By Aylin Woodward

The annual winter solstice in the Northern Hemisphere, which this year occurs on Dec. 21, marks the first day of winter and the shortest day of the year in that half of the world. It coincides with the summer solstice in the Southern Hemisphere, which marks the start of summer and the year's longest day there.

In the months leading up to the Northern Hemisphere's winter solstice, the sun's rays gradually shift southward from the equator until the sun reaches its most southerly point in the sky at the Tropic of Capricorn, a mapmaker's line that parallels the equator at 23.5 degrees south latitude. After a brief pause at that line on the solstice, the sun appears to reverse direction and begins its journey northward.

The word solstice comes from the Latin words sol, meaning sun, and sistere, meaning to stop.

For millennia, humanity has used this moment in the sun's apparent motion as a means of seasonal timekeeping, according to Ed Krupp, director of the Griffith Observatory in Los Angeles.

"It gives people a signal about the cosmic order that surrounds them and seems to govern their lives, and also gives them an opportunity to anticipate what is coming next," Dr. Krupp said.

When is the winter solstice?

The 2022 winter solstice comes on Wednesday, Dec. 21, at 9:48 p.m. Coordinated Universal Time $(UTC)$, the international standard time used by astronomers. That is 1:48 p.m. Pacific Time and 4:48 p.m. Eastern Time.

This precise moment in time is when the sun reaches its southernmost position in the sky, according to Dr. Krupp.

Though the exact dates can vary by a day or two every year, solstices occur every June and December. They occur at the same time all over the world, though with contrasting effects 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 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.

The Northern Hemisphere's winter solstice marks the moment each year when Earth's tilt brings the North Pole farthest from the sun. After that point, days in the Northern Hemisphere begin to lengthen and continue to do so until the summer solstice in June, which marks the longest period of daylight there.

Any planet with a tilt goes through seasonal changes and has solstices, according to Shannon Schmoll, director of Michigan State University's Abrams Planetarium. Mars, whose tilt is similar to Earth's, is one example.

How do solstices differ from equinoxes?

The winter and summer solstices in the Northern Hemisphere mark the days when this part of Earth gets the least or most daylight, respectively, with contrasting effects in the Southern Hemisphere.

Equinoxes, by contrast, mark the days on which everywhere on Earth gets nearly the same amount of daylight and darkness. Those occur annually 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.

"There's a point where you're going to transition from being tilted toward to titled away," Dr. Schmoll said. "Equinoxes are those transition points."

How many hours of daylight are there during the winter solstice?

Though the winter solstice marks the shortest day of the year in the Northern Hemisphere, the hours of daylight and the hours of darkness in a particular location depend on its latitude.

The Arctic Circle is shrouded in darkness for nearly 24 hours on the winter solstice as the North Pole is tilted away from the sun. In contrast, the Antarctic Circle gets about 24 hours of daylight on the winter solstice as our planet's tilt brings the South Pole closest to the sun.

Places in North America will receive about eight to 10 hours of daylight on the winter solstice, compared with between 14 and 16 hours on the summer solstice. Los Angeles, for instance, will get just under 10 hours of daylight on this year's winter solstice, compared with the roughly 14.5 hours of daylight that it got on the summer solstice in June, according to Dr. Krupp.

On the winter solstice, the sun traces its lowest arc in the sky, and thus shines more obliquely on the Northern Hemisphere.

Why is the winter solstice considered so important?

Though experts now calculate solstice times and days using mathematical models for Earth's orbital movements, our ancestors began marking the solstices thousands of years ago.

"If you watch the sun's path in the summer, it's really high in the sky, and in winter it's really low," said Dr. Schmoll. "Humans saw that pattern happening, and there was a fear that the sun would disappear, or they knew it would come back but there would have to be some sort of ritual in order to coax it back up into the sky."

The rituals sometimes involved specific sites and monuments, such as 5,000-year-old Stonehenge on England's Salisbury Plain. Its features align with the sun's rays on the longest and shortest days of the year.

The stones that make up Stonehenge were shaped and set up to frame at least two important events in the annual solar cycle -- the midwinter sunset on the winter solstice and the midsummer sunrise on the summer solstice, according to Jennifer Wexler, properties historian with English Heritage, a charity that manages Stonehenge.

The winter solstice may have been more important than the summer solstice for the builders of Stonehenge, she said, adding that the former was a crucial time when the sun's return hung in the balance and ideas of rebirth were intertwined with celebrations marking the day.

Excavations from a Stone Age settlement 2 miles away from the monument suggest that people held huge feasts around this time of year, Dr. Wexler said.

How is the winter solstice celebrated today?

Tens of thousands of people typically gather at locations around the world to observe the solstices, marking the days before and after with festivals and ritual celebrations.

This year, as it has for more than two decades, English Heritage will allow people to enter the Stonehenge site to celebrate the winter solstice. Though such access was suspended during the coronavirus pandemic in 2020, about 2,500 celebrants were in attendance last December.

"Many people who have spiritual or religious beliefs relating to the monument and the past come to celebrate the turning of the year and the changing seasons," Dr. Wexler said, adding that members of Druid, Wicca and pagan communities undertake ceremonies that include prayers and songs.

This explanatory article may be periodically updated.

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

(END) Dow Jones Newswires

December 21, 2022 06:02 ET (11:02 GMT)

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