The Northern Hemisphere will encounter its briefest day and longest evening of the year on Tuesday, as the sun arrives at its most southerly point in the sky.
Winter Solstice 2021:
What is it?
The winter solstice, also called the hiemal solstice, hibernal solstice, and brumal solstice, occurs when either of Earth’s poles reaches its maximum tilt away from the Sun. This happens twice yearly, once in each hemisphere.
When is it?
The 2021 winter solstice takes place on Tuesday, Dec. 21, at 3:59 Coordinated Universal Time (UTC), the international standard time used by astronomers. That’s 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.
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.
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.
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’s summer.”
During this year’s winter solstice, the continental U.S. will get eight to 10 hours of daylight, compared to between 14 and 16 hours on the summer solstice.
“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.