solstice occur anyway, and how have people observed it through history? Read on for everything you need to know about the December solstice.

Earth's tilt is the reason for the season.