At Large : Cosmic wobbling and the end of one of those summers
A president, Illumination, The Fair, fireworks, a hurricane, Labor Day, back to school, autumn equinox. That's the familiar late summer rhythm of Martha's Vineyard. We know it, we expect it, we don't complain. Or, maybe we do a little. Really, we complain all the time. Also, this year, add an earthquake, and the hurricane's name is Irene.
These mornings, when he awakens at 5:30 am, his usual time, sunrise is a half hour away. In a few weeks, the equinox will be upon us, and one day he will arise in summer and go to bed in the fall. He has lived through the event and will again, perhaps.
The sun's early gleam lit the dark windows and limned the waving oaks and the rusting swamp maple out back. The trees did as they were told by the wind, unconcerned with the underlying astronomical principles at work in the universe, which includes us. Unconcerned also with the progress of Irene, with which the computer forecasting models are slowly coming to grip, so that, within a few hours of Irene's arrival, they — and we, unaided by computers — will be able to predict its course.
In the kitchen, as the sky lit itself in preparation for the big solar moment, he dumped coffee in the press, filled it from the hot water tap.
While the coffee made itself, he got out the old, royal blue Britannica — the paper one, not the one on CD. Unlike the thoughtless trees, he wondered what lay in store for him.
He read: "either of the two moments in the year when the sun is exactly above the equator and day and night are of equal length ... " That sounded familiar.
Coffee was ready. He poured it, black, into the large, big-bellied mug with the reclining English mastiff on the side, and he read some more.
"Also, either of the two points in the sky where the ecliptic (the sun's annual pathway) and the celestial equator intersect ... The autumnal equinox falls about Sept. 23, as the sun crosses the celestial equator going south."
Oh, he thought, that equator, not the one on all the globes.
The coffee was stronger than he had planned, and there were still a few moments before the sun flies by. He added milk. The dogs were sitting expectantly. What did they want? A walk, or some food.
Before he could decide, his eye fell on this: "Some astronomical coordinates ... are measured from the vernal equinox. It is sometimes called the first point of Aries because it was at the beginning of that constellation some 2,000 years ago. The term is still used, though precession of the equinoxes has moved the vernal equinox into Pisces."
If these equinoxes are preceding, he thought, how does anyone know that the moment will be at precisely 6:31 am on that fateful day? Are there no seconds involved here? How could there not be?
That's when he came upon the business about wobble. We're wobbling, the equinoxes are preceding, there is really a great deal going on. These dogs will just have to wait.
Although we haven't thought much about it, everybody has known about it since before 129 BC when the Greek astronomer Hipparchus "noticed that the positions of the stars were shifted in a systematic way from earlier Babylonian (Chaldean) measures." But it wasn't the celestial arrangement that was moving, it was earth, our observatory and vantage point.
Precession, the third-discovered motion of the earth, along with daily rotation and annual revolution, "consists of a cyclic wobbling in the orientation of the earth's axis of rotation with a period of almost 26,000 years."
The gravitational influences of the sun and the moon do it to us, and because of this wobbling the celestial poles of the earth trace out circles on the sky, and the "equinoxes drift westward along the ecliptic at the rate of 50.2 arc seconds annually as the celestial equator moves with the earth's precession."
Wobbling and drifting (in a fashion not unlike his own early morning progressions) the earth and the sun have conspired over this equinoctial instant. We count on it, distant and unknowable as it is. It means something to us, although while we may have recorded its measurement we have not approached the why of it.
He recalled an arrival at Bermuda years ago after a stormy, mostly cloudy sailing trip. The sun had been elusive.
It was night as the yacht approached the narrow entrance channel and its tumbling sea buoy. The navigator, without global positioning system, radar, or radio direction finder, mostly without starsights, and even days without sunsights, had somehow found this speck on the wobbling, watery globe. For which he remembered being terribly grateful. And as he recalled the gladdening events of that dark, heaving evening, he briefly lost track of the time.
The radio-controlled clock, which nightly gets in touch with Colorado to square itself with its atomic master, read 6:00. The sun had risen, at last, as time rushed on toward autumn. Each sunrise and sunset was a triumph of celestial precision over wavering and uncomprehending mortal vigilance. Only the dogs had maintained their appealing focus, insisting, as such creatures always do, on just exactly what matters most.