AccuWeather reports that today, Dec. 22, this year's winter solstice, will be the darkest day of the year. Even though the earliest sunset is not on the day of the solstice (in fact it is earlier in December), it is the darkest day of the year because it's the shortest amount of time between sunrise and sunset.
This is caused by the Earth's relation to the sun, as well as the angle of the Earth's tilt.
"The Earth is at a constant angle to the Sun of 23.5 degrees. That axis is always pointing in the same directions so at some times of the year the sun is shining most brightly on the Southern Hemisphere in our winter, and in our summer, when it comes the other way, the sun is shining on the Northern Hemisphere more," said AccuWeather.com Chief Meteorologist Elliot Abrams. "On the day when the sun appears to be as far south as it ever gets, that's the winter solstice."
The amount of sunlight for the solstice varies by region. In New York, for example, there are roughly nine hours and 13 minutes of sunlight for the solstice. In Phoenix, on the other hand, there are nine hours and 57 minutes of sunlight.
"Astronomically, the sun is thought of as a point source. We define sunrise and sunset as when the disc of the sun makes contact with the horizon," said AccuWeather.com Expert Senior Meteorologist Dan Kottlowski. "So, if we're defining the winter solstice, we use the times of day when the disc of the Sun hits the horizon, not when the center of the sun hits the equator. It's all about perception."
While the solstice is the darkest day of the year, it's not the coldest. The oceans take a long time to heat up and cool down, and by December they still have some of their summer warmth. Typically, the coldest days of winter do not occur for another month and a half. On the flip side of the seasons, the summer does not hit its hottest point until August, long after the summer solstice in June, according to National Geographic.
"That's a 'budget situation,'" explained Abrams. "If there is more energy going out from the Earth (from the Northern Hemisphere) than coming in, it will get colder and vice versa. So, even at the end of December or beginning of January, the sun is still low in the sky and it doesn't add enough heat to reverse the cooling that's taking place. So, as we get to the end of January or beginning of February it finally gets high enough in the sky that it does start to reverse and we get our spring."
Abrams added, "In fact, I think there's a New England phrase that goes something like 'as the days get longer, the cold gets stronger.' In fact, the coldest weather ever recorded in the Northeast was Feb. 9, 1934. Philadelphia dropped to 11 below zero."
The word solstice literally means "sun stands still" in Latin.