Winter Solstice: Why Dec. 21 Is the Shortest Day — And How Ancient Romans Celebrated

The winter solstice — the shortest day of the year in the Northern Hemisphere — typically falls on Dec. 21. While it can feel like a day to endure, many ancient cultures treated the solstice as a turning point worth celebrating.

History and Traditions

For the ancient Romans, the solstice inspired the festival Dies Natalis Solis Invicti, the "Birthday of the Invincible Sun." By the third century A.D., that observance included widespread illumination and as many as 30 chariot races. The Roman calendar placed the solstice on Dec. 25, a date later adopted by early Christians for the celebration of Christ's birth — a date not specified in the Bible.

What Causes the Shortest Day?

The changing length of daylight is not caused by the sun getting "weaker" but by Earth's axial tilt. Our planet spins on an axis tilted by roughly 23 degrees. That tilt — which some scientists suggest may have resulted from a collision with a Mars-sized body named Theia about 4.5 billion years ago — means different hemispheres receive more-direct sunlight at different times of the year.

How It Looks Locally

In New Jersey on Dec. 21 there are about 9½ hours of daylight: sunrise near 7:15 a.m. and sunset around 4:30 p.m. By contrast, on the summer solstice (around June 20) that location receives roughly 14½ hours of daylight.

"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 in Paterson. "I really love it, being able to set my telescope up by 5 o'clock and getting to work by 6... The winter solstice is wonderful. We love it."

Opposite Seasons Across the Equator

When the Northern Hemisphere experiences its shortest day, the Southern Hemisphere has its longest. Countries like Australia and New Zealand celebrate the beginning of summer at this time of year.

Originally published on NorthJersey.com under the headline: "When is shortest day of year? Get ready for winter solstice."