Newsweek areticle by Aristos Georgiou Photo by © Gary Hershorn/Getty Image
The December solstice this year will occur at 5:02 a.m. ET on Monday, December 21, marking the shortest day of the year and the beginning of astronomical winter in the Northern Hemisphere.
While we tend to think that the term “winter solstice” refers to an entire day, it technically only refers to a specific moment in time—when the Earth’s North Pole is pointing furthest away from the sun.
The day on which the December solstice occurs in the Northern Hemisphere has the fewest daylight hours and the longest night. From this date onwards, the days start to get longer again.
The reason the day of the winter solstice is so short in the Northern Hemisphere can be explained by the fact that Earth is tilted on its axis by 23.5 degrees relative to the plane of the planet’s orbit around the sun.
Thanks to this phenomenon, the Earth’s Northern Hemisphere is pointed as far away from the sun as possible at the December solstice. At the same time, the Southern Hemisphere is pointed furthest towards our star.
On December 21, the sun will take its lowest possible path across the sky after rising in the east and setting in the west, providing the Northern Hemisphere with fewer hours of light than any other day this year.
The December solstice marks the start of “astronomical winter.” This astronomical definition of winter is based on the movement of our planet around the sun.
“Meteorological winter,” on the other hand, begins three weeks earlier on December 1. This meteorological definition of winter is based on annual temperature cycles, with the mercury in the Northern Hemisphere typically falling to its lowest average levels in December, January and February.
Because of Earth’s tilt, different parts of our planet receive the sun’s most direct rays over the course of the year, giving rise to the seasons. When Earth’s North Pole is tilted away from the sun, the Northern Hemisphere experiences winter because it is receiving less direct radiation.
The opposite scenario is true when the Northern Hemisphere is tilted toward the sun. During this period the hemisphere experiences summer.
As you move further away from the equator either north or south, the effect of Earth’s tilt on the seasons becomes more pronounced. While Earth’s equatorial regions do experience different seasons—wet and dry—there is little variation in temperature throughout the year, unlike at higher latitudes.
The December solstice usually occurs on the 21st or 22nd day of the month but on rare occasions it falls on the 20th or 23rd.