Sunday, 22 December 2019 at 0419 UTC — or Saturday, 21 December at 11:19 PM Eastern Standard Time, if you prefer — was the December Solstice, what we usually call the “first day of winter” in the Northern Hemisphere. We usually call the June Solstice the “first day of summer,” and of course it’s the opposite in the Southern Hemisphere. And, of course, like every year people are thinking back to the first snowfall or the first cold weather, and saying “Wait, what?”
There’s a good reason it’s confusing: we actually observe two different sets of seasons — Astronomical and Meteorological.
Long before we had any rules and special names, humans must have observed that over the course of a year, the weather grew colder for one part of the year, and then became warmer again. When we were primarily an African species, we probably noticed changes in precipitation most of all — closer to the equator, the length of the day doesn’t change much. But people kept exploring. extending their range to the north and south, and the farther from the Equator they ranged, the more dramatic the change became. The weather got colder in the winter, relatively hotter in the summer; days got shorter in the cold season and longer in the warm season, and for a lot of cultures, this settled down into four seasons.
The sequence of warm, cooling, cold, warming show up all over the world as the seasons. Meteorologists are primarily concerned with, well, weather, and so define their seasons by the weather. On our Gregorian calendar, the meteorological seasons are agreed to be: Spring in March, April, and May; Summer in June, July, and August; Autumn, September, October, and November; and Winter is December, January, and February.
There’s another phenomenon that’s important to us, and much more important to our ancestors, and that’s the position of the Sun. As we get closer to the cold part of the year, the sun rises to a lower and lower point; it seems like it might be going away. So someone, shaman or just some curious person, started marking how high the Sun went every day; over time, it became clear the sun traces the same pattern every year. In December, the Sun reliably stopped moving south and started to come back, an occasion of much relief and a fair bit of debauchery among our pagan ancestors. (And some not-so-Pagan: it’s probably not a coincidence that Christian celebration of Christmas migrated to the same season.)
That was the December solstice, that is “Sun stop.” Of course, there’s a similar point in June, which is the June solstice.
If you observe the Sun at the same time every day from one spot on Earth, that motion traces a pattern known as an analemma. Amazingly, the first analemma photograph was only taken in 1978.
To us now, except for astronomers, the solstice is just a fun fact, a curiosity. But to our ancestors, it must have been an amazing discovery — that the motion of the sun over the year was predictable, and we humans could predict it and over time come to understand it.
The observation of the solstice may have been the beginning of science and the end of assuming magic ruled the world.