Winter solstice — when it is and what it means

https://www.wsj.com/articles/winter-solstice-2021-when-is-what-to-know-11640024457?

Winter Solstice 2021: When It Is and How It Is Observed

The Northern Hemisphere will experience its shortest day and longest night of the year on Tuesday, as the sun reaches its most southerly point in the sky

By Aylin Woodward  Updated Dec. 20, 2021 9:52 pm ET

The winter solstice in the Northern Hemisphere marks the first day of winter in that part of the world—and the year’s shortest day and longest night there. It coincides with the summer solstice in the Southern Hemisphere, which marks the start of summer and the year’s longest day in that part of the world.

Solstices happen every June and December, though the exact dates vary by a day or two each year. The word solstice comes from the Latin words sol, meaning sun, and sistere, meaning to stop—which reflects our host star’s seemingly brief pause in the sky on the solstice before reversing direction. 

In the months leading up to the Northern Hemisphere’s winter solstice, which this year occurs on Dec. 21, the sun’s rays gradually shift southward from the equator until the sun reaches its most southerly point in the sky over Earth at the Tropic of Capricorn, a mapmaker’s line that parallels the equator at 23.5 degrees south latitude. 

The 2018 winter solstice was celebrated during an early morning gathering at Stonehenge in the U.K.PHOTO: BEN BIRCHALL/PA WIRE/ZUMA PRESS
Aymara indigenous people in Bolivia raise their hands at sunrise for the winter solstice, which marks the new year in their calendar.PHOTO: AIZAR RALDES/AGENCE FRANCE-PRESSE/GETTY IMAGES

Everywhere on Earth north of that line will get fewer than 12 hours of daylight on the winter solstice. Everywhere south of the line will get more than 12 hours. Following the solstice, the sun’s rays begin their six-month journey back northward.



When is the winter solstice happening?

The 2021 winter solstice takes place on Tuesday, Dec. 21, at 3:59 p.m. Coordinated Universal Time (UTC), the international standard time used by astronomers. That is 7:59 a.m. Pacific Time and 10:59 a.m. Eastern Time.

Every solstice occurs simultaneously at all points on the globe, though the effects contrast on opposite sides of the equator. 

What is the science behind the winter solstice? 

The axis around which Earth spins isn’t perpendicular to our planet’s orbit around the sun. Rather, Earth’s rotational axis is tilted at an angle of 23.5 degrees. That tilt causes different parts of the Earth to receive more or less sunlight as the planet traces its annual elliptical orbit around our host star—in other words, the tilt gives Earth its seasons. Planets with no such tilt, like Mercury, lack seasons.

On the winter solstice, our planet’s tilt brings the South Pole closest to the sun—and the Antarctic Circle gets 24 hours of daylight. The North Pole is tilted away from the sun, and the Arctic Circle is shrouded in darkness for nearly a full day.

James O’Donoghue, a planetary scientist with the Japan Aerospace Exploration Agency, likened our planet’s tilting phenomenon to a nodding head. 

“At certain points in the year, we’re going to be kind of nodding our head towards the sun,” he said. “That is summer.”

During this year’s winter solstice, the continental U.S. will get eight to 10 hours of daylight, compared with between 14 and 16 hours on the summer solstice.

The sun rose behind the Temple of Karnak in Luxor, Egypt, during the winter solstice in 2013.PHOTO: KHALED DESOUKI/AGENCE FRANCE-PRESSE/GETTY IMAGES

“On top of that, the intensity of sunlight is lower as well” on the winter solstice and during the winter months because the sun is shining obliquely on the Northern Hemisphere, Dr. O’Donoghue said.

How do solstices differ from equinoxes?

Solstices mark the days when the sun’s arc through the sky is the farthest north or south from the equator.

Equinoxes mark the days on which everyone on Earth “is getting pretty much the same amount of daylight and nighttime,” Dr. O’Donoghue said. Those occur in March and September, as one hemisphere shifts toward the sun and the other away from the sun. “Equinox” comes from the Latin words “aequi” and “nox,” meaning equal and night, respectively.

Why is the winter solstice important?

The winter solstice in the Northern Hemisphere marks the point at which the season’s short days start to lengthen—continuing until the summer solstice in June, when there is the longest period of daylight and the shortest period of darkness there. 

Sites and monuments that mark the solstice, like Stonehenge in England, show that their builders anticipated the arrival of spring, with its warmer weather and increased daylight.

“Marking the shortest day and the longest day tells you something about the people that built Stonehenge. They are farmers, they are people who have got herds of cattle and pigs, and they’re growing crops,” said Susan Greaney, an archaeologist with English Heritage, which manages the Stonehenge site. “So obviously, the changing seasons and the turning of the year was really significant for them.”

Many rituals and celebrations that occur on or around the solstice focus on “cleaning out the bad things from the preceding year and beginning again,” said Dr. Natalie Kononenko, a retired professor of Ukrainian ethnography at the University of Alberta in Canada.

“There’s a human sense of the reality that this is a point of change and transition,” she said.

This article will be updated.

Write to Aylin Woodward at aylin.woodward@wsj.com

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