Today is the winter solstice and the first day of winter in the Northern Hemisphere. It's all due to Earth's tilt, which ensures that the shortest day of every year falls around December 21.
But it's not all about astronomy.Since ancient times people have marked the winter solstice with countless cultural and religious traditions—it's no coincidence the modern holiday season surrounds the first day of winter.
Solstice in Space: Astronomy of the First Day of Winter
During the winter solstice the sun hugs closer to the horizon than at any other time during the year, yielding the least amount of daylight annually. On the bright side, the day after the winter solstice marks the beginning of lengthening days leading up to the summer solstice.
"Solstice" is derived from the Latin phrase for "sun stands still."
That's because—after months of growing shorter and lower since the summer solstice—the sun's arc through the sky appears to stabilize, with the sun seeming to rise and set in the same two places for several days. Then the arc begins growing longer and higher in the sky, reaching its peak at the summer solstice.
The solstices occur twice a year (around December 21 and June 21), because Earth is tilted by an average of 23.5 degrees as it orbits the sun—the same phenomenon that drives the seasons.
During the warmer half of the year in the Northern Hemisphere, the North Pole is tilted toward the sun. The northern winter solstice occurs when the "top" half of Earth is tilted away from the sun at its most extreme angle of the year.
Being the shortest day of the year, the winter solstice is essentially the year's darkest day, but it's not the coldest.
Because the oceans are slow to heat and cool, in December they still retain some warmth from summer, delaying the coldest of winter days for another month and a half. Similarly, summer doesn't hit its heat peak until August, a month or two after the summer solstice.
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