Winter officially starts Thursday, December 21, 2017 at 11:28 AM.
This day each year is known as the Winter Solstice. The 21st is the shortest day of the year with only 9 hours, 12 minutes and 52 seconds of daylight.
The short day length is created as the earth orbits the sun. The earth is tilted on an axis at 23.5° tilt angle. This tilt will slowly pivot with the earth as it orbits the sun, which takes a year.
You may have noticed the sun deeper toward the southern horizon through the fall. This is because the earth is traveling its orbit around the sun on a tilted axis.
As the earth orbits the sun through the late summer and fall, it will cast a more direct angle toward the Southern Hemisphere. It will stop its southern migration at 23.5° south of the equator at the Tropic of Capricorn.
Since the earth is tilted on a 23.5° angle, it makes sense. It will only pivot to that angle through the year.
This less direct sunlight casts a larger shadow on the earth, creating longer nights and shorter days.
December 21 is the turning point of the year. From this date on, the earth will continue to orbit the sun on its axis, causing the more direct sun angles to lift north toward the equator on March 20, 2018 at 12:15 PM for the Equinox.
The Equinox features equal day and equal night hours. It will then continue to move north until June 21, 2018 at 6:07 AM for the Summer Solstice — the longest day of the year with 15 hours, 8 minutes and 15 seconds of daylight. That is the turning point of summer and the longer day of the year.
You would think our temperatures will be at the coldest on the Winter Equinox but the truth is that there is a lag time in the cold. Our coldest time of the year is almost a month later, around January 14 each year.