The winter solstice is the shortest day of the year in the Northern Hemisphere.

Astronomical winter begins Sunday, Dec. 21 in the Northern Hemisphere. Here's how the solstice works.

When is winter? Weather forecasters in the Northern Hemisphere will tell you Dec. 1 through the end of February, which is known as meteorological winter. But based on Earth's tilt and orbit around the sun, it's Dec. 21, 2025, through March 20, 2026.

This is astronomical winter, which begins on the winter solstice with the shortest day of the year north of the equator and ends with the equinox (or "equal night") signifying the beginning of spring.

This year, the winter solstice occurs at 10:03 a.m. EST (15:03 UTC) on Sunday, Dec. 21, according to Time and Date. It marks the shortest day of the year and the first day of astronomical winter in the Northern Hemisphere, because Earth's rotational axis is tilted by 23.5 degrees. On the December solstice, Earth's northern axis is tipped away from the sun, causing the fewest hours of daylight in the Northern Hemisphere.

Sun stands still

Stonehenge was built to align with the sun at the solstice.

The solstice marks the point at which the sun appears to stop its southward movement and begins moving northward again in the sky. The solstice — Latin for "sun still," — is the moment when the sun rises and sets at its most southerly points on the horizon as seen from the Northern Hemisphere.

At noon on Dec. 21, the sun will be above the Tropic of Capricorn, a line of latitude approximately 23.5 degrees south of the equator that runs through Argentina, Australia, Botswana, Brazil, Chile, Madagascar, Mozambique, Namibia, Paraguay and South Africa.

Everything is reversed for the Southern Hemisphere, where it's the summer solstice south of the equator. Earth's southern axis is tipped toward the sun, causing the most hours of daylight and the shortest night of the year in the Southern Hemisphere.

These extremes are felt most of all at the planet's poles; the sun does not rise at the North Pole and does not set at the South Pole on the solstice. (Hence, "sun still.")

The winter solstice has long been celebrated as the rebirth of the sun in the Northern Hemisphere because it is when the sun is at its lowest in the sky, and in its wake, the days begin to get longer. The most famous celebration is at Stonehenge, a 5,000-year-old structure in England built to align with the sun at the solstice, according to English Heritage.