In case you didn’t notice, the sun slipped into the Southern Hemisphere on Wednesday to deliver the blessings of spring and then summer to all things living below the equatorial belt. Those of us left in the bending light of its wake must now bank wood in our sheds, stock soup in our freezers, and books in our Kindles and shelves for the coming months of darkening and decrease.
For planets like ours that rotate on an axis tilted relative to its path around the sun, our heliocentric orbit is an endless Hokey Pokey dance of hemispheric leanings in and out. And for living things that have evolved on earth over the past 3.5 billion years, that really is what it’s all about.
Actually, there isn’t anyone who didn’t notice the autumnal equinox this week. We may have missed reading about it, or hearing about it, or thinking about it, but deep in our brains we noticed.
Equinox means equal night, and twice a year, in March and September, the sun spends roughly an equal amount of time above and below the horizon. It is not exactly equal, thanks to our atmosphere’s refractory properties, which bend the sun’s light so it appears to be above the horizon for a few minutes when it has, in fact, already set. The shifting angle and duration of sunlight, as subtle as it is hour by hour and day by day, is registered by the eyes of living creatures, and for some it triggers biological changes and instinctive behaviors to reproduce, or to migrate, or perhaps to put up some soup stock.
All organisms on earth, from the single-celled amoebae to humans, demonstrate a 24-hour, or circadian, rhythm that has evolved genetically. It influences, among other things, sleep cycles, body temperature, hormone levels, and even digestion. All this is controlled in humans by a tiny bundle of neurons called the suprachasmatic (-expialidocious?) nuclei located in the hypothalamus just below where the optic nerves cross in the brain.
Not everyone’s clock ticks to the same drummer. The average human circadian cycle generated by these neurons is 24 hours and 11 minutes. For some people (night owls) it is a little longer, for others (early birds) it is shorter, according to Dr Domien Beersma, who conducts research in “chronobiology” in the Netherlands. But if everybody’s internal circadian clock runs roughly 11 minutes longer than 24 hours each day, wouldn’t our body clocks loose a day every four months or so?
It turns out our eyes have evolved, not just for creating representational images of the world in our brains through the rods and cones in our retinas. There is a third type of retinal receptor that has nothing to do with vision. When stimulated with light, it provides those suprachasmatic nuclei with information that resets our circadian clocks every day and orients us to our position on the annual calendar. Daylight matters to us, quite literally, in ways we do not see.
Of course, this idea that the sun is coming and going with the seasons is a human conceit. We are the ones spinning in our daily and annual circuits, not the sun. We convert our dizzy view of things to a stable, centered view, perhaps to avoid the inevitable existential motion sickness that attends the reality of our situation.
Meanwhile, our life-giving star has dug its spurs into its own journey, galloping around the galaxy subject to seasons and cycles far beyond our ken.