This evening at 23:03 UT winter officially starts in the northern hemisphere with the winter solstice.
The solstice occurs when the northern hemisphere of the Earth is inclined at a maximum angle (23° 26′) away from the Sun and results in the shortest period of daylight. In fact, for residents within the Arctic Circle, 24 hours of darkness occurs and for those on the Tropic of Capricorn, the Sun will be directly overhead. After the winter solstice, the daytimes lengthen and the nights become lighter and shorter.
The opposite occurs in the southern hemisphere that sees the summer solstice at this same time. The reverse of this event occurs with the northern solstice on June 21st 2015 at 16:38 UT.