Earth dances joyously to cosmic beat


December 19, 1992|By TOM HORTON

The days are thy footprints by the seasons thou renews creation.

-- from a hymn

to the Egyptian Sun God Just two more days and the tide here will be at low ebb. Not the liquid, moon-driven surges we normally associate with the bay, two highs and two lows every 24 hours; but a grander wave of light that troughs and crests six months apart across the hemisphere.

To its rhythms move events in the life of the Chesapeake as diverse as the flowering of underwater grasses and the spawning of rockfish; the arrival of eagles and the exodus of eels.

We call Tuesday, Dec. 21, the winter solstice. It is the shortest day of the year, a day when darkness reigns in our region for nearly 15 hours. This can be depressing, and it is probably no coincidence that our major celebrations during this period -- Christmas and Hanukkah -- incorporate plenty of lights to cheer us up.

In fact, it was not until the second or third century A.D. that inEurope Christmas began to overtake the pagan festival held around the solstice to worship Mithras, the Persian (also Roman and Hindu) god of light.

Even as Tuesday's long darkness signals the beginning of three months of winter, it also means that the climb back toward spring has begun. And, in species of life spread from the deep Atlantic to the tropics, a subtle trigger has clicked, beginning a countdown that will return them to the Chesapeake.

Solstice literally means "sun-stop," the point when our planet reaches the farthest boundary of its elliptical orbit, placing the Northern Hemisphere as far from the sun's radiation as it ever gets.

As of sunrise Wednesday, Dec. 22, we begin swinging back; the days will start to lengthen incrementally. The tide of light will not cease rising until June 21, the sun-soused summer solstice, when the day will be nearly 5 1/2 hours longer than now. The halfway points between the solstices, around March 21 and Sept. 21, are called the equinoxes, and the days then are 12 hours, exactly as long as the nights.

All of this cosmic clockwork is just that -- a superb clock, a precise and unvarying timing mechanism, worldwide in scope. And timing is critical to much of life on Earth.

A tree must know, somehow, when to bud; a bird, wintering hundreds or thousands of miles from its nesting grounds, cannot afford to undertake an exhausting migration only to find ice has not moved off the land or pond where it feeds. A fish needs its return to a spawning river to coincide with a season conducive to survival of its eggs.

No bird or fish can forecast the weather over vast distances; and weather can vary wildly from one season to another, and even be influenced profoundly for years at a time by events such as Mount Pinatubo's spewing volcanic ash into the atmosphere, resulting in worldwide cooling.

But light's clock is invariable. You can bet on the solstices and the equinoxes -- bet the survival of your race, no less. So it is no surprise that a lot of the planet's life has evolved to dance with the day length.

"The skirts of the sun's robe, passing over the ocean, stir the deep and its mysterious people move on the fringes of the light," wrote the naturalist Henry Beston. Indeed, as early as sunrise next Wednesday we can take heart in the fact that the lengthening daylight has begun ushering in the exuberant migrations of upstream-running fish that will, come April, May and June, revitalize every Chesapeake tributary.

Out in the cold Atlantic, the shad, striped bass, mullet, river herrings and other species will begin to sense the reversal in light's tide. To return to their Chesapeake spawning grounds, the fish will "cue" on a number of factors -- temperature, currents, movements of prey and even the unique odors that apparently emanate from each river and stream. But longer days prime the pump; the light registers on the pineal gland, a sort of atavastic eye in the skull. This in turn stimulates production of hormones, growth of sexual organs and other changes that essentially translate into an unspoken, but insistent, command: "Time to be moving."

In birds, you can see this light-induced traveling urge so clearly that German experimenters who first observed it coined a word, zugunruhe, meaning "migratory restlessness."

Some birds will heed it sooner than others. By January, bald eagles will be winging north into the bay region, staking out territory and building nests. Great blue herons won't be far behind, arriving to nest on the Potomac usually within a week of Valentine's Day.

The bay's winter residents, such as swans and other waterfowl, respond to changing day length -- or photoperiodism, as the phenomenon is known -- by timing their leaving to exploit the near round-the-clock daylight of the brief but glorious arctic summer.

The farther north you go, the longer is the year's longest day, and the briefer is its shortest one. In the polar regions, June sees 24 hours of daylight, and December descends into total darkness. Only on the equator are day and night always equal.

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