What is the winter solstice and why does it matter?

This weekend’s winter solstice occurs quite early, on December 21 at 1:19 a.m. PT, initiating the start of astronomical winter.

Sunrise will be at 7:55 a.m. and sunset at 4:20 p.m., for a total of 8 hours and 25 minutes for the length of the day. When the sun is out, the day also features the longest shadow of the year since the peak sun angle above the horizon is at its lowest point, only about 19.7 degrees above the horizon. The winter solstice also marks the longest night of the year.

The winter solstice is the moment when the sun is centered over the Tropic of Capricorn in the Southern Hemisphere at 23.5 degrees south, thanks to the tilt of the Earth as it rotates around the sun throughout the year.

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In contrast, for the Southern Hemisphere, this day marks its longest period of daylight, just like our summer solstice in mid-June. Also of interest, those above the Arctic Circle will have no daylight at all on the winter solstice, as well as for several days on either side of the solstice.

From a weather perspective, the seasons lag the winter and summer solstices since it takes time for the sun to warm the Earth’s surface, ground and water.

Historically, January is our coldest month of the year, even as daylight hours start to increase. The same goes for July, our warmest month of the year, even as hours of daylight begin to shrink following the summer solstice in mid-June, with 16 hours and 2 minutes of daylight.

Winter solstice over the centuries

Early humans tracked the days by observing the sun as it cast shadows at different times of the year. Historians believe Stonehenge in England, built during the Stone Age, was constructed to track the sun’s annual progress across the sky.

Many countries around the world have celebrated the solstice over the centuries, including the Romans, Scandinavians, Albanians, Indians, English, Irish, Europeans, Iranians, Japanese, and other parts of Eastern Asia. In the Southern Hemisphere, many celebrate their summer solstice.

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After December 21, daylight hours will begin to grow longer, at first slowly by mere seconds, and then by the spring equinox in March, peaking at around three and a half minutes per day. The increase of daylight will then gradually slow, ending on the summer solstice in mid-June with just over 16 hours of daylight.

As I often joked just after the winter solstice, when working at the National Weather Service: summer is coming!

Ted Buehner is the KIRO Newsradio meteorologist. You can read more of Ted’s stories here and follow him on X.