Winter solstice marks shortest day, longest night and a turning point toward longer days

Winter Solstice marks the shortest day of the year

The solstice is a turning point that has been observed and celebrated by cultures around the world for thousands of years.

SAN FRANCISCO - Sunday, Dec. 21 is the winter solstice, marking the shortest day and longest night of the year. It is a turning point that has been observed and celebrated by cultures around the world for thousands of years.

How the solstice works:

The annual astronomical event occurs because of the way Earth moves through space, said Andrew Fraknoi, a professor of astronomy at the University of San Francisco’s Fromm Institute. As the planet orbits the sun, its axis is tilted, causing seasonal changes in daylight.

"We’re not going around the sun with our head held high," Fraknoi said. "In the winter, we lean away from the sun. The sun is low in the sky for us and doesn’t spend much time with us. In the summer, we lean into the sun and get more sunlight each day."

That tilt, he said, dates back billions of years to a violent collision early in the solar system’s history, when a small planet struck Earth and knocked it off balance. Unlike planets such as Venus and Jupiter, which orbit without a pronounced tilt, Earth’s angled axis creates the changing seasons and opposite seasonal patterns in the Northern and Southern hemispheres.

For astronomers today, the winter solstice remains significant because of its impact on daily life and human activity, Fraknoi said. Many aspects of society still depend on sunlight, making the shortest day of the year an important marker.

"After today, the days will get longer and the nights will get shorter," he said. "We’re making our way back toward summer and the longest day of the year."

Solstice in history:

Long before modern science explained the phenomenon, ancient cultures recognized the solstice as a moment of renewal. 

Sites such as Stonehenge in England are believed to have been designed, in part, to mark the sun’s position on the winter solstice.

"It was a signal that it’s going to get better — time for a celebration," Fraknoi said, noting that many societies created rituals and gatherings to lift spirits during the darkest time of the year.

That legacy continues today, he said, in the widespread use of light as a symbol during winter holidays, from Christmas lights to menorahs.

"Even ancient cultures were very much aware of the sky and what was happening," Fraknoi said.

As the solstice passes, daylight slowly begins to return — a change that has offered hope and inspiration across civilizations for millennia.