Dec. 21 marks the winter solstice in the Northern Hemisphere. It's the shortest day of the year, and it's rooted in astronomical and religious significance. Many early cultures celebrated this as the day the sun "came back," commencing a period of longer days. Here are five things to know about the winter solstice.
1. It's the Day the Sun Stands Still — Sort Of
The word "solstice" comes from the Latin word 'solstitium,' which means "the sun stands still." We know it doesn't, literally (right, flat earthers?), but this is the day when the sun reaches the southernmost point as we see it from Earth and then "reverses" direction.
Since the sun doesn't actually move, here is what's actually happening: Earth doesn't orbit upright, but is instead tilted on its axis by 23.5 degrees. This tilt is what causes the four seasons. At the time of the winter solstice, the Northern Hemisphere is leaning the most away from the sun for the year.
2. One Man's Winter Solstice Is Another Man's Summer Solstice
The winter solstice in the Northern Hemisphere is known as the summer solstice in the Southern Hemisphere. In the Northern Hemisphere, this is the shortest day of the year, and after that, the days start to get longer and the nights shorter. In the Southern Hemisphere, Dec. 21 is the longest day of the year, and the days begin to shorten and the nights lengthen afterward.
Earth's closest point to the sun actually comes in early January. It may seem surprising that it's not summer in the Northern Hemisphere at that time but the difference between the sun at its farthest point (in July) and at its closest is just 3.3 percent, not enough to change seasons. It's the tilt of Earth's axis and not its orbit that gives the seasons.
3. It's a Specific Point in Time
Although most people will count all of Dec. 21 as the solstice, it is actually at a very specific time — the exact moment when the sun is directly over the Tropic of Capricorn (the imaginary latitudinal line between the equator and the Antarctic circle). In 2017, that time is 16:28 UTC or 11:28 EST. The date of the solstice can vary too — it's not always Dec. 21. It can also be the 20th or 22nd, and occasionally the 23rd. The last time the solstice was on Dec. 23 was in 1903, and the next will be 2303.
4. Christmas Celebrations Have Solstice Celebration Roots
You may know that many Christmas traditions are rooted in paganism, but the actual feast day of Christmas is very closely linked to a pagan Roman festival called Saturnalia. (Although the Bible doesn't give a date, historians believe Jesus was really born in the springtime rather than winter because the shepherds were watching their flocks outdoors at night.) Saturnalia was a weeklong festival to the god Saturn — the god of the sun, agriculture and time — which began on Dec. 17. Saturnalia was a time of feasting, merrymaking and gift-giving. It was so popular that many of its customs were incorporated into the celebration of Christmas when Christianity became the main religion of the West. Many cultures have a solstice festival that honors the return of the sun or a sun god.
5. It's a Time for Super-long Shadows
In December in the Northern Hemisphere, the sun is at its lowest arc across the sky which means long shadows from its light. The shadow you cast on the winter solstice is the longest you can make all year. Give it a try at noontime and see!