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 towards the long, warm days of summer.  

The solstice occurs on Tuesday, December 22 at 4:48 UTC—that's late Monday night across most of North America. 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 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