On December 21st, 2015 we learned about

Shedding light on the solstice’s stake in the start of winter

In addition to counting down to Christmas, my first grader has been diligently reminding us about when winter really starts. She’s aware of the solstice, but there’s some understandable confusion about why the start of winter means more sunshine, not less. All this is understandable, because by many counts the season and the daylight are related, but different, things.

Winter by orbit

The solstice on the 21st of December marks the when the North Pole is the farthest it can get from the Sun. Since or planet’s axis is tilted around 23º, the northern hemisphere and southern hemispheres don’t get equal exposure to the Sun. Depending where we are in our yearly orbit, the north or the south is going to be getting more or less sunlight. The December solstice is the North’s day of minimal sunlight, spending more time pointed away from the Sun’s light (and of course, it’s the South’s longest day.) Since more direct sunlight brings warmth, darker days means colder temperatures, and thus our tipped planet gets to enjoy seasonal weather.

Winter by weather

The importance of the tipped axis and sun exposure in seasons would then seem to support the idea of December 21st being the First Day of Winter, since… well, obviously it’s a big deal! It’s got to be something, right? From a meteorological perspective, it’s actually a bit late in the year to be the start of cold weather in the Northern Hemisphere. Since meteorologists look at winter as the months with the coldest average temperatures, they usually mark the start of winter with December 1st, extending until the start of March. This fits closer to our daily experience in many places, since winter weather often turns up well before the last week of the year.

Winter by daylight

There’s another metric to consider when measuring the onset of winter, which nicely combines the above ideas. The solstice marks the shortest day of the year in the Northern Hemisphere, but it’s also a turning point when the days start getting longer (for the most part). If winter is the measure of the shortest days of the year, the solstice suddenly becomes the apex of the season, falling right in the middle. Historically, this has been a popular way to measure the year, with many cultures marking winter as the darker days after the Fall equinox, up until the Spring equinox, both of which are equally light and dark.

Using light to measure the seasons seems to have fallen out of favor thanks to artificial lighting. What had once forced major, if cyclical, changes in lifestyle can now be nearly ignored. At least until you’re the one out shoveling the driveway in the dark.

Source: Don’t Believe the Hype: Winter Does Not Begin Tonight by Amos Zeeberg, Nautilus

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