Field Notes—

Winter Solstice: A Triumph For Light

On The Darkest Day Of The Year

By Curtiss Clark

Two years ago, at dawn on a cold December morning, Kate and I piled rocks and stones into two cairns under an old maple tree in the yard. The tree stands at the crest of the hill that our property straddles. We were marking two points on a line to the southeast — a line that when extrapolated to the horizon hit the deep red bull’s-eye of the rising sun. It was the beginning of the shortest day of the year, the winter solstice. The sunrise was at the southern terminus of its annual journey up and down the eastern horizon.

People have been engaging in similar activities for thousands of years. Long before there was a month of December, more than 5,000 years ago, a remarkable tomb was constructed in a kidney-shaped mound atop a hill in what is today Newgrange, Ireland, about 30 miles northwest of Dublin. The cruciform chamber at the heart of the mound lies at the end of a narrow 62-foot passage. On the winter solstice, at four and a half minutes after sunrise, the rays of the sun slip through a slit above the door and pierce the passageway, illuminating carved designs in the chamber.

Modern day astronomers have calculated an adjustment for the slight change in the earth’s tilt over the past 5,000 years and found the same effect would have occurred for the tomb’s Stone Age builders four and a half minutes earlier, precisely at sunrise.

When and where the sun rises and sets isn’t so critical to us these days. Our homes and workplaces are well lit, and notwithstanding all the articles on seasonal affective disorder that crop up at this time of year, most of us make it through the dark days of winter without any lasting ill effects. Frankly, it seems like the toughest days of winter don’t come right at the beginning of the season, when there are the fewest hours of sunlight, but later on in February and early March when the days are longer but winter’s icy grip seems to have us by the throat.

Now, in December, we use the darkness as an adornment, wrapping holiday lights in its deep mystery and taking refuge in the heat and comfort of flickering firelight. Our days are swaddled in twilight at this time of year, and there is a magic to it.

Once you draw a bead on the winter solstice, as we have done with our cairns, you will notice how long it takes the sun to slow and turn around. The sun rises in roughly the same place on the horizon for nearly a week before it shifts its momentum northward toward the vernal equinox. The word solstice comes to us from the Latin sol stetit, which means “the sun stands still.”

 Using our backyard line-of-sight method, we would never know that the winter solstice occurs this year at precisely 7:22 pm Eastern Time on December 21. That day will be 9 hours and 11 minutes long, and the sun will climb just 25 degrees above the horizon and will shine with a power of 455 watts per square meter at noon. Compare that with the summer solstice on June 21 when the day will be 15 hours and 10 minutes long, and the sun will be 72 degrees above the horizon at noon generating 1,167 watts per square meter at noon.

These calculations are made using modern mathematics and physics and are beyond my reckoning. (I got them by plugging our longitude and latitude into an online photoperiod calculator.) But there is a light-measuring device entombed deep in the brains of mammals that notes the changing tilt of the world in relation to the sun. It triggers a variety of behaviors from breeding and feeding to migrations and hibernations.

The interplay of light and dark travels down the narrow passageway of the optic nerve from the retina through the hypothalamus to the tiny chamber of the pineal gland. Instead of revealing carved designs, however, light regulates the production of the hormone melatonin by the pineal. Melatonin production is suppressed during the day and released at night. Researchers have found that this hormone plays a key role in regulating both circadian (daily) rhythms, including sleep and wakefulness, and circannual cycles that induce specific seasonal behaviors at the same time each year.

The length of the days, when combined with changing temperatures, exert a profound influence on all living things. But what about us? We humans are encased in our illuminated living spaces during cold dark periods. Are we still affected by the ancient rhythms and cycles of the world as our ancestors were 5,000 years ago? The researchers say yes … and no.

As everyone knows, we are susceptible to seasonal behaviors, like cutting down fir trees and dragging them inside in December. But human biology doesn’t appear to pay much attention to the calendar. We don’t migrate to breeding grounds, or hibernate, or feel compelled to fast or feed on a seasonal schedule.

Melatonin does suffuse our blood each night, but for about half of us, our melatonin levels are not affected in the least by the length of days. That would be the half of us who are men. Women are another story.

Dr Thomas A. Wehr, a leading researcher on human biological clocks working at the National Institute of Mental Health ten years ago, studied “photoperiodicity” in humans and found that melatonin levels in men was the same at the winter solstice as it was at the summer solstice. Women, on the other hand, showed marked differences in the production of melatonin corresponding with the tilting of the world from dark to light in its annual circuit of the sun.

Dr Wehr’s study suggests that men may be more sensitive to artificial lighting than women. His findings also tie in with statistics that show that women are far more likely to suffer from seasonal affective disorder. Yet there is something in the blood of men and women alike that draws us to reflect on the universal at this time of year.

The triumph of the sun over darkness is celebrated year after year on the winter solstice. It is easy to understand how the pagan rites observed at Newgrange in Ireland, at Stonehenge in England, and at countless other locations on the globe, less durable and long forgotten, led directly to the choice of December 25 as the appropriate date to celebrate the historically undetermined date of the birth of Christ. The light of the world returns again and again in midwinter. Weather permitting, Kate and I will be standing in line with two cairns on the morning of December 21 awaiting its arrival.