The winter solstice — the shortest day of the year — falls around Dec. 21, bringing reduced daylight (about 9½ hours in New Jersey versus roughly 14½ hours on the June summer solstice). Ancient peoples celebrated the solstice as the rebirth of the sun; Romans marked Dec. 25 with festivals that later influenced the Christian calendar. The change in daylight is caused by Earth’s ~23° axial tilt (possibly from a Theia impact), which gives the Southern Hemisphere summer while the North experiences its shortest day.
Why Dec. 21 Is the Shortest Day of the Year — And Why Ancient Cultures Celebrated It
The winter solstice setting sun creates a lens flare circle at Mt. Sinai Harbor, in Mt. Sinai, New York on Dec. 21, 2024.
The winter solstice — the shortest day and longest night of the year — arrives around Dec. 21. For many people it's a chilly stretch to endure, but for ancient cultures it was a turning point worth celebrating: the sun had begun to "return."
Ancient Celebrations and a Calendar Connection
On the Roman calendar the solstice inspired the festival Dies Natalis Solis Invicti, the "Birthday of the Invincible Sun." In the third century A.D. that holiday was marked with lights and even 30 chariot races. Because Romans observed a winter celebration on Dec. 25, early Christians — whose Bible does not specify a date for Jesus' birth — adopted that day for Christmas as their influence grew.
What Happens on Dec. 21
In the modern calendar the shortest day typically falls on Dec. 21. That means reduced daylight everywhere in the Northern Hemisphere. For example, in New Jersey on Dec. 21 there are roughly 9½ hours of daylight, with sunrise near 7:15 a.m. and sunset around 4:30 p.m.. By contrast, the summer solstice (around June 20) brings about 14½ hours of daylight in the same location.
“Obviously, from an astronomer's point of view, the winter solstice is delightful, because we have more time to observe,” said Gary Swangin, Warren County astronomer-at-large and former director of the Newark Museum Planetarium and the Panther Academy Planetarium. “I really love it — being able to set my telescope up by 5 o'clock and getting to work by 6.”
Why Day Length Changes — Not the Sun, But Earth
The changing length of daylight is caused by Earth's axial tilt, not the sun moving closer or farther. Earth spins on an axis tilted about 23 degrees relative to its orbital plane. That tilt means different hemispheres lean toward or away from the sun at different points in the year.
Some scientists propose that this tilt resulted from a colossal early collision: a Mars-sized object called Theia may have struck Earth about 4.5 billion years ago, ejecting material that later formed the moon and leaving Earth tilted. Whatever the origin, the tilt is what causes seasons and the variations in day length — so when the Northern Hemisphere has its shortest day, the Southern Hemisphere receives its most direct sunlight and celebrates the longest day.
What To Look For
Whether you treat Dec. 21 as a quiet observance or a festive moment, it's a good time to notice the sky: earlier darkness means stargazers can set up telescopes sooner, and many cultures use the solstice for seasonal rituals and lights. Enjoy the long night — spring and longer days return gradually from here.
Originally published by NorthJersey.com.
