With its many museums, world-class planetarium and magnificent Central Park, this great city offers just about everything for science fans and nature buffs, but a clear nighttime sky is seldom among them. No, New Yorkers have little opportunity to experience astronomical events, but December 21 at 11:49 p.m. is one of those rare moments we can all enjoy. That is the moment of the winter solstice, but just what does that mean?
Of Equinoxes Hither and Yon
Our ancestors, both here and abroad, were far more familiar with the night sky than most of us living now. They were attuned to the seasons because their sustenance depended on following planting schedules not by a smartphone calendar, but by frosts and freezes. Pluck any 17th century New Yorker—or even Peter Stuyvesant himself, from New Netherland—from his comfortable surroundings and set him in today’s Manhattan. That old New Yorker will know more about the coming solstice, and the recent autumnal equinox, than any 100 New Yorkers today.
“Equinox” comes to us from Latin (aequus, or equal, and nox, or night), and means “equal night,” because the two equinoxes are moments in earth’s revolution around the sun when each hemisphere enjoys nearly equal hours of nighttime and daytime. The moment itself is the exact second an imaginary plane extending from earth’s imaginary waistband, the equator, would slice through the center of our sun. At an equinox, the terminator (the shadow line separating day from night) is exactly perpendicular to the equator. You can see it in pictures!
In the northern hemisphere, which enjoys an opposite cycle of seasons from the southern hemisphere, our autumnal equinox was September 23 at 4:22 a.m. (you probably snoozed through it). That was the southern hemisphere’s vernal, or spring, equinox. New York’s vernal equinox comes on March 21 at 12:31 a.m. (set your alarm), when your friends in South America will celebrate their autumnal equinox.
Of Solstices North and South
“Solstice” comes to us from Latin (sol, our sun, and sistere, meaning to stand still), to mark the moment when the sun is directly over the Tropic of Cancer (in the northern hemisphere) or the Tropic of Capricorn (in the southern hemisphere), much as it was directly over the equator to mark the equinoxes.
The earth is tilted on its axis by 23.5 degrees (due to our most recent, primordial collision with something really big), so we have four seasons. At the equator, these seasons bisect as “dry season” and “wet season,”each roughly six months long. Moving away from the equator, seasons become more pronounced. Here in New York, we bask in long summer days, relax in cool autumn evenings, chill to long winter nights, and awake to fresh spring mornings. As we move closer to the two poles, the seasons shrink into roughly six months of light (summer) and six months of near-dark (winter). Arctic and antarctic springs and autumns are little more than very short periods of thawing and refreezing.
The Tropics of Cancer and Capricorn lie 23.5 degrees away, north and south, from the equator.
To be more geographically neutral than saying "winter" solstice or "summer" solstice, we can refer to each solstice as the June and December solstices. Our first day of winter is the southern hemisphere’s first day of summer; they break out the woolens when we break out the Speedos.
Since few of us today fully appreciate the skies and our own solar system, NASA provides visualizations of both equinoxes and both solstices, so you need not even stir from your bed this December 21 to witness the wonder of it all.