Don't Panic: Winter Solstice is here!

The winter solstice for those in the Northern Hemisphere has finally arrived! It occurred today at 12:42 UT (that's Universal Time, equivalent to Greenwich Meridian Time, or GMT), or 7:42am EST (6:42am CST).

So what is the solstice, why does it mark the start of winter (in the north), and why do we have all of these big celebrations at this time of year?

The seasons of the year are caused by the tilt of the earth (23.45 degrees) relative to the plane of earth's orbit, the ecliptic.


(image courtesy MSNBC)
This tilt causes the daily arc that the sun traces in the sky to change from day to day throughout the year. In the Northern hemisphere the sun's daily arc is highest at the summer solstice (June 21-22) and lowest at the winter solstice (Dec. 21-22). (If you want to know why the hours of day and night aren't exactly equal at the equinoxes, see my previous article: Vernal and Autumnal Equinoxes.)

(image courtesy Sandburg Center for Sky Awareness)
The term "solstice" comes from the Latin sol stetit, meaning "sun standing still". To the ancients, the day-to-day drift of the sun's arc would appear to stop drifting at the times of the solstice.

Many ancient cultures placed a lot of importance on the winter solstice (again in the northern hemisphere). Many held worship ceremonies to sun gods to encourage the sun to stop its southward travel and march back north again. These peoples would celebrate and rejoice the time of the winter solstice because it marked new life with the increasing daylight. Many Christmas and Hanukkah traditions are based on ancient winter solstice traditions, including the burning of the Yule log (this information from an article at the Boulder Public Library).

One curiosity with which I have always struggled is that the Winter Solstice is considered to mark the beginning of winter. I have always felt that the Winter Solstice should mark the middle of winter instead of its beginning. This would put the beginning of winter roughly in the first week of November, rather than the 3rd week of December.

Interestingly enough, ancient cultures believed the solstice to mark the middle of winter. Ancient Chinese calendars, for example, mark the "Start of Winter/Summer/Spring/Fall" to be offset by several weeks from "Autumnal/Vernal Equinoxes" and "Winter/Summer Solstices".

Although you may have heard that the Winter Solstice is the "official" start of winter, there is no official body that makes this designation. In Alaska, the beginning of winter is considered to be on the first full day where the air temperature does not rise above 0 degrees Celsius (freezing point of water). In Albany, New York, the start of winter is considered to be after the first snowfall. Meteorologists like to define the three coldest months, December, January and February as the the three months of winter for data collection and research purposes.

Part of the problem is that the season of Winter is defined differently by different cultures. So relating the start of winter to an exact astronomical event like the solstice is what is currently accepted by modern news media, although this practice truly is without meteorological, agricultural or historical merit.

So have a Happy Winter Solstice Celebration, whatever your culture!

Good Night!