With the passing of the December solstice yesterday, the season officially shifts from autumn to winter, which may be news to some folks around these parts who have lately been battered by waves of cold and snow. We call it the “winter” solstice here, but in the Southern Hemisphere, the opposite calendrical condition prevails, and on that date Spring officially gives way to Summer.
The solstice (from Latin roots meaning “sun” and “to make stand,” respectively) is a regular astronomical event, driven by the Earth’s oblique tilt with respect to its orbit about the sun, which causes the sun’s apparent elevation in the sky to rise and fall day over day, month over month. Depending on which Hemisphere you happen to reside in, at the solstice the days are as short, or as long, as they are going to be throughout the course of the year, and for the next six months, give or take a few hours, they will trend the other way until the opposite condition occurs, in June. And so on.
What seems like an imperfection, this slight yet obvious tilt–a proper planet would stand up straight–is in reality a remnant of the early and rather violent period of our solar system’s formation. The smoothly spherical planets we see today were formed long ago by the accretion of many smaller bodies through numberless collisions. One of those collisions most likely tore off the chunk of rock and ferrous metal that became our moon, as it permanently nudged our planet off the vertical. The other planets of our local group show various degrees of tilt: Saturn leans 27 degrees, Uranus a remarkable 82, and Venus is tipped almost all the way over at 177 degrees.
Traditionally, the December solstice occurs on the twenty first day of that month. But the exact moment of this occurrence is never the same from year to year, dependent as it is on many interacting oscillations; wobbles upon wobbles. The axis of rotation precesses–spins around a centerpoint–exactly as does a top, on a time frame of about twenty two thousand years. The amount of axial tilt varies between about twenty degrees and about twenty four on a time frame of about forty thousand years. The Earth’s orbit about the sun varies between nearly circular and mildly elliptical on a time scale of about four hundred thousand years. Even that unit of time we call a year, defined as a complete revolution of the Earth about the sun, changes on an even larger time scale. We do not notice these slight changes, of course, because our life spans are far too short to directly perceive them. Were you to come back in eleven thousand years or so, you would find that the winter and summer solstice had traded places on the calendar. But come back in yet another eleven thousand and they would be right back where they “belong,” calendrically speaking.
As is our habit, we humans infuse the solstice with meaning. Sunlight is energy, upon which all living things depend. The loss of light means the loss of life itself. The winter solstice marks the end of the time of slow dying, and a welcome resurgence of the vital, life-giving force. So the solstice is both a nadir and an inflection point, upon which we may pin our fragile hopes.