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What is the winter solstice? Here’s what you need to know.

Once a year, an astronomical alignment ushers in this seasonal change. The winter solstice is celebrated across the world—and shrouded in myth.

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This year, the northern winter solstice falls on December 21 at 10:59 a.m. ET (December 21 at 3:59 UT). South of the Equator, this same moment marks the unofficial beginning of summer. Solstices occur at the same time around the world, but their local times vary with time zones. 

Traditionally, summer and winter solstices helped mark the changing of the seasons—along with their counterparts, the spring and autumnal equinoxes. However, today’s meteorologists officially use temperature records instead to draw lines between the seasons. So what exactly are solstices—and how have they been celebrated throughout history? Here’s all you need to know.

What are solstices?

Solstices occur because Earth’s axis of rotation is tilted about 23.4 degrees relative to Earth's orbit around the sun. This tilt drives our planet's seasons, as the Northern and Southern Hemispheres get unequal amounts of sunlight over the course of a year. From March to September, the Northern Hemisphere is tilted more toward the sun, driving its spring and summer. From September to March, the Northern Hemisphere is tilted away, so it feels like autumn and winter. The Southern Hemisphere's seasons are reversed.

On two moments each year—what are called solstices—Earth's axis is tilted most closely toward the sun. The hemisphere tilted most toward our home star sees its longest day, while the hemisphere tilted away from the sun sees its longest night. During the Northern Hemisphere’s summer solstice—which always falls around June 21—the Southern Hemisphere gets its winter solstice. Likewise, during the Northern Hemisphere's winter...

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