For people who like turning points, the winter solstice ranks right up there, bringing the end of short days and the return of more sun – ever so slowly.
"It's kind of a gut thing. People don't like the dark," says Deborah Byrd, editor in chief of EarthSky.org.
"After Christmas, you start to notice that the sun is setting later. Even a few minutes later is such a relief," she says.
The winter solstice arrives at 5:11 p.m. on Dec. 21, marking the shortest day of the year. On that day the United States will get just nine hours and 32 minutes of daylight.
"Up until winter solstice, the sun is moving southward from day to day. As it approaches solstice its southward march slows down," says Benjamin Burress, an astronomer at the Chabot Space & Science Center in Oakland.
At the solstice the sun stops going south and pauses, motionless. "Then after solstice, it is again moving northward in the sky each day," he says. Solstice means "stationary sun."
The solstice occurs because the Earth is tipped on its axis 23.5 degrees. In the northern hemisphere in the summer, the axis is pointing its most toward the sun on June 21. On that day the most light reaches us and we experience the longest day of the year and warmer temperatures: the summer solstice.
The reverse is true on Dec. 21. Then the axis is pointing its most away from the sun, bringing less light and colder temperatures. That is the winter solstice, the longest night of the year.
It's the opposite in the southern hemisphere, where Dec. 21 marks the longest day of the year and June 21 the shortest.
The midpoints, on March 20 and Sept. 22, are known as the equinoxes. On these days the axis is exactly in between and night and day are the same length, 12 hours.
All these dates loom large in myth and folklore.
"Culturally, the solstices and equinoxes are typically used to denote either the beginnings of the seasons or the center points of the seasons," as in England, says Rick Kline, with the Spacecraft Planetary Imaging Facility at Cornell University in Ithaca, N.Y.
"Christmas, Hanukkah, Kwanzaa and other holidays have arisen out of the solstices, equinoxes and the midpoints between them," he says.
The solstice is regarded by many traditional societies as the turning point of the year and a time of great concern, says Edwin Krupp, director of the Griffith Observatory in Los Angeles. It is a "cosmological crisis point, in which the outcome of the coming year would be determined," he says.
Many cultures have elaborate rituals at solstice time to ensure the return of the sun. In China the emperor would ascend the Temple of Heaven in Beijing to offer burned sacrifices as an intermediary between heaven and earth, Krupp says.
It is no coincidence that Christmas and the solstice occur near each other, he says.
"First, we don't really know when Christ was born, it's that simple," says Krupp. The Dec. 25 date was chosen by the church several centuries after the birth of Christ.
"It had its antecedents in Rome, which already had a celebration called Dies Natalis Solis Invictus, the Birthday of the Unconquerable Sun," he says.
For early peoples, for whom the sun was all light and all warmth, the solstice loomed large.
But even for us it's a difficult time. "Between clock-change day and when people take down their Christmas lights, if I can get through that month, then I'm OK," Byrd says.