The winter solstice marks the start of a new season.
In the Northern Hemisphere, it is the official start of winter. During our solstices, either the Northern or Southern Hemisphere is tilted toward or away from the sun. For winter in the Northern Hemisphere, the North Pole is tilted farthest away from the sun. This means then that the Southern Hemisphere gets the most incoming solar energy.
When is it?
Dec. 21 at 11:28 a.m. is the exact moment that the Northern Hemisphere is tilted farthest from the sun. The night is longest around the solstice so today will be the shortest day, but there is something to look forward to — the days will now slowly get longer.
How do the days change?
On Dec. 20, the day length was 9 hours and 21 minutes. By Jan. 31, the day length increases to 10 hours and 9 minutes. Each year there is a winter and summer solstice as well as a fall and spring equinox.
Didn't winter already start?
It depends on who you ask. Remember the astronomical seasons are based on the Earth's relation to the sun during orbit. Meteorologically speaking, winter began on Dec. 1 because seasons are classified this way based on the annual temperature cycle. Only climatologists and meteorologists use this classification. It is still astronomical seasons that are widely observed.