December 21 is the winter solstice, the shortest day of the year

In the Northern Hemisphere the December solstice (wiki) marks the longest night and shortest day of the year with the latest dawn and the sun at its lowest point in the sky. I seem to remember that when I was a kid, the first day of each new season was always considered to be the 21st (of March for Spring, June for Summer, September for Autumn, and December for Winter). I guess it's more accurate now.

In seed-time learn, in harvest teach, in winter enjoy.

~William Blake

Winter is the king of showmen,

Turning tree stumps into snowmen

And houses into birthday cakes

And spreading sugar over lakes.

Smooth and clean and frosty white,

The world looks good enough to bite.

That’s the season to be young,

Catching snowflakes on your tongue.

Snow is snowy when it’s snowing,

I’m sorry it’s slushy when it’s going.

~ Ogden Nash

I wonder if the snow loves the trees and fields, that it kisses them so gently? And then it covers them up snug, you know, with a white quilt; and perhaps it says "Go to sleep, darlings, till the summer comes again.

~ Lewis Carroll, Alice's Adventures in Wonderland & Through the Looking-Glass

What good is the warmth of summer, without the cold of winter to give it sweetness.

~ John Steinbeck, Travels with Charley: In Search of America

“Nothing burns like the cold.”

~ George R.R. Martin, A Game of Thrones

Nature has many scenes to exhibit, and constantly draws a curtain over this part or that. She is constantly repainting the landscape and all surfaces, dressing up some scene for our entertainment. Lately we had a leafy wilderness; now bare twigs begin to prevail, and soon she will surprise us with a mantle of snow. Some green she thinks so good for our eyes that, like blue, she never banishes it entirely from our eyes, but has created evergreens.

~ Henry David Thoreau, Walden

The winter solstice (wiki) produces the shortest day of the year in the northern hemisphere, when darkness descends over the land to its greatest extent. For centuries, humankind has greeted the solstice with mixed emotions - with fear that the darkness might not really recede; and with hope that the cycling of the seasons would again reassert itself. 

Of the winter season, American poet Ezra Pound (1885-1972) complained,

"Winter is icumen in,
Lhude sing Goddamn,
Raineth drop and staineth slop,
And how the wind doth ramm!
Sing: Goddamn."*

* N.B. This is actually a parody of an anonymous 13th-century English song:

"Sumer is icumen in,
Lhude sing cuccu!
Groweth sed and bloweth med,
And springeth the wude nu.
Sing cuccu."

Here's a brief explanation on the mechanics of solstices and equinoxes:

And a two minute NatGeo video:

The two revolutions, I mean the annual revolutions of the declination and of the centre of the Earth, are not completely equal; that is the return of the declination to its original value is slightly ahead of the period of the centre. Hence it necessarily follows that the equinoxes and solstices seem to anticipate their timing, not because the sphere of the fixed stars moves to the east, but rather the equatorial circle moves to the west, being at an angle to the plane of the ecliptic in proportion to the declination of the axis of the terrestrial globe.

~ Nicolaus Copernicus (wiki)

Also at this time of year:

Saturnalia (wiki):

The winter solstice festival Saturnalia began on December 17 and lasted for seven days in In Ancient Rome. These Saturnalian banquets were held from as far back as around 217 BCE to honor Saturn, the father of the gods.

Saturnalia by Antoine-Francois Callet (1741-1823) Musée du Louvre

The holiday was celebrated with a sacrifice at the Temple of Saturn, in the Roman Forum, and a public banquet, followed by private gift-giving, continual partying, and a carnival atmosphere that overturned Roman social norms.

The festival was characterized as a free-for-all when all discipline and orderly behavior was ignored.

Wars were interrupted or postponed, gambling was permitted, slaves were served by their masters and all grudges and quarrels were forgotten.

The Saturnalia would degenerate into a week-long orgy of debauchery and crime – giving rise to the modern use of the term 'saturnalia', meaning a period of unrestrained license and revelry. A mock 'king' was even chosen from a group of slaves or convicts and was allowed to behave as he pleased for seven days (until his eventual ritual execution).

Halcyon Days (wiki):

The seven days preceding, and the seven days following the shortest day, or the winter-solstice, were called by the ancients the Halcyon Days. This phrase, so familiar as expressive of a period of tranquility and happiness, is derived from a fable, that during the period just indicated, while the halcyon bird or king-fisher was breeding, the sea was always calm, and might be navigated in perfect security by the mariner. The name halcyon is derived from two Greek words: the sea & to conceive; and, according to the poetic fiction, the bird was represented as hatching her eggs on a floating nest, in the midst of the waters. Dryden thus alludes to the notion:

'Amidst our arms as quiet you shall be,
As halcyons brooding on a winter's sea.'

More on the Greek myth of Halcyon and Ceyx here.

Related posts: 

Autumnal equinox science, videos, quotes, poems, Vivaldi and Copernicus.

Spring is here: science, myths, "spring spheres" and more.