Had enough winter? Read on; now is a good time to reflect on how the solar system is trying to make you happy.
You know the basics, of course, about how the earth’s tilted access conspires with our orbit around the sun to produce long, warm days in summer and short, cold ones in winter. But as with so many obvious things in the natural world, this one merits a closer look, especially in the wake of a blizzard that made a depressing mess of the Island.
We’re well along in the process of lengthening days, and if you’re on any kind of regular schedule, it’s easy to observe. My personal benchmark is the 3:45 boat, which I take to Falmouth each Thursday. Not so long ago, the sun set before the boat had made its turn west outside Vineyard Haven’s harbor. Just few weeks after that, it dipped behind the waves as we approached Naushon, and now we hit Woods Hole in full sunlight.
A bit harder to notice is that the rate at which day length changes is variable, slowest around the solstices and fastest near the equinoxes (in late September and March). Over the next five weeks or so, each day will become more than three minutes longer than its predecessor. At that rate, a full week produces change you can hardly miss.
Of course as they days lengthen, the points on the horizon at which the sun rises and sets move northward. This, too, can be easy to notice (for me, the sun is suddenly in my eyes as I sit at my computer with my first cup of coffee in the morning). And correspondingly, the highest angle the sun reaches above the horizon, at noon, steadily increases.
This last point, again, is hard to overlook, but it produces a subtle result that has enormous implications in the natural world. As the sun gets higher, shining more directly down on the earth’s surface at our latitude, its rays pass through fewer miles of the earth’s atmosphere before they reach the ground. Losing less of its strength to water vapor, dust, and other obstacles in the air, the higher sun produces much brighter light and greater heat.
Longer days and brighter sun, combined, vastly increase the amount of solar energy we receive each day. For warm-blooded animals like birds, life becomes easier because they need less energy (in the form of food) to maintain their body temperature. The eased “energy budget” for birds allows them devote less time and effort to merely surviving and more to secondary priorities like socializing or establishing a territory. And for cold-blooded animals, like insects, the change can be even more dramatic: many types of insects use the strengthening sun to heat themselves up to operating temperature, meaning, in a very real sense, that their lives resume after a hiatus of several months.
Day length also prompts physical changes in wildlife (and probably in humans, too). For many species (probably including human beings), longer days produce hormonal changes, altering physiology and behavior. If, as Tennyson has it, in spring a young man’s fancy lightly turns to thoughts of love, for birds the effect is more like instant adolescence. Under the influence of longer days, many species of birds become territorial, aggressive, and obsessed with sex. Mating and preparing to raise young turns into the main focus of their lives.
You might say, then, that bird song is sunlight condensed into sound, and a predictable train of species join the chorus. Carolina wrens never totally quit singing in the winter, but as soon as we turn the corner of the winter solstice, lengthening days make them sing more. In early January, chickadees begin to chime in; later in the month, they’re joined by house finches, and as February proceeds, song sparrows tune up, and then their complex song is joined by the clear whistle of lecherous male cardinals. I love this process; the season’s first song from a new species never fails to lift my spirits.
Plants, too, respond. Buds swell, as cellular division resumes and leaves and flowers for the approaching season begin to grow. Seeds from last year, realizing in some dim, seedy kind of way that they are spending more time thawed and less time frozen, begin to germinate. Brown twigs aloft and desiccated leaf rosettes on the ground take on hints of color.
It is hard to tell which is the more potent force: the resolute change of day length and sun angle, constant from year to year and therefore offering wildlife a wholly reliable stimulus to respond to; or the secondary effect of longer days and brighter sun, in the form of erratically but generally increasing warmth. But between the two, rest assured that a most wonderful process is solidly under way: the onset of spring, with its predictable but still infinitely varying changes as the natural world wakes up.