The astronomical beginning of the winter season is signaled by Earth's tilt and the amount of sunlight received in the Northern Hemisphere.
In the Northern Hemisphere, the solstice that occurs in December marks the official, astronomical start of winter. A list of the exact dates and times of the winter solstice is below.
- December 22, 2007 at 1:08 am EST
- December 21, 2008 at 7:04 am EST
- December 21, 2009 at 12:47 pm EST
- December 21, 2010 at 6:38 pm EST
- December 22, 2011 at 12:30 am EST
- December 21, 2012 at 6:11 am EST
Determining the Winter Solstice
Each year there are two solstices, summer and winter, and two equinoxes, spring and fall. The winter solstice that occurs in December for the Northern Hemisphere is the summer solstice for the Southern Hemisphere.
The meaning of the word "solstice" is that the sun stands still. Indeed, if you have been watching the sun during sunrise and sunset over the past few months, you will have noticed it rising and setting farther and farther south each day. On the solstice the sun appears to stop and stand still, no longer journeying toward the south each day.
In actuality, the sun has not been moving farther south but the Earth has been orbiting the Sun. As it does the North Pole is sometimes tilted toward the sun, and sometimes its tilted away. At the winter solstice Earth's Northern Hemisphere is tilted as far away as it will get. Over the rest of the winter, the sun will very slowly begin to return toward the north.
Earthly Conditions as a Result of the Solstice
In the winter time, those in the Northern Hemisphere receive fewer hours of sunshine and less direct rays of sunshine, which combine to make it the coldest season. Many people wonder why the "first" day of winter isn't the middle point of winter and why it's not the coldest on the solstice. It is understandable to wonder why, if the December solstice marks the shortest day and the farthest south the sun will get, that it hasn't been colder leading up to the solstice and that it doesn't get warmer more quickly as the sun moves north again.
The answer lies in the Earth's atmosphere. It is a cumulative effect. In the summer, it takes the atmosphere a while to warm up from receiving the additional radiation, and therefore it is not hottest at the summer solstice. In the same way, the atmosphere has taken some time to cool down so that it is not the coldest at the winter solstice. The same thing can be noticed in the atmosphere on a daily basis. It is not warmest during the day when the sun is highest but in late afternoon. It is not coldest in the middle of the night but instead just before the dawn.
