Today marks the winter solstice, marking the day with the fewest hours of sunlight during the year. The 2016 winter solstice started at 4:44 a.m.
The winter solstice takes place every year because the Earth is tilted at an average of 23.5 degrees, causing the Northern and Southern Hemispheres to receive unequal amounts of sunlight. This results in the solstices and seasons, according to National Geographic.
Each hemisphere’s colder half of the year happens with it’s tilted away from the sun, and winter solstice marks the point when that half of the Earth is tilted away from the sun at its most extreme angle. This point falls in December for the Northern Hemisphere and June for the Southern Hemisphere.
A lack of exposure to the sun’s rays causes winter solstice to be the darkest day of the year, though not the coldest. Winter solstice is also not the day with the earliest sunset. That comes about a week or two before the solstice.
For more information on the shortest day of the year, click here.