From astronomy to religion, the year's shortest day brings enduring mysteries. Why does the solstice occur anyway, and how have people observed it through history? Read on for everything you need to know about the December solstice. (Full story:
  1. The Solstice From Space
    Earth's tilt is the reason for the season. Our planet orbits the sun while tilted at an average of 23.5 degrees, so the Northern and Southern Hemispheres receive unequal amounts of sunlight. This causes both the solstices and the seasons. Lack of exposure to the sun's rays makes the winter solstice the darkest day of the year.
  2. Earliest Sunset? Not on the Solstice
    Still, most of us actually see the year's earliest sunset a week or two before the solstice. Why? Because the sun and our human clocks don't keep exactly the same time. We've organized our days into precise 24-hour segments but the Earth doesn't spin on its axis so precisely. So while the time from noon to noon is always exactly 24 hours the time between solar noons, the moment each day when the sun reaches its highest peak, varies.
  3. Can I See the Solstice?
    The sun's arc across the sky has been steadily dropping lower and becoming shorter since June. Now, at the north's winter solstice, it has reached its lowest possible arc—so low that in the few days surrounding the solstice it appears to rise and set in the same place. That phenomenon produced the Latin phrasing from which the word solstice was derived, meaning “sun stands still.” The sun's low angle also means that your noontime shadow is the longest of the year during the winter solstice.
  4. Ancient Solstice Sites
    In the ancient world a number of monuments were built to commemorate the solstice. One example is Newgrange, a huge Stone Age tomb mound built in the Irish countryside around 3200 B.C., about 1,000 years before Stonehenge. Ancient Egypt's sprawling temple of Karnak was constructed in alignment with the winter solstice at Luxor more than 4,000 years ago. Similar alignments can be seen from Angkor Wat to Machu Picchu.
  5. Christmas Connection?
    For Christians the solstice has long been overshadowed by Christmas. But to historian David Gwynn, the proximity of the events may not be an accident. One theory holds that Christmas was set on December 25 to replace a Roman holiday based on the pagan cult of “Sol Invictus” (the unconquered sun), says Gwynn.
  6. Still Celebrating the Sun
    Some ancient solstice celebrations continue to the present. Iran's Yalda festival marks the day when Mithra, an angel of light, was thought to have been born. China's Dōngzhì festival marks the time when winter's darkness begins to give way to light. Scandinavians celebrated Juul, or Yule, a multi-day feast marking the sun god's return. In Britain, Druids observed the solstice by cutting mistletoe. Today some of these traditions are still observed by modern pagans.