The annual winter solstice begins winter for the northern hemisphere today at 12:11 p.m. EST. At that time the sun appears directly overhead at the southernmost point in our sky, along the Tropic of Capricorn, at 23.5 degrees south latitude. With sun in this position we get our shortest amount of daylight for the year. This is because the Earth’s North Pole is at its maximum tilt from the sun. Locations north of the equator see the sun follow its lowest and shortest arc across the southern sky. For the next six months, the days again grow longer as the sun spends more time each day above the horizon.
After reading this description of the northern hemisphere's relative distance from the sun and the shortest day, you might think that winter solstice would be the coldest day too. Not so. As you can see by the image of the St. Johns weather indicator, temperatures here aren't event crisp. St. Johns in northeast Florida doesn't typically see a substantial dip in temperature for another three weeks - just long enough to give the grandkids a nice warm visit with grandma and grandpa before starting school in January. And Fruit Cove is not unique. Northern hemisphere temperatures post-winter-solstice will decline for several more weeks to come. Why is that?
And while we're at it, what is a "solstice"? The word itself is derived from two Latin words: "sol" means "sun" and "sistere" means to "stand still". Now you know the origin of the words, but you may be even more confused than before. To describe the sun as "standing still" is surely a puzzlement. These days we know that the belief that the sun moves around the earth is itself an old misconception, based on the perception that it moves across the sky (along the other heavenly bodies observed from earth). We now know the sun, moon, and stars only appear to move across the sky from horizon to horizon because the earth is rotating in a west-to-east direction once every 24 hours or so, causing night and day. Astronomers since Copernicus have proven that the earth's movement is the cause of night and day. But "solstice" refers, not to the sun's "movement" east-to-west, but to its daily position north-to-south at its high point each day. This position change is caused not by the earth's rotation, but because the earth tips back and forth, from north-to-south-to-north, once a year. Winter solstice is the moment when the tipping changes direction from north to south, when the sun's apparent position stops "retreating" south along the sky. For six months thereafter, the sun appears to be at its highest each day in a more northern point in the sky, until the summer solstice, when the earth changes the direction of its own tipping motion, and the sun appears daily in an ever more southern high point.
So the "standing still of the sun" at solstice is actually a "standing still of the earth" (or more correctly, "changing the direction of earth's tilt"). It begins to sound a little like the science fiction movie, "the Day the Earth Stood Still". And while that story had no basis in fact, solstices are real and they occur twice a year, ushering in both summer and winter. The summer and winter solstices also are the cause of the longest and shortest hours of daylight. And in between each solstice is a celestial event called, "equinox," (equal day-and-night). The vernal and autumnal equinoxes that issue in spring and fall respectively are also caused by the earth's rotation and tipping. (Look up this blog for a future posting on equinoxes.) Heavenly events like these are significant for farmers. Equinoxes and solstices are also important to world religions, architecture, and culture in general. Some Christmas carols and traditions refer to the pagan winter solstice celebration of burning a "yule" log at Yuletide.
But what about the "lands down under"? Do they experience the same phenomenons? Yes and no.
So perhaps it's interesting to note that solstices and equinoxes define the start of the seasons, but is it necessary to know when they occur? Can't one just remember that winter begins on December 21? The long answer is, "no". The reason for the answer is that solstices and equinoxes occur at slightly different times each year. Nature does not set her events by our calendar. Given enough time for the earth's position to change relative to the sun (and therefore, for the sun's position in the sky to vary slightly from year to year), the winter solstice can occur anywhere between December 20 and December 23. It just takes years and years for the event to actually slip or advance by one day and thus reset the start of a particular season. Many of us mere mortals may never experience the change in dates for the seasons because planets generally change slowly.
So to all us residents of the northern hemisphere, winter has officially arrived. Unless you are referring to "meteorological winter". In which case, you're about three weeks late. Meteorological winter refers to the coldest three months of the year. In the northern hemisphere it means that meteorological winter begins on December 1 and ends on the last day of February (the 28th or 29th depending upon leap years). Let's not even touch the global warming debate...
One CAN say with certainty, however, that the start of winter is in the eye (or the thermometer) of the beholder.
