Winter solstice is the shortest day of the year in the Northern Hemisphere and marks the official beginning of winter. The sun is at its lowest in our sky because the North Pole of our tilted planet is pointing away from it.
[Got a question? Send us an email and we'll look for an expert who can crack it.]
Timing of the winter solstice varies each year. In 2013 and 2014, it occurs on Dec. 21 but in 2015 it falls on Dec. 22 Universal Time.
If you're already sick of winter, look at the solstice this way: The days are now getting longer. In six months, Earth will be on the other side of the sun and residents of the Northern Hemisphere will enjoy the longest day of the year.
Check out this amazing series of photographs that reveals the sun's shifting position in our sky throughout the year.